example -D macro

macro_compile.cpp

#include <iostream>
#ifndef SIZE_ARRAY
#define SIZE_ARRAY 4
#endif 

int main(){
    int array[SIZE_ARRAY] = {0};
    std::cout << "num of elements: " << sizeof(array)/sizeof(array[0]) << "\n";
    return 0;
}

macro_compile.txt

cpp_tutorial# g++ macro_example.cpp  -o a 
cpp_tutorial# ./a
num of elements: 4
cpp_tutorial# g++ macro_example.cpp  -o a -DSIZE_ARRAY 
cpp_tutorial# ./a
num of elements: 1
cpp_tutorial# g++ macro_example.cpp  -o a -DSIZE_ARRAY=10
cpp_tutorial# ./a
num of elements: 10