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827 строки
30 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
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<title>Language Compatibility</title>
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<link type="text/css" rel="stylesheet" href="menu.css" />
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<link type="text/css" rel="stylesheet" href="content.css" />
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<style type="text/css">
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</style>
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</head>
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<body>
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<!--#include virtual="menu.html.incl"-->
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<div id="content">
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<!-- ======================================================================= -->
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<h1>Language Compatibility</h1>
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<!-- ======================================================================= -->
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<p>Clang strives to both conform to current language standards (C99,
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C++98) and also to implement many widely-used extensions available
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in other compilers, so that most correct code will "just work" when
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compiler with Clang. However, Clang is more strict than other
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popular compilers, and may reject incorrect code that other
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compilers allow. This page documents common compatibility and
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portability issues with Clang to help you understand and fix the
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problem in your code when Clang emits an error message.</p>
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<ul>
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<li><a href="#c">C compatibility</a>
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<ul>
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<li><a href="#inline">C99 inline functions</a></li>
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<li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
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<li><a href="#lvalue-cast">Lvalue casts</a></li>
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<li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
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<li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li>
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<li><a href="#inline-asm">Inline assembly</a></li>
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</ul>
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</li>
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<li><a href="#objective-c">Objective-C compatibility</a>
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<ul>
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<li><a href="#super-cast">Cast of super</a></li>
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<li><a href="#sizeof-interface">Size of interfaces</a></li>
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<li><a href="#objc_objs-cast">Internal Objective-C types</a></li>
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<li><a href="#c_variables-class">C variables in @class or @protocol</a></li>
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</ul>
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</li>
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<li><a href="#c++">C++ compatibility</a>
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<ul>
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<li><a href="#vla">Variable-length arrays</a></li>
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<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
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<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
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<li><a href="#undep_incomplete">Incomplete types in templates</a></li>
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<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
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<li><a href="#default_init_const">Default initialization of const
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variable of a class type requires user-defined default
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constructor</a></li>
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<li><a href="#param_name_lookup">Parameter name lookup</a></li>
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</ul>
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</li>
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<li><a href="#objective-c++">Objective-C++ compatibility</a>
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<ul>
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<li><a href="#implicit-downcasts">Implicit downcasts</a></li>
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</ul>
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<ul>
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<li><a href="#class-as-property-name">Using <code>class</code> as a property name</a></li>
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</ul>
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</li>
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</ul>
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<!-- ======================================================================= -->
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<h2 id="c">C compatibility</h3>
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<!-- ======================================================================= -->
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<!-- ======================================================================= -->
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<h3 id="inline">C99 inline functions</h3>
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<!-- ======================================================================= -->
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<p>By default, Clang builds C code according to the C99 standard,
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which provides different semantics for the <code>inline</code> keyword
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than GCC's default behavior. For example, consider the following
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code:</p>
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<pre>
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inline int add(int i, int j) { return i + j; }
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int main() {
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int i = add(4, 5);
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return i;
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}
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</pre>
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<p>In C99, <code>inline</code> means that a function's definition is
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provided only for inlining, and that there is another definition
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(without <code>inline</code>) somewhere else in the program. That
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means that this program is incomplete, because if <code>add</code>
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isn't inlined (for example, when compiling without optimization), then
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<code>main</code> will have an unresolved reference to that other
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definition. Therefore we'll get a (correct) link-time error like this:</p>
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<pre>
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Undefined symbols:
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"_add", referenced from:
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_main in cc-y1jXIr.o
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</pre>
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<p>By contrast, GCC's default behavior follows the GNU89 dialect,
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which is the C89 standard plus a lot of extensions. C89 doesn't have
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an <code>inline</code> keyword, but GCC recognizes it as an extension
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and just treats it as a hint to the optimizer.</p>
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<p>There are several ways to fix this problem:</p>
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<ul>
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<li>Change <code>add</code> to a <code>static inline</code>
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function. This is usually the right solution if only one
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translation unit needs to use the function. <code>static
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inline</code> functions are always resolved within the translation
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unit, so you won't have to add a non-<code>inline</code> definition
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of the function elsewhere in your program.</li>
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<li>Remove the <code>inline</code> keyword from this definition of
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<code>add</code>. The <code>inline</code> keyword is not required
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for a function to be inlined, nor does it guarantee that it will be.
