C + + preprocessing directive # define, conditional compilation #ifdefine

In this paper, we mainly record the pre-C + + preprocessor directives, and the common preprocessing directives are:

#空指令, without any effect

#include包含一个源代码文件

#define定义宏

#undef取消已定义的宏

#if如果给定条件为真, the following code is compiled

#ifdef如果宏已经定义, the following code is compiled

#ifndef如果宏没有定义, the following code is compiled

#elif如果前面的 # If the given condition is not true and the current condition is true, compile the following code

#endif结束一个 # If ... #else条件编译块

#error停止编译并显示错误信息

The most common form of conditional compilation commands is:

#ifdef identifiers
Procedure Section 1
#else
Procedure Section 2
#endif

Cases:

BOOL #define ture 1#define false 0#endif

In the early VC, the bool variable is represented by 1,0, which can be defined so as to ensure program compatibility

It is important to use #ifdef and #ifndef in header files to prevent double-defined errors.

// main.cpp File  "cput.h" "put.h" int main () {    cput ();    Put ();     " Hello world! " << Endl;     return 0 ;}

 // cput.h header file   #include  <iostream>using  namespace   STD;  int   Cput () {cout  <<  " hello world!      " << Endl;  return  0  ;}  

// put.h Header File  "cput.h"int  put () {    cput ();     return 0 ;}

Compilation error, two times in Main.cpp contains Cput.h

Attempt to simulate the restoration of the compilation process;

When the compiler compiles main.cpp

//pre-compilation loads the header file into the main.cpp file first//Expand # include "cput.h" content#include <iostream>using namespacestd;intCput () {cout<<"Hello world!"<<Endl; return 0;}//Expand # include "put.h" content//Put.h contains the cput.h first unfold#include <iostream>using namespacestd;intCput () {cout<<"Hello world!"<<Endl; return 0;}intput () {cput (); return 0;}intMain () {cput ();    Put (); cout<<"Hello world!"<<Endl; return 0;}

It is obvious that after merging the expanded code, two times the Cput () function is defined;

If you change the cput.h to the following form:

#ifndef _cput_h_ #define _cput_h_<iostream>usingnamespace  std; int Cput () {    "Hello world! " << Endl;     return 0 ;} #endif

When the compiler compiles main.cpp, the merged main.cpp file will be like this:

#ifndef _cput_h_#define_cput_h_#include<iostream>using namespacestd;intCput () {cout<<"Hello world!"<<Endl; return 0;}#endif#ifndef _cput_h_#define_cput_h_#include<iostream>using namespacestd;intCput () {cout<<"Hello world!"<<Endl; return 0;}#endifintput () {cput (); return 0;}intMain () {cput ();    Put (); cout<<"Hello world!"<<Endl; return 0;}

This compilation succeeds by running, because the cput.h contained in the expanded put.h does not take effect, and the macro is defined earlier _cput_h_

C + + preprocessing directive # define, conditional compilation #ifdefine