C++ Header Guards

• • Description:
  • 例一：定義同名函數(shù)
  • 例二：重復調(diào)用頭文件
  • 防衛(wèi)式聲明：

Description:

C++ 中的條件 Header Guards(防衛(wèi)式聲明)是條件編譯指令，有助于避免由于程序員的錯誤多次定義相同的函數(shù)或變量時出現(xiàn)的錯誤。 根據(jù) C++，當一個函數(shù)或一個變量被多次定義時，它會產(chǎn)生一個錯誤。

例一：定義同名函數(shù)

代碼：

#include <iostream> using namespace std;void complex() {cout<<"this number is complex number!"<<endl;return; }void complex() {cout<<"this number is also complex number!"<<endl;return; } int main() {complex();return 0; }

Output：

E:\TSWorld程序\complex.cpp:10:6: error: 'void complex()' previously defined here

分析：
定義了兩個同名函數(shù)，所以會報錯。

例二：重復調(diào)用頭文件

Program 1：fruit.h

// C++ program to create fruit class // file named as "fruit.h" #include <iostream> #include <cstring> using namespace std;// Fruit Class class Fruit {string name;string color;public:void input() {name = "apple";color = "red";}void display() {cout<< name <<" color is " << color <<endl;} };

Program 2: apple.h

// C++ program // file named as "apple.h" #include <iostream> #include <cstring> #include "fruit.h" using namespace std;class apple {Fruit a; public:void apple_input() {a.input();}void apple_display() {a.display();} };

Program 3: main.cpp

// C++ program // file named as "main.cpp" # include "fruit.h" # include "apple.h" # include <iostream> using namespace std;int main() {apple a;a.apple_input();a.apple_display();return 0; }

Output:

In file included from E:\TSWorld程序\main.cpp:3:0: E:\TSWorld程序\fruit.h:10:7: error: previous definition of 'class Fruit'class Fruit {

分析：
main.cpp 和 apple.h 都調(diào)用的 fruit.h，重復調(diào)用了頭文件。

防衛(wèi)式聲明：

Program 1：fruit.h

// C++ program to create fruit class // file named as "fruit.h" #ifndef _FRUIT_ #define _FRUIT_#include <iostream> #include <cstring> using namespace std;// Fruit Class class Fruit {string name;string color;public:void input() {name = "apple";color = "red";}void display() {cout<< name <<" color is " << color <<endl;} };#endif

Program 2: apple.h

// C++ program // file named as "apple.h" #ifndef _APPLE_ #define _APPLE_#include <iostream> #include <cstring> #include "fruit.h" using namespace std;class apple {Fruit a; public:void apple_input() {a.input();}void apple_display() {a.display();} };#endif

Program 3: main.cpp

// C++ program // file named as "main.cpp" # include "fruit.h" # include "apple.h" # include <iostream> using namespace std;int main() {apple a;a.apple_input();a.apple_display();return 0; }

Output:

apple color is red

總結(jié)

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