在同一个头文件中定义底层的顶层类初始化

问题描述

可能是一个愚蠢的问题。很抱歉，但我想执行以下操作，

connectionState currentState;

class connectionState {
    public:
        .......
        .......
};

而是

class connectionState {
    public:
        .......
        .......
};

connectionState currentState;

前者根本不编译，而后者编译。那么，没有办法做到这一点吗？

解决方法

对此有前向声明。前向声明也解决了循环依赖的问题：

class connectionState; // forward class declaration
extern connectionState currentState; // forward global extern variable declaration,now possible because connectionState is declared as a typename/class name
static connectionState currentState; // forward global static variable declaration 

class connectionState { ... }; // implementation/definition,can access currentState
connectionState currentState = ...; // definition/initialization
connectionState currentState {...}; // definition/initialization