使用未定义类型 [英] Use of undefined type

问题描述

#include <iostream>

class Core;
class State;

int main (){
        std::cin.get();
        return 0;
}

class State{
public:
    State(Core* core){
        core->setState();
    }
};

class Core{
public:
    Core(){
        State state(this);
    }
    void setState(){
        std::cout << "setting state" << std::endl;
    }
};

我得到使用未定义类型错误。我想如果我转发声明这两个类，它会解决这个问题，但我不能弄清楚。它是只是愚蠢的c ++语法，我错过了？

I keep getting the "use of undefined type" error. I thought that if I forward declare both of the classes, it would fix the problem but I can't figure it out. Is it just silly c++ syntax that I'm missing?

编辑：对不起的gamestate打字错误，我把它改为状态，它仍然产生错误。 / p>

Sorry about the gamestate typo, I've changed it to State and it still produces the error.

推荐答案

在 State :: State ，您使用 Core 。您可以在您的示例中轻松修复此问题：

In State::State, you are using Core before it is actually defined. You can fix this easily in your example:

class State{
public:
    State(Core* core);
};

class Core{
   // This stays the same...
};

State::State(Core* core)
{
   core->setState();
}

在实践中更常见的是将这些函数实现为单独的实现（ .cpp ）文件，在这种情况下，转发声明将按照您的预期工作。

It's much more common in practice to have the implementation of these functions in a separate implementation (.cpp) files, in which case the forward declarations would work as you've expected.

那种情况：

// State.h
class Core

class State{
public:
    State(Core* core);
};

并

// Core.h
#include "State.h"

class Core{
public:
    Core(){
        State state(this);
    }

    void setState(){
        std::cout << "setting state" << std::endl;
    }
};

和实现文件：

// State.cpp
#include "State.h"
#include "Core.h"

State::State(Core* core)
{
   core->setState();
}

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