How to return "void value"?

Question

Let's say I have a class as

template <class T>
class c_array {
private:
    T* m_arr;
    int m_length = 0;

public:
    T pop() {
        if (m_length==0) {
            //???
        }

        m_length--;
        return m_arr[m_length];
    }
};

What should I return if length==0?

Returning NULL gives a warning:

"warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null]"

Trying

std::vector<int> vect;

int b = vect.pop_back();

Gives the error "error: void value not ignored as it ought to be"

You need to return a valid `T`. You'll note that the `std::vector::pop_back` doesn't return the poppped element for (presumably) this reason. Nowadays, using [`std::optional`](https://en.cppreference.com/w/cpp/utility/optional) can be idiomatic. — Brian61354270, Feb 07 '21 at 14:38

eerorika · Answer 1 · 2021-02-07T14:49:52.900

What should I return if length==0?

There are a few typical ways to handle this.

Don't perform any check and do the same regardless of size and specify that non emptiness is a pre condition. If the caller violates the pre condition, then behaviour is undefined. This is the most efficient alternative, but also least safe.
Abort the program. This isn't very convenient.
Instead of returning a value, throw an exception when the element doesn't exist.
This doesn't apply to pop in particular, but to similar functions that keep the element in container: Instead of returning an object in general, return an iterator. In case element doesn't exist, return an iterator that represents non-existing element. Typically, iterator to element past the end is used.

How to return "void value"?

1 Answers1