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Some compilers ignore it completely. Clang treats it as a mild
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suggestion from the programmer.</li>
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<li>Provide an external (non-<code>inline</code>) definition
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of <code>add</code> somewhere else in your program. The two
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definitions must be equivalent!</li>
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<li>Compile with the GNU89 dialect by adding
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<code>-std=gnu89</code> to the set of Clang options. This option is
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only recommended if the program source cannot be changed or if the
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program also relies on additional C89-specific behavior that cannot
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be changed.</li>
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</ul>
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<p>All of this only applies to C code; the meaning of <code>inline</code>
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in C++ is very different from its meaning in either GNU89 or C99.</p>
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<!-- ======================================================================= -->
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<h3 id="vector_builtins">"missing" vector __builtin functions</h3>
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<!-- ======================================================================= -->
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<p>The Intel and AMD manuals document a number "<tt><*mmintrin.h></tt>"
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header files, which define a standardized API for accessing vector operations
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on X86 CPUs. These functions have names like <tt>_mm_xor_ps</tt> and
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<tt>_mm256_addsub_pd</tt>. Compilers have leeway to implement these functions
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however they want. Since Clang supports an excellent set of <a
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href="../docs/LanguageExtensions.html#vectors">native vector operations</a>,
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the Clang headers implement these interfaces in terms of the native vector
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operations.
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</p>
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<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to
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builtin function calls, like <tt>__builtin_ia32_paddw128</tt>. These builtin
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functions are an internal implementation detail of GCC, and are not portable to
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the Intel compiler, the Microsoft compiler, or Clang. If you get build errors
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mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p>
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<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC
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architectures respectively. For these, make sure to use the <arm_neon.h>
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and <altivec.h> headers.</p>
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<p>For x86 architectures this <a href="builtins.py">script</a> should help with
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the manual migration process. It will rewrite your source files in place to
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use the APIs instead of builtin function calls. Just call it like this:</p>
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<pre>
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builtins.py *.c *.h
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</pre>
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<p>and it will rewrite all of the .c and .h files in the current directory to
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use the API calls instead of calls like <tt>__builtin_ia32_paddw128</tt>.</p>
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<!-- ======================================================================= -->
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<h3 id="lvalue-cast">Lvalue casts</h3>
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<!-- ======================================================================= -->
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<p>Old versions of GCC permit casting the left-hand side of an assignment to a
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different type. Clang produces an error on similar code, e.g.,</p>
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<pre>
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lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
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supported
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(int*)addr = val;
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^~~~~~~~~~ ~
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</pre>
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<p>To fix this problem, move the cast to the right-hand side. In this
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example, one could use:</p>
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<pre>
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addr = (float *)val;
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</pre>
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<!-- ======================================================================= -->
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<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3>
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<!-- ======================================================================= -->
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<p>Clang disallows jumps into the scope of a <tt>__block</tt>
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variable. Variables marked with <tt>__block</tt> require special
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runtime initialization. A jump into the scope of a <tt>__block</tt>
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variable bypasses this initialization, leaving the variable's metadata
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in an invalid state. Consider the following code fragment:</p>
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<pre>
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int fetch_object_state(struct MyObject *c) {
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if (!c->active) goto error;
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__block int result;
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run_specially_somehow(^{ result = c->state; });
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return result;
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error:
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fprintf(stderr, "error while fetching object state");
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return -1;
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}
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</pre>
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<p>GCC accepts this code, but it produces code that will usually crash
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when <code>result</code> goes out of scope if the jump is taken. (It's
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possible for this bug to go undetected because it often won't crash if
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the stack is fresh, i.e. still zeroed.) Therefore, Clang rejects this
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code with a hard error:</p>
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<pre>
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t.c:3:5: error: goto into protected scope
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goto error;
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^
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t.c:5:15: note: jump bypasses setup of __block variable
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__block int result;
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^
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</pre>
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<p>The fix is to rewrite the code to not require jumping into a
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<tt>__block</tt> variable's scope, e.g. by limiting that scope:</p>
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<pre>
