C++ Header Guards

• • Description:
  • 例一：定義同名函數
  • 例二：重復調用頭文件
  • 防衛式聲明：

Description:

C++ 中的條件 Header Guards(防衛式聲明)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或變量時出現的錯誤。 根據 C++，當一個函數或一個變量被多次定義時，它會產生一個錯誤。

例一：定義同名函數

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，所以會報錯。

例二：重復調用頭文件

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 都調用的 fruit.h，重復調用了頭文件。

防衛式聲明：

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

以上是生活随笔為你收集整理的C++面向对象(1)：防卫式声明的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。