How to avoid duplication on template function return type?

Question

Is there a way to avoid duplication and improve readability on template functions return type ?

Here is an example

template <typename FunctionType>
std::enable_if_t<
    !std::is_void_v<std::invoke_result_t<FunctionType, MyClass*>>,
    std::optional<std::invoke_result_t<FunctionType, MyClass*>>
> CallIfValid(MyClass* instance, FunctionType func)
{
    using InvocationType = std::invoke_result_t<FunctionType, MyClass*>;
    if (instance != nullptr)
    {
        return func(instance);
    }
    else
    {
        return std::optional<InvocationType>();
    }
}

Notice how std::invoke_result_t<FunctionType, MyClass*> ends up duplicated twice in return type, and also a third time in method body.

Any suggestions or trick I am not seeing here ?

Thanks

Answer 1

I have the same problem. In my personal opinion there isn't a good real solution, not for the general case. But there are some mitigations/workarounds you can employ. In your example you can add a default template parameter. Also since you specified the return type, you don't need to repeat the type in the return expression:

template <typename FunctionType, class InvocationType  = std::invoke_result_t<FunctionType, MyClass*>>
std::enable_if_t<
    !std::is_void_v<InvocationType >,
    std::optional<InvocationType >
> CallIfValid(MyClass* instance, FunctionType func)
{
    if (instance != nullptr)
    {
        return func(instance);
    }
    else
    {
        return std::nullopt; // if you want to be explicit (I personally prefer this)
        // return {}; // if you want to be terse
    }
}