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{
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__block int result;
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run_specially_somehow(^{ result = c->state; });
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return result;
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}
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</pre>
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<!-- ======================================================================= -->
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<h3 id="block-variable-initialization">Non-initialization of <tt>__block</tt>
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variables</h3>
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<!-- ======================================================================= -->
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<p>In the following example code, the <tt>x</tt> variable is used before it is
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defined:</p>
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<pre>
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int f0() {
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__block int x;
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return ^(){ return x; }();
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}
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</pre>
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<p>By an accident of implementation, GCC and llvm-gcc unintentionally always
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zero initialized <tt>__block</tt> variables. However, any program which depends
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on this behavior is relying on unspecified compiler behavior. Programs must
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explicitly initialize all local block variables before they are used, as with
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other local variables.</p>
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<p>Clang does not zero initialize local block variables, and programs which rely
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on such behavior will most likely break when built with Clang.</p>
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<!-- ======================================================================= -->
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<h3 id="inline-asm">Inline assembly</h3>
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<!-- ======================================================================= -->
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<p>In general, Clang is highly compatible with the GCC inline assembly
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extensions, allowing the same set of constraints, modifiers and operands as GCC
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inline assembly.</p>
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<p>On targets that use the integrated assembler (such as most X86 targets),
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inline assembly is run through the integrated assembler instead of your system
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assembler (which is most commonly "gas", the GNU assembler). The LLVM
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integrated assembler is extremely compatible with GAS, but there are a couple of
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minor places where it is more picky, particularly due to outright GAS bugs.</p>
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<p>One specific example is that the assembler rejects ambiguous X86 instructions
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that don't have suffixes. For example:</p>
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<pre>
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asm("add %al, (%rax)");
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asm("addw $4, (%rax)");
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asm("add $4, (%rax)");
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</pre>
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<p>Both clang and GAS accept the first instruction: because the first
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instruction uses the 8-bit <tt>%al</tt> register as an operand, it is clear that
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it is an 8-bit add. The second instruction is accepted by both because the "w"
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suffix indicates that it is a 16-bit add. The last instruction is accepted by
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GAS even though there is nothing that specifies the size of the instruction (and
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the assembler randomly picks a 32-bit add). Because it is ambiguous, Clang
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rejects the instruction with this error message:
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</p>
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<pre>
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<inline asm>:3:1: error: ambiguous instructions require an explicit suffix (could be 'addb', 'addw', 'addl', or 'addq')
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add $4, (%rax)
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^
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1 error generated.
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</pre>
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<p>To fix this compatibility issue, add an explicit suffix to the instruction:
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this makes your code more clear and is compatible with both GCC and Clang.</p>
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<!-- ======================================================================= -->
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<h2 id="objective-c">Objective-C compatibility</h3>
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<!-- ======================================================================= -->
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<!-- ======================================================================= -->
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<h3 id="super-cast">Cast of super</h3>
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<!-- ======================================================================= -->
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<p>GCC treats the <code>super</code> identifier as an expression that
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can, among other things, be cast to a different type. Clang treats
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<code>super</code> as a context-sensitive keyword, and will reject a
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type-cast of <code>super</code>:</p>
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<pre>
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super.m:11:12: error: cannot cast 'super' (it isn't an expression)
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[(Super*)super add:4];
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~~~~~~~~^
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</pre>
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<p>To fix this problem, remove the type cast, e.g.</p>
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<pre>
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[super add:4];
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</pre>
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<!-- ======================================================================= -->
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<h3 id="sizeof-interface">Size of interfaces</h3>
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<!-- ======================================================================= -->
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<p>When using the "non-fragile" Objective-C ABI in use, the size of an
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Objective-C class may change over time as instance variables are added
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(or removed). For this reason, Clang rejects the application of the
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<code>sizeof</code> operator to an Objective-C class when using this
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ABI:</p>
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<pre>
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sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
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non-fragile ABI
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int size = sizeof(NSArray);
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^ ~~~~~~~~~
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</pre>
