Certain functions for which a definition is supplied by the implementation are replaceable . A C++ program may provide a definition with the signature of a replaceable function, called a replacement function . The replacement function, if provided, is used instead of the default version supplied by the implementation. Such replacement occurs prior to program startup.

If a declaration of the replacement function does not satisfy any of the following conditions, the program is ill-formed, no diagnostic is required:

• It is not inline.
• It is attached to the global module.
• It has C++ language linkage.
• It has the same return type as the replaceable function.
• If the replaceable function is declared in a standard library header, it would be valid as a redeclaration of the declaration in that header.

Standard library

The following standard library functions are replaceable, and the description of function semantics apply to both the default version defined by the C++ standard library and the replacement function defined by the program:

Example

#include <cstddef>
#include <new>
#include <print>

// replacement function
void* operator new(std::size_t count)
{
    std::print("Replaced!");
    return nullptr;
}

int main()
{
    int* ptr = new int; // invokes the replacement version defined by the program
}