预编译指令

Posted 2021-07-10 Jqivin

#if
#ifdef
#ifndef
#define
#undef
#elif
#else
#error
#endif

#include<iostream>

#if 0
#define MAX 10
#elif !defined(MAX)
#define MAX 20
#else
#define MAX 30
#endif

#define JQW 1
#ifndef JQW
#define JQW 100
#else
#undef JQW
#define JQW 200
#endif

#ifdef W
#undef w
#else
#define W 10
#endif 

#if !defined(__cplusplus)
#error C++ compiler required.
#endif
int main()
{
	std::cout << MAX << std::endl;
	std::cout << JQW << std::endl;
	std::cout << W << std::endl;
}