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<p>Code that relies on the size of an Objective-C class is likely to
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be broken anyway, since that size is not actually constant. To address
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this problem, use the Objective-C runtime API function
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<code>class_getInstanceSize()</code>:</p>
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<pre>
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class_getInstanceSize([NSArray class])
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</pre>
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<!-- ======================================================================= -->
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<h3 id="objc_objs-cast">Internal Objective-C types</h3>
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<!-- ======================================================================= -->
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<p>GCC allows using pointers to internal Objective-C objects, <tt>struct objc_object*</tt>,
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<tt>struct objc_selector*</tt>, and <tt>struct objc_class*</tt> in place of the types
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<tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> respectively. Clang treats the
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internal Objective-C structures as implementation detail and won't do implicit conversions:
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<pre>
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t.mm:11:2: error: no matching function for call to 'f'
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f((struct objc_object *)p);
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^
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t.mm:5:6: note: candidate function not viable: no known conversion from 'struct objc_object *' to 'id' for 1st argument
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void f(id x);
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^
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</pre>
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<p>Code should use types <tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt>
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instead of the internal types.</p>
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<!-- ======================================================================= -->
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<h3 id="c_variables-class">C variables in @interface or @protocol</h3>
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<!-- ======================================================================= -->
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<p>GCC allows the declaration of C variables in
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an <code>@interface</code> or <code>@protocol</code>
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declaration. Clang does not allow variable declarations to appear
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within these declarations unless they are marked <code>extern</code>.</p>
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<p>Variables may still be declared in an @implementation.</p>
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<pre>
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@interface XX
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int a; // not allowed in clang
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int b = 1; // not allowed in clang
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extern int c; // allowed
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@end
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</pre>
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<!-- ======================================================================= -->
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<h2 id="c++">C++ compatibility</h3>
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<!-- ======================================================================= -->
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<!-- ======================================================================= -->
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<h3 id="vla">Variable-length arrays</h3>
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<!-- ======================================================================= -->
|
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<p>GCC and C99 allow an array's size to be determined at run
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time. This extension is not permitted in standard C++. However, Clang
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supports such variable length arrays in very limited circumstances for
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compatibility with GNU C and C99 programs:</p>
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<ul>
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<li>The element type of a variable length array must be a POD
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("plain old data") type, which means that it cannot have any
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user-declared constructors or destructors, any base classes, or any
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members of non-POD type. All C types are POD types.</li>
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<li>Variable length arrays cannot be used as the type of a non-type
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template parameter.</li> </ul>
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<p>If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:
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<ol>
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<li>replace the variable length array with a fixed-size array if you can
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determine a reasonable upper bound at compile time; sometimes this is as
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simple as changing <tt>int size = ...;</tt> to <tt>const int size
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= ...;</tt> (if the initializer is a compile-time constant);</li>
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<li>use <tt>std::vector</tt> or some other suitable container type;
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or</li>
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<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
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just remember to <tt>delete[]</tt> it.</li>
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</ol>
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<!-- ======================================================================= -->
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<h3 id="dep_lookup">Unqualified lookup in templates</h3>
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<!-- ======================================================================= -->
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<p>Some versions of GCC accept the following invalid code:
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<pre>
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template <typename T> T Squared(T x) {
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return Multiply(x, x);
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}
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int Multiply(int x, int y) {
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return x * y;
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}
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int main() {
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Squared(5);
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}
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</pre>
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<p>Clang complains:
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<pre> <b>my_file.cpp:2:10: <span class="error">error:</span> use of undeclared identifier 'Multiply'</b>
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return Multiply(x, x);
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<span class="caret"> ^</span>
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<b>my_file.cpp:10:3: <span class="note">note:</span> in instantiation of function template specialization 'Squared<int>' requested here</b>
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Squared(5);
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<span class="caret"> ^</span>
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</pre>
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<p>The C++ standard says that unqualified names like <q>Multiply</q>
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are looked up in two ways.
