Weak symbol in MSVC

weak.cpp

#include <stdio.h>

#define DEFINE_WEAK_VARIABLE(Type, Name) \
    extern "C" Type Name; \
    extern "C" Type Name##Default; \
    __pragma(comment(linker, "/alternatename:" #Name "=" #Name "Default")) \
    extern "C" Type Name##Default \

#define DEFINE_WEAK_FUNCTION(Type, Name, Params) \
    extern "C" Type Name Params; \
    extern "C" Type Name##Default Params; \
    __pragma(comment(linker, "/alternatename:" #Name "=" #Name "Default")) \
    extern "C" Type Name##Default Params\

DEFINE_WEAK_VARIABLE(const char*, WeakValue) = "hello";
DEFINE_WEAK_FUNCTION(void, error_log, ()) {}

// Uncomment to see string definition replace weak ones.
// const char* WeakValue = "world";
// void error_log() { printf("log\n"); }

int main() {
    error_log();
    printf("WeakValue=%s\n", WeakValue);
}

Ref:

• https://devblogs.microsoft.com/oldnewthing/20200731-00/?p=104024
• https://stackoverflow.com/questions/2290587/gcc-style-weak-linking-in-visual-studio

You can also use __declspec(selectany).