C ++函数中静态变量的生命周期是多少？ [英] What is the lifetime of a static variable in a C++ function?

问题描述

如果变量在函数的作用域中声明为 static ，它只会初始化一次，并在函数调用之间保留其值。它的寿命究竟是什么？它的构造函数和析构函数何时被调用？

If a variable is declared as static in a function's scope it is only initialized once and retains its value between function calls. What exactly is its lifetime? When do its constructor and destructor get called?

void foo() 
{ 
    static string plonk = "When will I die?";
}

---

对于那些想知道我为什么问这个问题的人如果我已经知道答案？

推荐答案

函数的生命周期 static 变量从 ^[0] 第一次开始，程序流遇到声明，并在程序终止时结束。这意味着运行时必须执行一些簿记，以便只有当它被实际构造时才会破坏它。

The lifetime of function static variables begins the first time^[0] the program flow encounters the declaration and it ends at program termination. This means that the run-time must perform some book keeping in order to destruct it only if it was actually constructed.

此外，因为标准说静态对象的析构函数必须以它们的构造 ^[1]

Additionally since the standard says that the destructors' of static objects must run in the reverse order of the completion of their construction^[1] and the order of construction may depend on the specific program run, the order of construction must be taken into account.

示例

struct emitter {
    string str;
    emitter(const string& s) : str(s) { cout << "Created " << str; << endl; }
    ~emitter() { cout << "Destroyed " << str << endl; }
};

void foo(bool skip_first) 
{
    if (!skip_first)
        static emitter a("in if");
    static emitter b("in foo");
}

int main(int argc, char*[])
{
    foo(argc != 2);
    if (argc == 3)
        foo(false);
}

输出：

C：> sample.exe

在foo中创建

在foo中销毁

C:>sample.exe
Created in foo
Destroyed in foo

C：> sample.exe 1

创建于如果

创建于foo

在foo中销毁

摧毁在

C:>sample.exe 1
Created in if
Created in foo
Destroyed in foo
Destroyed in if

C：> sample.exe 1 2

创建于foo

创建于如果

如果被破坏

在foo中销毁

C:>sample.exe 1 2
Created in foo
Created in if
Destroyed in if
Destroyed in foo

[0] code>由于 C ++ 98 [2] 没有引用多个线程如何在多线程环境中的行为是未指定的，可以是问题在于 Roddy 提及。

[0] Since C++98^[2] has no reference to multiple threads how this will be behave in a multi-threaded environment is unspecified, and can be problematic as Roddy mentions.

[1] C ++ 98 节 3.6.3.1 [basic.start.term]

[2] code>在C ++ 11静态被初始化在线程安全的方式，这也被称为 魔术统计 。

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