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<p>First, the compiler does <i>unqualified lookup</i> in the scope
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|
where the name was written. For a template, this means the lookup is
|
|
done at the point where the template is defined, not where it's
|
|
instantiated. Since <tt>Multiply</tt> hasn't been declared yet at
|
|
this point, unqualified lookup won't find it.
|
|
|
|
<p>Second, if the name is called like a function, then the compiler
|
|
also does <i>argument-dependent lookup</i> (ADL). (Sometimes
|
|
unqualified lookup can suppress ADL; see [basic.lookup.argdep]p3 for
|
|
more information.) In ADL, the compiler looks at the types of all the
|
|
arguments to the call. When it finds a class type, it looks up the
|
|
name in that class's namespace; the result is all the declarations it
|
|
finds in those namespaces, plus the declarations from unqualified
|
|
lookup. However, the compiler doesn't do ADL until it knows all the
|
|
argument types.
|
|
|
|
<p>In our example, <tt>Multiply</tt> is called with dependent
|
|
arguments, so ADL isn't done until the template is instantiated. At
|
|
that point, the arguments both have type <tt>int</tt>, which doesn't
|
|
contain any class types, and so ADL doesn't look in any namespaces.
|
|
Since neither form of lookup found the declaration
|
|
of <tt>Multiply</tt>, the code doesn't compile.
|
|
|
|
<p>Here's another example, this time using overloaded operators,
|
|
which obey very similar rules.
|
|
|
|
<pre>#include <iostream>
|
|
|
|
template<typename T>
|
|
void Dump(const T& value) {
|
|
std::cout << value << "\n";
|
|
}
|
|
|
|
namespace ns {
|
|
struct Data {};
|
|
}
|
|
|
|
std::ostream& operator<<(std::ostream& out, ns::Data data) {
|
|
return out << "Some data";
|
|
}
|
|
|
|
void Use() {
|
|
Dump(ns::Data());
|
|
}</pre>
|
|
|
|
<p>Again, Clang complains about not finding a matching function:</p>
|
|
|
|
<pre>
|
|
<b>my_file.cpp:5:13: <span class="error">error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'ns::Data const')</b>
|
|
std::cout << value << "\n";
|
|
<span class="caret">~~~~~~~~~ ^ ~~~~~</span>
|
|
<b>my_file.cpp:17:3: <span class="note">note:</span> in instantiation of function template specialization 'Dump<ns::Data>' requested here</b>
|
|
Dump(ns::Data());
|
|
<span class="caret">^</span>
|
|
</pre>
|
|
|
|
<p>Just like before, unqualified lookup didn't find any declarations
|
|
with the name <tt>operator<<</tt>. Unlike before, the argument
|
|
types both contain class types: one of them is an instance of the
|
|
class template type <tt>std::basic_ostream</tt>, and the other is the
|
|
type <tt>ns::Data</tt> that we declared above. Therefore, ADL will
|
|
look in the namespaces <tt>std</tt> and <tt>ns</tt> for
|
|
an <tt>operator<<</tt>. Since one of the argument types was
|
|
still dependent during the template definition, ADL isn't done until
|
|
the template is instantiated during <tt>Use</tt>, which means that
|
|
the <tt>operator<<</tt> we want it to find has already been
|
|
declared. Unfortunately, it was declared in the global namespace, not
|
|
in either of the namespaces that ADL will look in!
|
|
|
|
<p>There are two ways to fix this problem:</p>
|
|
<ol><li>Make sure the function you want to call is declared before the
|
|
template that might call it. This is the only option if none of its
|
|
argument types contain classes. You can do this either by moving the
|
|
template definition, or by moving the function definition, or by
|
|
adding a forward declaration of the function before the template.</li>
|
|
<li>Move the function into the same namespace as one of its arguments
|
|
so that ADL applies.</li></ol>
|
|
|
|
<p>For more information about argument-dependent lookup, see
|
|
[basic.lookup.argdep]. For more information about the ordering of
|
|
lookup in templates, see [temp.dep.candidate].
