在构造函数C中使用枚举

Question

I'm trying to create a class "Apple" that has two values 1. int n 2. enum color

but my code doesnt work and I get an "No matching constuctor for Initialization" Error

I don't know what is the best way to do it.

#include <iostream>
#include<stdexcept>

using namespace std;

class Color{
public:
   enum color{r,g};
};

class Apple: public Color {
    int n;
    Color c;
public:
    Apple(int n,Color color){
        if(n<0)throw runtime_error("");
        this->n=n;
        this->c=color;
    }
    int n_return(){return n;}
};
int main(){
    try{
        const Apple a1{10,Color::g};
        cout << a1.n_return();}
    catch(runtime_error&){
        cout<<"ER\n";
    }
    return 0;
}

I don't want to change anything in main.

Additionally, how can I set the default color of apple to g in the constructor for when there is no color given?

Answer 1

Your Color c; member in class Apple is referring to the Color base class instead of the color enumerator defined inside it. Having said that, from a design perspective, you seem to be inheriting an Apple from a Color . I believe what you intended for is having each Apple instance hold a color value. For that, you want composition , not inheritance -- as an Apple is not a color , it is-a fruit : ) that has-a color.

Further, n_return() needs to be a const method for you to be able to invoke it from a const instance.

This is as closest to your original code that fills the points raised regarding syntax and design, so you could easily isolate the differences. main() stays the same:

#include <iostream>
#include<stdexcept>

using namespace std;

enum class Color{r,g};

class Apple{
    int n;
    Color c;
public:
    Apple(int n,Color color){
        if(n<0)throw runtime_error("");
        this->n=n;
        this->c=color;
    }
    int n_return() const {return n;}
};
int main(){
    try{
        const Apple a1{10,Color::g};
        cout << a1.n_return();}
    catch(runtime_error&){
        cout<<"ER\n";
    }
    return 0;
}

Note I've changed your enum to enum class . The general reasons for which you can read about here .

If you want to set the default Color for your Apple upon construction in case it isn't specified, then you can write the declaration for it like this:

// Apple has Color `g` by default
Apple(int n,Color color = Color::g){//...

So you can do this:

const Apple a1{10};

and get your Color::g colored apple.

Answer 2

As the comments have already pointed out, you're making an (empty) class Color and defining a scoped enum within it. All of this is unnecessary cruft; all you need is the enum. Replace your class Color with

enum class Color{r,g};

and don't do : public Color in the Apple declaration.

Unrelated, but necessary to get your code working as written: You declare your Apple variable as const but then call a non- const method on it. In order to make this work, you'll need your n_return to look like this.

int n_return() const {return n;}

Notice the const keyword here, to allow the method to be used on const variables.