codeql/change-notes/1.21/analysis-cpp.md

# Improvements to C/C++ analysis

## General improvements

## New queries

| **Query**                   | **Tags**  | **Purpose**                                                        |
|-----------------------------|-----------|--------------------------------------------------------------------|
| Call to alloca in a loop (`cpp/alloca-in-loop`) | reliability, correctness, external/cwe/cwe-770 | Finds calls to `alloca` in loops, which can lead to stack overflow if the number of iterations is large.  Newly displayed on [LGTM](https://lgtm.com/rules/1508831665988/). |
| Call to function with fewer arguments than declared parameters (`cpp/too-few-arguments`) | correctness, maintainability, security | Finds all cases where the number of arguments is fewer than the number of parameters of the function, provided the function is also properly declared/defined elsewhere. Results are displayed by default on [LGTM](https://lgtm.com/rules/1508860726279/). |
| Call to a function with one or more incompatible arguments (`cpp/mismatched-function-arguments`) | correctness, maintainability | Finds all cases where the types of arguments do not match the types of parameters of the function, provided the function is also properly declared/defined elsewhere. Results are not displayed by default on [LGTM](https://lgtm.com/rules/1508849286093/). |
| Use of dangerous function (`cpp/dangerous-function-overflow`) | reliability, security, external/cwe/cwe-242 | Finds calls to `gets`, which does not guard against buffer overflow. These results were previously detected by the `cpp/potentially-dangerous-function` query. Results for both queries are displayed by default on LGTM. |

## Changes to existing queries

| **Query**                  | **Expected impact**    | **Change**                                                       |
|----------------------------|------------------------|------------------------------------------------------------------|
| Buffer not sufficient for string (`cpp/overflow-calculated`) | Fewer results | This query no longer reports results that would be found by the 'No space for zero terminator' (`cpp/no-space-for-terminator`) query. |
| Call to function with extraneous arguments (`cpp/futile-params`) | Improved coverage | Query has been generalized to find all cases where the number of arguments exceeds the number of parameters of the function, provided the function is also properly declared/defined elsewhere. |
| Commented-out code (`cpp/commented-out-code`) | More correct results | Commented-out preprocessor code is now detected by this query. |
| Comparison result is always the same | Fewer false positive results | The range analysis library is now more conservative about floating point values being possibly `NaN`. |
| Constructor with default arguments will be used as a copy constructor (`cpp/constructor-used-as-copy-constructor`) | Lowered severity and precision | The severity and precision of this query have been reduced to "warning" and "low", respectively. This coding pattern is used intentionally and safely in a number of real-world projects. Results are no longer displayed on LGTM unless you choose to display them. |
| Dead code due to goto or break statement (`cpp/dead-code-goto`) | Fewer false positive results | Functions containing preprocessor logic are now excluded from this analysis. |
| Memory is never freed (`cpp/memory-never-freed`) | More correct results | Support added for more Microsoft-specific allocation functions, including `LocalAlloc`, `GlobalAlloc`, `HeapAlloc` and `CoTaskMemAlloc`. |
| Memory may not be freed (`cpp/memory-may-not-be-freed`) | More correct results | Support added for more Microsoft-specific allocation functions, including `LocalAlloc`, `GlobalAlloc`, `HeapAlloc` and `CoTaskMemAlloc`. |
| Mismatching new/free or malloc/delete (`cpp/new-free-mismatch`) | Fewer false positive results | Fixed an issue where functions were being identified as allocation functions inappropriately.  This correction also affects `cpp/new-array-delete-mismatch` and `cpp/new-delete-array-mismatch`. |
| No space for zero terminator (`cpp/no-space-for-terminator`) | More correct results | This query now detects calls to `std::malloc`. |
| Overflow in uncontrolled allocation size (`cpp/uncontrolled-allocation-size`) | More correct results | This query has been reworked so that it can find a wider variety of results. |
| Resource not released in destructor (`cpp/resource-not-released-in-destructor`) | Fewer false positive results | Resource allocation and deallocation functions are now determined more accurately. |
| Use of potentially dangerous function | More correct results | Calls to `localtime`, `ctime` and `asctime` are now detected by this query. |
| Wrong type of arguments to formatting function (`cpp/wrong-type-format-argument`) | More correct results and fewer false positive results | This query now understands non-standard uses of `%L`. In addition, it more accurately identifies wide and non-wide string/character format arguments on different platforms.  Platform detection has also been made more accurate for this query. |
| Use of potentially dangerous function (`cpp/potentially-dangerous-function`) | Fewer results | Results relating to the standard library `gets` function have been moved into a new query (`cpp/dangerous-function-overflow`). |

## Changes to QL libraries
- The predicate `Declaration.hasGlobalName` now only holds for declarations that are not nested in a class. For example, it no longer holds for a member function `MyClass::myFunction` or a constructor `MyClass::MyClass`, whereas previously it would classify those two declarations as global names.
- In class `Declaration`, predicates `getQualifiedName/0` and `hasQualifiedName/1` are no longer recommended for finding functions by name. Instead, use `hasGlobalName/1` and the new `hasQualifiedName/2` and `hasQualifiedName/3` predicates. This improves performance and identifies names involving templates and inline namespaces more reliably.
- Additional support for definition by reference has been added to the `semmle.code.cpp.dataflow.TaintTracking` library, including:
    - Taint-specific edges for functions modeled in `semmle.code.cpp.models.interfaces.DataFlow`.
    - Flow through library functions that are modeled in `semmle.code.cpp.models.interfaces.Taint`. Queries can add subclasses of `TaintFunction` to specify additional flow.
- There is a new `FoldExpr` class, representing C++17 fold expressions.
- The member predicates `DeclarationEntry.getUnspecifiedType`, `Expr.getUnspecifiedType`, and `Variable.getUnspecifiedType` have been added. These should be preferred over the existing `getUnderlyingType` predicates.