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
Some versions of GCC accept the following invalid code:
|
|
|
|
<pre>
|
|
template <typename T> struct Base {
|
|
void DoThis(T x) {}
|
|
static void DoThat(T x) {}
|
|
};
|
|
|
|
template <typename T> struct Derived : public Base<T> {
|
|
void Work(T x) {
|
|
DoThis(x); // Invalid!
|
|
DoThat(x); // Invalid!
|
|
}
|
|
};
|
|
</pre>
|
|
|
|
Clang correctly rejects it with the following errors
|
|
(when <tt>Derived</tt> is eventually instantiated):
|
|
|
|
<pre>
|
|
my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
|
|
DoThis(x);
|
|
^
|
|
this->
|
|
my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
|
|
void DoThis(T x) {}
|
|
^
|
|
my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
|
|
DoThat(x);
|
|
^
|
|
this->
|
|
my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
|
|
static void DoThat(T x) {}
|
|
</pre>
|
|
|
|
Like we said <a href="#dep_lookup">above</a>, unqualified names like
|
|
<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
|
|
<tt>Derived</tt> is defined, not when it's instantiated. When we look
|
|
up a name used in a class, we usually look into the base classes.
|
|
However, we can't look into the base class <tt>Base<T></tt>
|
|
because its type depends on the template argument <tt>T</tt>, so the
|
|
standard says we should just ignore it. See [temp.dep]p3 for details.
|
|
|
|
<p>The fix, as Clang tells you, is to tell the compiler that we want a
|
|
class member by prefixing the calls with <tt>this-></tt>:
|
|
|
|
<pre>
|
|
void Work(T x) {
|
|
<b>this-></b>DoThis(x);
|
|
<b>this-></b>DoThat(x);
|
|
}
|
|
</pre>
|
|
|
|
Alternatively, you can tell the compiler exactly where to look:
|
|
|
|
<pre>
|
|
void Work(T x) {
|
|
<b>Base<T></b>::DoThis(x);
|
|
<b>Base<T></b>::DoThat(x);
|
|
}
|
|
</pre>
|
|
|
|
This works whether the methods are static or not, but be careful:
|
|
if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
|
|
dispatch!
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="undep_incomplete">Incomplete types in templates</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
The following code is invalid, but compilers are allowed to accept it:
|
|
|
|
<pre>
|
|
class IOOptions;
|
|
template <class T> bool read(T &value) {
|
|
IOOptions opts;
|
|
return read(opts, value);
|
|
}
|
|
|
|
class IOOptions { bool ForceReads; };
|
|
bool read(const IOOptions &opts, int &x);
|
|
template bool read<>(int &);
|
|
</pre>
|
|
|
|
The standard says that types which don't depend on template parameters
|
|
must be complete when a template is defined if they affect the
|
|
program's behavior. However, the standard also says that compilers
|
|
are free to not enforce this rule. Most compilers enforce it to some
|
|
extent; for example, it would be an error in GCC to
|
|
write <tt>opts.ForceReads</tt> in the code above. In Clang, we feel
|
|
that enforcing the rule consistently lets us provide a better
|
|
experience, but unfortunately it also means we reject some code that
|
|
other compilers accept.
|
|
|
|
<p>We've explained the rule here in very imprecise terms; see
|
|
[temp.res]p8 for details.
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="bad_templates">Templates with no valid instantiations</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
The following code contains a typo: the programmer
|
|
meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
|
|
|
|
<pre>
|
|
template <class T> class Processor {
|
|
...
|
|
void init();
|
|
...
|
|
};
|
|
...
|
|
template <class T> void process() {
|
|
Processor<T> processor;
|
|
processor.innit(); // <-- should be 'init()'
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
Unfortunately, we can't flag this mistake as soon as we see it: inside
|
|
a template, we're not allowed to make assumptions about "dependent
|
|
types" like <tt>Processor<T></tt>. Suppose that later on in
|
|
this file the programmer adds an explicit specialization
|
|
of <tt>Processor</tt>, like so:
|
|
|
|
<pre>
|
|
template <> class Processor<char*> {
|
|
void innit();
|
|
};
|
|
</pre>
|
|
|
|
Now the program will work — as long as the programmer only ever
|
|
instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
|
|
it's hard, and sometimes impossible, to diagnose mistakes in a
|
|
template definition before it's instantiated.
|
|
|
|
<p>The standard says that a template with no valid instantiations is
|
|
ill-formed. Clang tries to do as much checking as possible at
|
|
definition-time instead of instantiation-time: not only does this
|
|
produce clearer diagnostics, but it also substantially improves
|
|
compile times when using pre-compiled headers. The downside to this
|
|
philosophy is that Clang sometimes fails to process files because they
|
|
contain broken templates that are no longer used. The solution is
|
|
simple: since the code is unused, just remove it.
