如何在C ++中返回自己的结构？

Question

I'm having trouble creating a function that returns my own struct.

Header:

    #ifndef FOOD_H
    #define FOOD_H
    #include <string>

    class Food
    {
    public:
        Food();
        ~Food();
    public:
        struct Fruit {
            std::string name;

        };
        struct Person {
            Fruit favorite;
            Fruit setFavorite(Fruit newFav);
        };

    public:
        Fruit apple;
        Fruit banana;

        Person Fred;
    };
    #endif

CPP:

    #include "Food.h"

    Food::Food()
    {
    }

    Food::~Food()
    {
    }

    Fruit Food::Person::setFavorite(Fruit newFav)
    {
        return newFav;
    }

Main:

    #include "Food.h"
    #include <iostream>

    int main() {
        Food fd;    
        fd.Fred.favorite = fd.Fred.setFavorite(fd.apple);
        std::cout << fd.Fred.favorite.name;
        system("pause");
    }

My errors are:

E0020 identifier "Fruit" is undefined Food.cpp 11

E0147 declaration is incompatible with "Food::Fruit Food::Person::setFavorite(Food::Fruit newFav)" (declared at line 17 of Food.h) Food.cpp 11

How do I fix these and is there a better way to write this code?

Answer 1

 identifier "Fruit" is undefined 

This error says that there is no definition for Fruit .

You have defined a class Fruit that is nested within Food . Therefore the fully qualified name of the class is Food::Fruit as can be seen from the other error message:

 declaration is incompatible with "Food::Fruit Food::Person::setFavorite(Food::Fruit newFav)" ^^^^^^^^^^^ 

This error message tells you that the declaration Food::Person::setFavorite(Fruit newFav) is incompatible because that function is supposed to return Food::Fruit rather than Fruit (which is something that doesn't have definition).

---

Fruit can be used to refer to Food::Fruit within the context of the class Food . The definition of this function is outside of the class, so it is not within the context. It is not until the name of the function ( Food::Person::setFavorite ) that the context is established. You could use a trailing return type to avoid using the fully qualified type:

auto Food::Person::setFavorite(Fruit newFav) -> Fruit

Answer 2

In addition to the accepted answer, OP's class design could also be improved in my opinion. It seems that OP wants to create a fruit class which should have a is-a relationship with food class. Making it a member of food class doesn't seem right to me. Same thing applies to Person class which should be a separated class instead of being a member of food.

#include <string>

class Food
{
    std::string m_name;
        // other stuffs...
};

class Fruit : public Food
{
    // unique fruit members and functions...
};

class Person
{
    Fruit m_favorite;

public:
    void SetFavorite(Fruit favorite);
};

void Person::SetFavorite(Fruit favorite)
{
    m_favorite = favorite;
}

int main()
{
    Fruit apple;
    Person fred;
    fred.SetFavorite(apple);

    return 0;
}