Unable to return pointer to array C++ visual studio

Question

I am brand new to C++ and am struggling with the idea of using arrays and returning pointers.

I have a class Student in a .h file:

class Student {
    public:
        // stuff
        int* GetNumDaysPerCourse() const;
        // stuff

    private:
        //stuff
        int numDaysPerCourse[3];
        // stuff
};

And I am trying to implement it in a .cpp file:

int* Student::GetNumDaysPerCourse() const {
    return numDaysPerCourse;
}

But I am getting an error saying:

return value type does not match the function type

I am confused because looking at other questions, this looks like a valid way to do this.

So, how do I return a class member array using a getter in C++?

Answer 1

When I try to compile this code, I get this error:

error: invalid conversion from ‘const int*’ to ‘int*’

This is because GetNumDaysPerCourse() is const-qualified, so its this pointer is pointing at a const Student object, and thus its numDaysPerCourse member is treated as const data. You can't assign the address of const data to a pointer-to-non-const (without an explicit type-cast, that is, but don't do that), as that would grant permission for outside code to modify data that is (potentially) stored in read-only memory.

So, you need to either:

• drop the const qualifier from GetNumDaysPerCourse():

public:
    ...
    int* GetNumDaysPerCourse();
    ...

private:
    ...
    int numDaysPerCourse[3];
    ...
};

int* Student::GetNumDaysPerCourse() {
    return numDaysPerCourse;
}

• make GetNumDaysPerCourse() return a pointer-to-const:

public:
    ...
    const int* GetNumDaysPerCourse() const;
    ...

private:
    ...
    int numDaysPerCourse[3];
    ...
};

const int* Student::GetNumDaysPerCourse() const {
    return numDaysPerCourse;
}

• combine the two approaches to provide both const and non-const access to the same array, depending on whether the object that GetNumDaysPerCourse() is being called on is mutable or read-only:

public:
    ...
    int* GetNumDaysPerCourse();
    const int* GetNumDaysPerCourse() const;
    ...

private:
    ...
    int numDaysPerCourse[3];
    ...
};

int* Student::GetNumDaysPerCourse() {
    return numDaysPerCourse;
}

const int* Student::GetNumDaysPerCourse() const {
    return numDaysPerCourse;
}

Answer 2

If you don't need to modify the pointer after calling int *GetNumDaysPerCourse() const;

You can modify your class and your implementation as follows:

class Student {
public:
    // stuff
    const int *GetNumDaysPerCourse() const;
    // stuff

private:
    //stuff
    int numDaysPerCourse[3];
    // stuff
};

The .cpp file:

const int *Student::GetNumDaysPerCourse() const {
    return numDaysPerCourse;
}