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++?

Do you want people to be able to change values of the array that the pointer points to? — NathanOliver
– NathanOliver, Commented Aug 13, 2021 at 18:18
Please try to create a proper minimal reproducible example to show us, and then copy-paste the full and complete build output of that example into your question. — Some programmer dude
– Some programmer dude, Commented Aug 13, 2021 at 18:19
error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive] return numDaysPerCourse; is the error I see since it wasn't included in the question. — Retired Ninja
– Retired Ninja, Commented Aug 13, 2021 at 18:21
By the way, returning pointers to internal data could lead to problems if the caller doesn't use the pointer correctly (modify data it should not modify, go out of bounds, etc.). I recommend you use std::array<int, 3> for the array, and return either by constant reference or by value. — Some programmer dude
– Some programmer dude, Commented Aug 13, 2021 at 18:22
@Mr.Spock Okay. Then you will want option 2 from Remy Lebeau's answer — NathanOliver
– NathanOliver, Commented Aug 13, 2021 at 19:12

Remy Lebeau · Accepted Answer · 2021-08-13 18:46:19Z

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;
}

loic.lopez · Accepted Answer · 2021-08-13 18:27:04Z

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;
}

answered Aug 13, 2021 at 18:27

loic.lopez

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