为什么我的朋友功能无法访问私人成员？

Question

The addBudget function in AuxiliaryOffice class is a friend function of the budget class. But the compiler is giving me error, that it cannot access the private members of the Budget class.

class Budget;
class AuxiliaryOffice
{
private:
    double auxBudget;
public:
    AuxiliaryOffice()
    {
        auxBudget = 0;
    }
    double getDivisionBudget()
    {
        return auxBudget;
    }
    void addBudget(double a, Budget &ref)
    {
        ref.corpBudget += a;
        auxBudget += a;
    }
};
class Budget
{
private:
    static double corpBudget;
    double divisionBudget;
    friend void AuxiliaryOffice::addBudget(double, Budget&);
public:
    Budget()
    {
        divisionBudget = 0;
    }
    void addBudget(double a)
    {
        corpBudget += a;
        divisionBudget += a;
    }
    double getDivisionBudget() const
    {
        return divisionBudget;
    }
    double getCorpBudget() const
    {
        return corpBudget;
    }
};
double Budget::corpBudget = 0;

Answer 1

The error you get is stopping your code from compile before friendship is even an issue.

You declared Budget as an incomplete type:

class Budget;

And then wrote code that calls functions on it, before the class is defined. That is not allowed. You have to declare functions that use it, but NOT define it until after the class is defined.

That's why (one of the reasons why) we put class declarations in headers, and implementations in .cpp files. It also reduces coupling between files, and can help with build speeds.

Here's a compacted version of your code that compiles, with most of the unessential lines removed (as you should aim to do in future questions):

class Budget;
class AuxiliaryOffice {
    double auxBudget = 0;

public:
    void addBudget(double a, Budget &ref);
};

class Budget {
    // Unrelated, but useful to know:
    // inline statics can be initialized in the class like this, no
    // definition necessary in the .cpp file.
    inline static double corpBudget = 0; 
    friend void AuxiliaryOffice::addBudget(double, Budget&);
};

// if in .cpp, remove "inline", if in header, keep it
inline void AuxiliaryOffice::addBudget(double a, Budget &ref) {
    ref.corpBudget += a;
}

see it live: https://godbolt.org/z/JYDZMz

Answer 2

The problem is not about friendship, is about the order of definition. At the point you've defined AuxiliaryOffice::addBudget the definition of Budget is incomplete, hence the member corpBudget isn't defined yet.

Change it to:

class Budget;
class AuxiliaryOffice
{
private:
    double auxBudget;
public:
    AuxiliaryOffice()
    {
    auxBudget = 0;
    }
    double getDivisionBudget()
    {
    return auxBudget;
    }
    void addBudget(double a, Budget &ref);
};


class Budget
{
private:
    static double corpBudget;
    double divisionBudget;
    friend void AuxiliaryOffice::addBudget(double, Budget&);
public:
    Budget()
    {
        divisionBudget = 0;
    }
    void addBudget(double a)
    {
        corpBudget += a;
        divisionBudget += a;
    }
    double getDivisionBudget() const
    {
        return divisionBudget;
    }
    double getCorpBudget() const
    {
        return corpBudget;
    }
};
double Budget::corpBudget = 0;

inline void AuxiliaryOffice::addBudget(double a, Budget &ref)
{
    ref.corpBudget += a;
    auxBudget += a;
}

That will work.