c++ - 如何在 C++ 中转发类的声明？

Question

为什么 A 类和 B 类的前向声明不起作用？

#include <iostream>
using namespace std;

class A, B;    

class A {
public:
    A() {
        new B();
    }
};

class B {
public:
    B() {
        new A();
    }
};

int main () {
    A a = new A();
    B b = new B();
};

编译器错误:

classes.cpp:4:8: error: expected unqualified-id before ‘,’ token
classes.cpp:4:10: error: aggregate ‘A B’ has incomplete type and cannot be defined
classes.cpp: In constructor ‘A::A()’:
classes.cpp:9:7: error: expected type-specifier before ‘B’
classes.cpp:9:7: error: expected ‘;’ before ‘B’
classes.cpp: In function ‘int main()’:
classes.cpp:22:14: error: conversion from ‘A*’ to non-scalar type ‘A’ requested
classes.cpp:23:4: error: expected ‘;’ before ‘b’

Answer 1

像这样:

class A
{
public:
    A();
};

class B
{
public:
    B();
};

A::A() { new B(); }    // terrible code, leaks, serves no purpose
B::B() { new A(); }    // likewise