static members

cppreference static members

Syntax

	syntax	explanation
(1)	`static data_member`	Declares a static data member.
(2)	`static member_function`	Declares a static member function.

Explanation

NOTE: 下面对原文Explanation中的内容进行组织，添加了标题。

Declaration and definition

NOTE: 原文并没有这样的标题，这是我添加上去的，便于理解

The static keyword is only used with the declaration of a static member, inside the class definition, but not with the definition of that static member:

#include <iostream>
class X
{
    static int n; // declaration (uses 'static')
};

int X::n = 1; // definition (does not use 'static')

int main()
{
    std::cout << X::n << std::endl;
}
// g++ test.cpp

Incomplete type

#include <iostream>

struct Foo;
struct S
{
    static int a[]; // declaration, incomplete type
    static Foo x;   // declaration, incomplete type
    static S s;     // declaration, incomplete type (inside its own definition)
};

int S::a[10]; // definition, complete type
struct Foo
{
};

Foo S::x;     // definition, complete type
S S::s;       // definition, complete type

int main()
{
}
// g++ test.cpp

Refer to a static member

NOTE: 描述如何使用一个

#include <iostream>

struct X
{
    static void f(); // declaration
    static int n;    // declaration
};

int X::n = 7; // definition

void X::f() // definition
{
    n = 1; // X::n is accessible as just n in this scope
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

X g()
{
    return X();
} // some function returning X

void f()
{
    X::f();  // X::f is a qualified name of static member function
    g().f(); // g().f is member access expression referring to a static member function
}

int main()
{
    f();
}
// g++ test.cpp

Static member functions

Static member functions are not associated with any object. When called, they have no this pointer.

Static member functions cannot be virtual, const, or volatile.

The address of a static member function may be stored in a regular pointer to function, but not in a pointer to member function.

Static data members

There is only one instance of the static data member in the entire program with static storage duration, unless the keyword thread_local is used, in which case there is one such object per thread with thread storage duration (since C++11).

NOTE: static data members能够保证唯一性

inline static data member (since C++17)

struct X
{
    inline static int n = 1;
};

NOTE: 这是C++17引入的新特性，它的意图在cppreference inline specifier中进行了描述:

"Inline variables eliminate the main obstacle to packaging C++ code as header-only libraries"即"内联变量消除了将c++代码打包为仅头文件库的主要障碍"，简而言之，它是为了促进header-only librariy的。

inline definition of member function早就是C++的标准了，但是inline definition of static data member却没有得到C++标准的**完整**的支持((后面会解释此处为什么说是**完整**的)，所以，再次之前，"packaging C++ code as header-only libraries"其实是存在着一定的挑战的，往往需要programmer采用walkaround策略。

上面这段话中说: "inline definition of static data member却没有得到C++标准的**完整**的支持"，这是因为在C++17之前，可以通过Constant static members实现类似的inline definition of static data member。