How "#define _CRTDBG_MAP_ALLOC" gets replaced by the preprocessor when there is no value f-CodePudding

I was using CRTDBG library to identify memory leaks in visual studio 2019. I need to define "#define _CRTDBG_MAP_ALLOC" macro for getting the line number of the leak occurs. I am unclear that how the macro gets replaced when there is no value to be replaced ??

#define _CRTDBG_MAP_ALLOC
#include <iostream>
#include <crtdbg.h>
using namespace std;

int main()
{
    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
    int *ptr = (int*)malloc(sizeof(int));
    *ptr = 10;
    //free(ptr);
    return 0;
}

I am a new to c/c , Kindly help me on this.

CodePudding user response：

#define _CRTDBG_MAP_ALLOC just tells the preprocessor that _CRTDBG_MAP_ALLOC is defined.

Code can later check if its defined with a #if defined(_CRTDBG_MAP_ALLOC).

Note nowhere do we set or care or check what value its defined to. So doing something like if (_CRTDBG_MAP_ALLOC == 2) doesn't make sense.

But we dont need it to. It just has to exist.

CodePudding user response：

It's simply replaced by the empty string. E.g.

#define FOO 1
std::cout << FOO 1; // expands to 1 1, outputs 2
#undef FOO
#define FOO
std::cout << FOO 1; // expands to  1, outputs 1

But typically these things are checked with #ifdef.