一 static变量在class中的使用

class中声明static变量，s_value的存在不依赖class的object实例，只属于class。

class Something
{
public:
    static int s_value;   //声明
};
 
int Something::s_value = 1;   //定义
 
int main()
{
    Something first;
    Something second;
 
    first.s_value = 2;
 
    std::cout << first.s_value << '\n';
    std::cout << second.s_value << '\n';
    return 0;
}

输出

2
2

二 static 成员变量的定义和初始化

在上面的例子中，class中static int s_value; 只是给编译器声明有这个变量，还需要我们对变量进行定义，因为static成员不依赖于一个class object。如上面的例子，需要定义。

int Something::s_value = 1; // defines the static member variable

上面的定义实例化static变量，并初始化，如果不赋值的话，默认是0
有一点要注意，不要在头文件中对static变量进行实例化

三 内联初始化

如果一个static 成员是一个const类型或者const enum类型的话，可以在class中定义初始化。如下

class Whatever
{
public:
    static const int s_value = 4; // a static const int can be declared and initialized directly
};

从c++11开始，static constexpr成员也可以在类中进行实例化，如下：

#include <array>
 
class Whatever
{
public:
    static constexpr double s_value = 2.2; // ok
    static constexpr std::array<int, 3> s_array = { 1, 2, 3 }; // this even works for classes that support constexpr initialization
};

四 static成员函数

同static成员变量一样，static成员函数属于class不属于特定object，由于不属于特定oject因此没有*this指针

class Something
{
private:
    static int s_value;
public:
    static int getValue() { return s_value; } // static member function
};
 
int Something::s_value = 1; // initializer
 
int main()
{
    std::cout << Something::getValue() << '\n';
}

static 函数可以在class外定义，如下：

class IDGenerator
{
private:
    static int s_nextID; // Here's the declaration for a static member
 
public:
     static int getNextID(); // Here's the declaration for a static function
};
 
// Here's the definition of the static member outside the class.  Note we don't use the static keyword here.
// We'll start generating IDs at 1
int IDGenerator::s_nextID = 1;
 
// Here's the definition of the static function outside of the class.  Note we don't use the static keyword here.
int IDGenerator::getNextID() { return s_nextID++; } 
 
int main()
{
    for (int count=0; count < 5; ++count)
        std::cout << "The next ID is: " << IDGenerator::getNextID() << '\n';
 
    return 0;
}

五 c++不支持satic构造函数

C++不支持static构造函数，如果变量可以直接初始化如第一个例子，不需要在构造函数。下面的例子通过定义一个static class成员的方式对static变量进行初始化。

class MyClass
{
private:
    static std::vector<char> s_mychars;
 
public:
 
    class _init // we're defining a nested class named _init
    {
    public:
        _init() // the _init constructor will initialize our static variable
        {
            s_mychars.push_back('a');
            s_mychars.push_back('e');
            s_mychars.push_back('i');
            s_mychars.push_back('o');
            s_mychars.push_back('u');
        }
    } ;
 
private:
    static _init s_initializer; // we'll use this static object to ensure the _init constructor is called
};
 
std::vector<char> MyClass::s_mychars; // define our static member variable
MyClass::_init MyClass::s_initializer; // define our static initializer, which will call the _init constructor, which will initialize s_mychars