C++的static關(guān)鍵字

static關(guān)鍵字可以用于修改局部變量，函數(shù)，類的數(shù)據(jù)成員以及對(duì)象。靜態(tài)局部變量只初始化一次，然后在每次函數(shù)調(diào)用時(shí)都保持它的值。靜態(tài)成員函數(shù)可以直接用類來(lái)調(diào)用，不需要?jiǎng)?chuàng)建對(duì)象來(lái)調(diào)用。同時(shí)，C++也支持靜態(tài)對(duì)象。

C++的靜態(tài)對(duì)象(static object)

當(dāng)使用static關(guān)鍵字聲明一個(gè)對(duì)象時(shí)，此對(duì)象就變成了靜態(tài)對(duì)象。見下面兩個(gè)表達(dá)式。

Test t; // 棧上的對(duì)象 static Test t1; // 靜態(tài)對(duì)象

第一個(gè)表達(dá)式會(huì)在棧上分配一個(gè)對(duì)象。也被稱為自動(dòng)對(duì)象或局部對(duì)象。局部對(duì)象在每次聲明它時(shí)就會(huì)創(chuàng)建。而靜態(tài)對(duì)象只初始化一次，并且在整個(gè)程序的生命周期中都存在。

靜態(tài)對(duì)象保存在靜態(tài)存儲(chǔ)區(qū)。在程序結(jié)束時(shí)，它就會(huì)被銷毀。C++既支持局部靜態(tài)對(duì)象，也支持全局靜態(tài)對(duì)象。

下面例子演示了局部靜態(tài)對(duì)象的使用：
[cpp] view plaincopy print?

#include?<iostream>??

class?Test??

{??

public:??

????Test()??

????{??

????????std::cout?<<?"Constructor?is?executed\n";??

????}??

????~Test()??

????{??

????????std::cout?<<?"Destructor?is?executed\n";??

????}??

};??

void?myfunc()??

{??

????static?Test?obj;??

}?//?函數(shù)結(jié)束時(shí)，obj不會(huì)被消耗，因?yàn)槭庆o態(tài)對(duì)象。??

???

int?main()??

{??

????std::cout?<<?"main()?starts\n";??

????myfunc();????//?這里不會(huì)調(diào)用析構(gòu)函數(shù)，因?yàn)楹瘮?shù)體中分配的是靜態(tài)對(duì)象。??

????std::cout?<<?"main()?terminates\n";??

????return?0;??

}??

#include <iostream> class Test { public:Test(){std::cout << "Constructor is executed\n";}~Test(){std::cout << "Destructor is executed\n";} }; void myfunc() {static Test obj; } // 函數(shù)結(jié)束時(shí)，obj不會(huì)被消耗，因?yàn)槭庆o態(tài)對(duì)象。int main() {std::cout << "main() starts\n";myfunc(); // 這里不會(huì)調(diào)用析構(gòu)函數(shù)，因?yàn)楹瘮?shù)體中分配的是靜態(tài)對(duì)象。std::cout << "main() terminates\n";return 0; }

輸出:

main() starts Constructor is executed main() terminates Destructor is executed

從結(jié)果可以看到，只有在主函數(shù)main()結(jié)束時(shí)，才會(huì)調(diào)用到析構(gòu)函數(shù)。

如果刪除掉上面程序中的關(guān)鍵字static,則會(huì)得到下面結(jié)果：

main() starts Constructor is called Destructor is called main() terminates

全局靜態(tài)對(duì)象

參考下面例子： [cpp] view plaincopy print?

#include?<iostream>??

class?Test??

{??

public:??

????int?a;??

????Test()??

????{??

????????a?=?10;??

????????std::cout?<<?"Constructor?is?executed\n";??

????}??

????~Test()??

????{??

????????std::cout?<<?"Destructor?is?executed\n";??

????}??

};??

static?Test?obj;??

int?main()??

{??

????std::cout?<<?"main()?starts\n";??

????std::cout?<<?obj.a;?//注意：靜態(tài)對(duì)象可以調(diào)用它的所有成員，包括非靜態(tài)成員。??

????????????????????????//但如果是靜態(tài)函數(shù)，則只能調(diào)用靜態(tài)成員。??

????std::cout?<<?"\nmain()?terminates\n";??

????return?0;??

}??

#include <iostream> class Test { public:int a;Test(){a = 10;std::cout << "Constructor is executed\n";}~Test(){std::cout << "Destructor is executed\n";} }; static Test obj; int main() {std::cout << "main() starts\n";std::cout << obj.a; //注意：靜態(tài)對(duì)象可以調(diào)用它的所有成員，包括非靜態(tài)成員。//但如果是靜態(tài)函數(shù)，則只能調(diào)用靜態(tài)成員。std::cout << "\nmain() terminates\n";return 0; }

輸出:

Constructor is executed main() starts 10 main() terminates Destructor is executed 與50位技術(shù)專家面對(duì)面20年技術(shù)見證，附贈(zèng)技術(shù)全景圖

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