C/C++ define宏定义

## 连接前后两个值

#define connect(a, b) a##b
int c = combom(1, 2);
//c = 12;

# 给后面的字符添加双引号

#define combom(b) #b
std::string c = combom(laji);
cout << c << endl;

##和#在一个宏语句中只能使用一个

#@给后面添加单引号

用##和#综合拼接字符串

#include <iostream>
#include "test.h"

#define A(a,b) a##b
#define g(a) #a
#define h(a) g(a)

int main(int argc, const char * argv[]) {
    int a = A(1, 2);
    std::string b = g(A(2,3));
    std::string c = h(A(2,3));
    std::string d = g(h(A(2,3)));
    std::string e = h(g(h(A(2,3))));
    std::string f = h(h(h(A(2,3))));
    printf("a = %d\n",a);
    printf("b = %s\n",b.c_str());
    printf("c = %s\n",c.c_str());
    printf("d = %s\n",d.c_str());
    printf("e = %s\n",e.c_str());
    printf("f = %s\n",f.c_str());
    return 0;
}
/*
 *  a:  A(1,2)  ==> 12
 *  b:  g(A(2,3)) ==> "A(2,3)"  //如果遇到#开头直接“”替换，不管后面是什么
 *  c:  h(A(2,3)) ==> g(A(2,3)) ==> g(23) ==> "23"  //执行完外层的h()后不会马上执行产生的g(),而是先继续向里执行
 *  d:  g(h(A(2,3))) ==> "h(A(2,3))"    //如果遇到#开头直接“”替换，不管后面是什么
 *  e:  h(g(h(A(2,3)))) ==> "g(g(h(A(2,3))))" ==> g("h(A(2,3))") ==> ""h(A(2,3))""
 *  f:  h(h(h(A(2,3)))) ==> g(h(h(A(2,3)))) ==> g(g(h(A(2,3)))) ==> g(g(g(A(2,3)))) ==> g(g(g(23))) ==> g(g("23")) ==> g(""23"") ==> """23"""   //先从外往里，再从里往外
 */


//结果如下
a = 12
b = A(2,3)
c = 23
d = h(A(2,3))
e = "h(A(2,3))"
f = "\"23\""

#ifdef DEBUG

调试时的添加打印的用法

#include <iostream>
#undef TEST_DEBUG
#define TEST_DEBUG

int main(int argc, const char * argv[]) {
    using std::cout;
    using std::endl;
#ifdef TEST_DEBUG
    cout << "debug" << endl;
#endif
    
#ifndef TEST_DEBUG
    cout << "no debug" << endl;
#endif
    return 0;
}

#if #elif #else #endif

#include <iostream>
#define TEST 3

int main(int argc, const char * argv[]) {
    using std::cout;
    using std::endl;
#if TEST == 2
    cout << "test = " << 2 << endl;
#elif TEST == 3
    cout << "test = " << 3 << endl;
#else
    cout << "test = " << 100 << endl;
#endif

    return 0;
}

内置宏

int main(int argc, const char * argv[]) {
    using std::cout;
    using std::endl;
    printf("data = %s\n",__DATE__);         //编译时的日期
    printf("time = %s\n",__TIME__);         //编译时的时间
    printf("fun = %s\n",__FUNCTION__);      //函数名
    printf("line = %d\n",__LINE__);         //所在行数
    printf("line = %s\n",__FILE__);         //文件名
    return 0;
}

#error会在编译时打印语句，用xcode时，会在编写时就直接提示。

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