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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.

Core language It is implementation-defined whether the contract-violation handler ::handle_contract_violation is replaceable.

(since C++26)

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:

operator newoperator new[]	allocation functions (function)
operator deleteoperator delete[]	deallocation functions (function)
is_debugger_present (C++26)	checks whether a program is running under the control of a debugger (function)

Example

Uses a replacement allocation function:

Run this code

#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
}

Output:

Replaced!