Warning C26865

Memory allocated with '<allocation-function>' is being deallocated with '<wrong-deallocation-function>'. Use '<correct-deallocation-function>' instead.

This rule was added in Visual Studio 2026 18.0.

Remarks

This warning indicates that the memory was allocated with one family of allocation functions, but was freed with a deallocation function from a different family. This usage results in undefined behavior according to C/C++ and the Microsoft MSVC implementation.

The exact ramifications of this defect are difficult to predict. It might result in leaks for classes with destructors that perform memory deallocation. It could cause inconsistent behavior for classes with destructors that perform semantically significant operations, or memory corruptions and crashes. In other cases the mismatch might be unimportant, depending on the implementation of the compiler and its libraries. Analysis tools can't always distinguish between these situations.

If memory is allocated with one family of allocation functions, it should be freed with a matching deallocation function.

C26865 covers the following allocation/deallocation pairs:

• C++ scalar new (new) must be deallocated with scalar delete (delete).
• C++ array new (new[]) must be deallocated with array delete (delete[]).
• C/C++ malloc/calloc/realloc must be deallocated with free (or realloc).
• Windows HeapAlloc must be deallocated with HeapFree.
• Windows GlobalAlloc must be deallocated with GlobalFree.
• Windows LocalAlloc must be deallocated with LocalFree.
• Windows MIDL_user_allocate must be deallocated with MIDL_user_free.
• Component Object Model (COM) CoTaskMemAlloc must be deallocated with CoTaskMemFree.

Code analysis name: ALLOCATION_DEALLOCATION_MISMATCH

Example

The following sample code generates warning C26865:

void f() {
    int *pInt = (int *)calloc(10, sizeof(int));
    // code ...
    delete pInt; // C26865: Memory allocated with 'calloc' is being deallocated with 'delete'. Use 'free' instead.
}

void g() {
    char * str = new char[50];
    // code ...
    delete str; // C26865: Memory allocated with 'new[]' is being deallocated with 'delete'. Use 'delete[]' instead.
}

Manual memory management has many pitfalls in terms of memory leaks and exceptions. To avoid these kinds of potential leaks altogether, use the mechanisms that are provided by the C++ Standard Library (STL). These mechanisms include shared_ptr, unique_ptr, and containers such as vector. For more information, see Smart pointers and C++ Standard Library.