转发声明cpp无效使用不完整的类型

Question

请注意，我是在一个没有头文件的类文件中编写这个测试代码的，我正转发声明Counter2并尝试调用它上的方法，这会导致编译时错误，无效使用不完整的类型‘Counter2’，有没有解决这个错误的方法？在.cpp文件中是否有forwrd声明的意义，或者它只在头文件中有用。

//TypeConversion.cpp
#include <iostream>
using namespace std;
class Counter2;

class Counter {

public:
int count;


public :
const int* getCount() const{return &(this->count);}

void setCount() {

    this->count=10;

}

// Pre-increment
Counter operator++() {
    this->count++;

    return (*this);
}
//Post increment
Counter& operator++(int){
    Counter dummy;
    dummy.count=this->count;//(*this).n
    this->count++;
    return dummy;
}

 const Counter& modify() {
    this->count=111;
    return *this;
}

Counter()  {
    this->count=0;
    //this->array[5]={1,1,2,3,4};
}

/*Counter(Counter2 &counter2) {
    this->count= counter2.getA() + counter2.getB();
}*/

istream& operator>>(istream& in) {

    in>>this->count;
    return in;
}

void operator=(Counter2 &counter2) {
    this->count = counter2.getA() + counter2.getB();
}

/*  int& operator[](int index){
    return this->array[index];
}*/

};

class Counter2 {
private:
int a ,b;
public:
Counter2() {
    this->a=10;
    this->b=20;
}
Counter2(Counter &c) {
    this->a=c.count;
    this->b=c.count;
}

int getA()  {return this->a;}
int getB()  {return this->b;}

operator Counter() {
    Counter c;
    return c;
}

};

int main() {
cout << "!World!" << endl; // prints !World!
Counter2 c2;
Counter c=c2;
//c=c2;
cout<<endl<<*(c.getCount());
return 0;
}

Answer 1

在头文件中使用转发声明，例如，能够保存类的对象。但是一旦你想要访问这个类的方法或成员(在你的.cpp中)，你就必须在定义中包含正确的头文件。