How "#define _CRTDBG_MAP_ALLOC" gets replaced by the preprocessor when there is no value for substitution?

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.