C++中如何正确避免友元方法的循环依赖？

Question

我有2个班级：

MyString

和

LongNumber

。并且

LongNumber

类有一个

Foo

方法，可以访问

private

类的

MyString

字段，也就是说，

Foo

方法是

friend

类的

MyString

。所以我在这里写了一些代码：

// File: MyString.h

- - - - - - - - - - - - - -

#pragma once
// default includes with angular brackets

class LongNumber; // forward declaration

class MyString
{
    char* symbols;
    size_t capacity;
    size_t length;

    friend void LongNumber::Foo(MyString& str); // <-- error here

    // Methods
}

// File: MyString.cpp

- - - - - - - - - - - - - -

// including "MyString.h" and "LongNumber.h" in .cpp to avoid circular dependency
#include "MyString.h"
#include "LongNumber.h"
// default includes with angular brackets

// Implementations

// File: LongNumber.h

- - - - - - - - - - - - - -

#pragma once
// default includes with angular brackets

class MyString; // forward declaration

class LongNumber
{
    uint8_t sign;
    MyString digits;
    void Foo(MyString& str); // supposed to be a friend to the class MyString

    // Methods
}

// File: LongNumber.cpp

- - - - - - - - - - - - - -

// including "MyString.h" and "LongNumber.h" in .cpp to avoid circular dependency
#include "MyString.h"
#include "LongNumber.h"
// default includes with angular brackets

void LongNumber::Foo(MyString& str) // supposed to be a friend to the class MyString
{
    // some code
}

// other implementations

File: Main.cpp

- - - - - - - - - - - - - -

#include <iostream>
#include "MyString.h"
#include "LongNumber.h"

// #include "LongNumber.h"
// #include "MyString.h"
// reversed order won't help either

int main() {} // empty

我读过有关避免 C++ 中的循环依赖的内容。他们建议您在

.h

文件中转发声明类，并将所有

include

移动到

.cpp

文件中。我已经这样做了，但这对我不起作用。

当我编译这个时，我得到：

...MyString.h(11,26): error C2027: use of undefined type 'LongNumber'

导致此错误的代码行是：

friend void LongNumber::Foo(MyString& str);

编译器不关心我向前声明了类

LongNumber

。从某种意义上来说，它似乎看不到我的其他文件，因为如果我像这样修改

MyString.h

文件：

// File: MyString.h

- - - - - - - - - - - - - -

#pragma once
// default includes with angular brackets

class UnknownClass; // forward declaration, deliberately wrote a name that does not exist

class MyString
{
    char* symbols;
    size_t capacity;
    size_t length;

    friend void UnknownClass::Foo(MyString& str); // <-- error here

    // Methods
}

错误是一样的：

...MyString.h(11,28): error C2027: use of undefined type 'UnknownClass'

我觉得让编译器知道类

LongNumber

存在并已完成的唯一方法是将

#include "LongNumber.h"

放入头文件中，但这是不允许的，因为它会创建循环依赖。

我想要实现的目标可能吗？

Answer 1

您不能通过名称引用不完整类的方法。将

UnknownClass

变为

KnownClass

:

// KnownClass.h
struct MyString;

struct KnownClass {
    void foo(const MyString&);
};

// MyString.h
struct MyString {
    friend void KnownClass::foo(const MyString&);
};

// KnownClass.cpp

void KnownClass::foo(const MyString&) {
    // implementation
}