Predefined macros trong C++

Compiler cung cấp một số macros được định nghĩa sẵn để hỗ trợ log, trace

#include <iostream>
using namespace std;

void checkOS()
{
#if defined(__linux__)
    printf("Running on Linux\n");
#elif defined(_AIX)
    printf("Running on AIX\n");
#elif defined(_WIN32)
    printf("Running on Windows (32/64-bit)\n");
#elif defined(_WIN64)
    printf("Running on Windows 64-bit\n");
#else
    printf("Unknown OS\n");
#endif
}

void foo()
{
    printf("File    : %s\n", __FILE__);
    printf("Line    : %d\n", __LINE__);
    printf("Function: %s\n", __func__);   // standard C++11
    printf("Date    : %s\n", __DATE__);
    printf("Time    : %s\n", __TIME__);

#ifdef __GNUC__  // GCC only
    printf("Function        : %s\n", __FUNCTION__ );
    printf("Pretty function : %s\n", __PRETTY_FUNCTION__);
#endif
}
int main() {
	checkOS();
	foo();
    return 0;
}
[root@GIGABYTEU4 test]# g++ test.cpp -o test
[root@GIGABYTEU4 test]# ./test
Running on Linux
File    : test.cpp
Line    : 22
Function: foo
Date    : Aug 23 2026
Time    : 15:58:50
Function        : foo
Pretty function : void foo()
[root@GIGABYTEU4 test]#

Chúng ta có thể tự tạo logging macro bao gồm các marcos có sẵn

#include <iostream>
using namespace std;

#define LOG(fmt, ...) printf("[LOG] %s:%d (%s): " fmt "\n", __FILE__, __LINE__, __func__, ##__VA_ARGS__)

void foo()
{
    LOG("Hello World");
    LOG("Current year : %d", 2026);
}

int main() {
	foo();
    return 0;
}
[root@GIGABYTEU4 test]# g++ test.cpp -o test
[root@GIGABYTEU4 test]# ./test
[LOG] test.cpp:8 (foo): Hello World
[LOG] test.cpp:9 (foo): Current year : 2026
[root@GIGABYTEU4 test]#