gecko-dev/mfbt/TypeTraits.h

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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* Template-based metaprogramming and type-testing facilities. */
#ifndef mozilla_TypeTraits_h_
#define mozilla_TypeTraits_h_
/*
* These traits are approximate copies of the traits and semantics from C++11's
* <type_traits> header. Don't add traits not in that header! When all
* platforms provide that header, we can convert all users and remove this one.
*/
#include <wchar.h>
namespace mozilla {
/**
* Helper class used as a base for various type traits, exposed publicly
* because <type_traits> exposes it as well.
*/
template<typename T, T Value>
struct IntegralConstant
{
static const T value = Value;
typedef T ValueType;
typedef IntegralConstant<T, Value> Type;
};
/** Convenient aliases. */
typedef IntegralConstant<bool, true> TrueType;
typedef IntegralConstant<bool, false> FalseType;
namespace detail {
// The trickery used to implement IsBaseOf here makes it possible to use it for
// the cases of private and multiple inheritance. This code was inspired by the
// sample code here:
//
// http://stackoverflow.com/questions/2910979/how-is-base-of-works
template<class Base, class Derived>
struct BaseOfHelper
{
public:
operator Base*() const;
operator Derived*();
};
template<class Base, class Derived>
struct BaseOfTester
{
private:
template<class T>
static char test(Derived*, T);
static int test(Base*, int);
public:
static const bool value =
sizeof(test(BaseOfHelper<Base, Derived>(), int())) == sizeof(char);
};
template<class Base, class Derived>
struct BaseOfTester<Base, const Derived>
{
private:
template<class T>
static char test(Derived*, T);
static int test(Base*, int);
public:
static const bool value =
sizeof(test(BaseOfHelper<Base, Derived>(), int())) == sizeof(char);
};
template<class Base, class Derived>
struct BaseOfTester<Base&, Derived&> : FalseType {};
template<class Type>
struct BaseOfTester<Type, Type> : TrueType {};
template<class Type>
struct BaseOfTester<Type, const Type> : TrueType {};
} /* namespace detail */
template<bool Condition, typename A, typename B>
struct Conditional;
/*
* IsBaseOf allows to know whether a given class is derived from another.
*
* Consider the following class definitions:
*
* class A {};
* class B : public A {};
* class C {};
*
* mozilla::IsBaseOf<A, B>::value is true;
* mozilla::IsBaseOf<A, C>::value is false;
*/
template<class Base, class Derived>
struct IsBaseOf
: Conditional<detail::BaseOfTester<Base, Derived>::value, TrueType, FalseType>::Type
{};
namespace detail {
template<typename From, typename To>
struct ConvertibleTester
{
private:
static From create();
template<typename From1, typename To1>
static char test(To to);
template<typename From1, typename To1>
static int test(...);
public:
static const bool value =
sizeof(test<From, To>(create())) == sizeof(char);
};
} // namespace detail
/**
* IsConvertible determines whether a value of type From will implicitly convert
* to a value of type To. For example:
*
* struct A {};
* struct B : public A {};
* struct C {};
*
* mozilla::IsConvertible<A, A>::value is true;
* mozilla::IsConvertible<A*, A*>::value is true;
* mozilla::IsConvertible<B, A>::value is true;
* mozilla::IsConvertible<B*, A*>::value is true;
* mozilla::IsConvertible<C, A>::value is false;
* mozilla::IsConvertible<A, C>::value is false;
* mozilla::IsConvertible<A*, C*>::value is false;
* mozilla::IsConvertible<C*, A*>::value is false.
*
* For obscure reasons, you can't use IsConvertible when the types being tested
* are related through private inheritance, and you'll get a compile error if
* you try. Just don't do it!
*/
template<typename From, typename To>
struct IsConvertible
: Conditional<detail::ConvertibleTester<From, To>::value, TrueType, FalseType>::Type
{};
/**
* Conditional selects a class between two, depending on a given boolean value.
*
* mozilla::Conditional<true, A, B>::Type is A;
* mozilla::Conditional<false, A, B>::Type is B;
*/
template<bool Condition, typename A, typename B>
struct Conditional
{
typedef A Type;
};
template<class A, class B>
struct Conditional<false, A, B>
{
typedef B Type;
};
/**
* EnableIf is a struct containing a typedef of T if and only if B is true.
*
* mozilla::EnableIf<true, int>::Type is int;
* mozilla::EnableIf<false, int>::Type is a compile-time error.
*
* Use this template to implement SFINAE-style (Substitution Failure Is not An
* Error) requirements. For example, you might use it to impose a restriction
* on a template parameter:
*
* template<typename T>
* class PodVector // vector optimized to store POD (memcpy-able) types
* {
* EnableIf<IsPod<T>::value, T>::Type* vector;
* size_t length;
* ...
* };
*/
template<bool B, typename T = void>
struct EnableIf
{};
template<typename T>
struct EnableIf<true, T>
{
typedef T Type;
};
/**
* IsSame tests whether two types are the same type.
*
* mozilla::IsSame<int, int>::value is true;
* mozilla::IsSame<int*, int*>::value is true;
* mozilla::IsSame<int, unsigned int>::value is false;
* mozilla::IsSame<void, void>::value is true;
* mozilla::IsSame<const int, int>::value is false;
* mozilla::IsSame<struct S, struct S>::value is true.
*/
template<typename T, typename U>
struct IsSame : FalseType {};
template<typename T>
struct IsSame<T, T> : TrueType {};
/**
* Traits class for identifying POD types. Until C++11 there's no automatic
* way to detect PODs, so for the moment this is done manually. Users may
* define specializations of this class that inherit from mozilla::TrueType and
* mozilla::FalseType (or equivalently mozilla::IntegralConstant<bool, true or
* false>, or conveniently from mozilla::IsPod for composite types) as needed to
* ensure correct IsPod behavior.
*/
template<typename T>
struct IsPod : public FalseType {};
template<> struct IsPod<char> : TrueType {};
template<> struct IsPod<signed char> : TrueType {};
template<> struct IsPod<unsigned char> : TrueType {};
template<> struct IsPod<short> : TrueType {};
template<> struct IsPod<unsigned short> : TrueType {};
template<> struct IsPod<int> : TrueType {};
template<> struct IsPod<unsigned int> : TrueType {};
template<> struct IsPod<long> : TrueType {};
template<> struct IsPod<unsigned long> : TrueType {};
template<> struct IsPod<long long> : TrueType {};
template<> struct IsPod<unsigned long long> : TrueType {};
template<> struct IsPod<bool> : TrueType {};
template<> struct IsPod<float> : TrueType {};
template<> struct IsPod<double> : TrueType {};
template<> struct IsPod<wchar_t> : TrueType {};
template<typename T> struct IsPod<T*> : TrueType {};
/**
* IsPointer determines whether a type is a pointer type (but not a pointer-to-
* member type).
*
* mozilla::IsPointer<struct S*>::value is true;
* mozilla::IsPointer<int**>::value is true;
* mozilla::IsPointer<void (*)(void)>::value is true;
* mozilla::IsPointer<int>::value is false;
* mozilla::IsPointer<struct S>::value is false.
*/
template<typename T>
struct IsPointer : FalseType {};
template<typename T>
struct IsPointer<T*> : TrueType {};
} /* namespace mozilla */
#endif /* mozilla_TypeTraits_h_ */