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
|
|
constructor, C++ doesn't allow you to default construct a <tt>const</tt>
|
|
instance of it like this ([dcl.init], p9):
|
|
|
|
<pre>
|
|
class Foo {
|
|
public:
|
|
// The compiler-supplied default constructor works fine, so we
|
|
// don't bother with defining one.
|
|
...
|
|
};
|
|
|
|
void Bar() {
|
|
const Foo foo; // Error!
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
To fix this, you can define a default constructor for the class:
|
|
|
|
<pre>
|
|
class Foo {
|
|
public:
|
|
Foo() {}
|
|
...
|
|
};
|
|
|
|
void Bar() {
|
|
const Foo foo; // Now the compiler is happy.
|
|
...
|
|
}
|
|
</pre>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="param_name_lookup">Parameter name lookup</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>Due to a bug in its implementation, GCC allows the redeclaration of function parameter names within a function prototype in C++ code, e.g.</p>
|
|
<blockquote>
|
|
<pre>
|
|
void f(int a, int a);
|
|
</pre>
|
|
</blockquote>
|
|
<p>Clang diagnoses this error (where the parameter name has been redeclared). To fix this problem, rename one of the parameters.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2 id="objective-c++">Objective-C++ compatibility</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="implicit-downcasts">Implicit downcasts</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>Due to a bug in its implementation, GCC allows implicit downcasts
|
|
of Objective-C pointers (from a base class to a derived class) when
|
|
calling functions. Such code is inherently unsafe, since the object
|
|
might not actually be an instance of the derived class, and is
|
|
rejected by Clang. For example, given this code:</p>
|
|
|
|
<pre>
|
|
@interface Base @end
|
|
@interface Derived : Base @end
|
|
|
|
void f(Derived *p);
|
|
void g(Base *p) {
|
|
f(p);
|
|
}
|
|
</pre>
|
|
|
|
<p>Clang produces the following error:</p>
|
|
|
|
<pre>
|
|
downcast.mm:6:3: error: no matching function for call to 'f'
|
|
f(p);
|
|
^
|
|
downcast.mm:4:6: note: candidate function not viable: cannot convert from
|
|
superclass 'Base *' to subclass 'Derived *' for 1st argument
|
|
void f(Derived *p);
|
|
^
|
|
</pre>
|
|
|
|
<p>If the downcast is actually correct (e.g., because the code has
|
|
already checked that the object has the appropriate type), add an
|
|
explicit cast:</p>
|
|
|
|
<pre>
|
|
f((Derived *)base);
|
|
</pre>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3 id="class-as-property-name">Using <code>class</code> as a property name</h3>
|
|
<!-- ======================================================================= -->
|
|
|
|
<p>In C and Objective-C, <code>class</code> is a normal identifier and
|
|
can be used to name fields, ivars, methods, and so on. In
|
|
C++, <code>class</code> is a keyword. For compatibility with existing
|
|
code, Clang permits <code>class</code> to be used as part of a method
|
|
selector in Objective-C++, but this does not extend to any other part
|
|
of the language. In particular, it is impossible to use property dot
|
|
syntax in Objective-C++ with the property name <code>class</code>, so
|
|
the following code will fail to parse:</p>
|
|
|
|
<pre>
|
|
@interface I {
|
|
int cls;
|
|
}
|
|
+ (int)class;
|
|
@end
|
|
|
|
@implementation I
|
|
- (int) Meth { return I.class; }
|
|
@end
|
|
<pre>
|
|
|
|
<p>Use explicit message-send syntax instead, i.e. <code>[I class]</code>.</p>
|
|
|
|
</div>
|
|
</body>
|
|
</html>
|