C ++: class declaration, the class definition, class implementation, class uses

---

#include <iostream>
using namespace std;

//类声明
class A;

//类定义
class A
{
    int m;    //数据成员声明
    int f1(); //成员函数声明
    int f2(); //成员函数声明及定义，类实现
    {
        return 1;
    }
};

//类实现
int A::f1()
{
    return 0;
}

int main()
{
    A a; //类使用，实例化类
    return 0;
}

to sum up:

1. Class declaration (forward declaration). For a class, previously defined after it declares that it is an incomplete type. It can only be used to define a reference and a pointer to it, or incomplete statement types as parameters or function return values.
2. Class definition. Objects can only be created after the class definition, the class member access. When the class is considered completed when the class is defined, i.e., a member of the class declaration, the class itself is an incomplete type, it is not the type of data members of the class type, but may be a class type of a reference or pointer.
3. Class implementation. Is a property class data members, member functions of the class method. Implementation class that is implemented or defined member functions.
4. Class use. The use of classes, i.e. the various operations to the class object is performed.