Using std::optional in a C++11 context

Question

I'm writing a small C++11 library in which I believe std::optional would be a nice addition in some functions which can return nullptr. However, std::optional is a C++17 feature. Since being C++11 is a requirement, I'm looking for ways to use std::optional while keeping compatibility.

I found that feature macros can be tested. I suppose I could use it to detect whether std::optional is available... but what's the best approach when it isn't?

Should I provide my own std::optional implementation?

Return nullptr when std::optional isn't available? (Likely to mess my code.)

Or give up on the idea and keep returning nullptr only?

Hoitytoity

What about boost::optional? – Yonatan

@Yonatan Possible, but I'd rather keep the library depending only on the STL itself. – Hoitytoity

If C++11 is a requirement, you cannot use std::optional. Choose one. – Jaimie

You can always make your own. AFAIK it does not rely on any C++17 features, it just wasn't added until then. – Shetrit

Another thing you can do is use std::unique_ptr – Shetrit

Why, std::optional is not that complex. Find some reliable implementation and/or write some template code. You won't get that class from anywhere else in C++11. – Card

If you have code that works correctly without std::optional just use it. Don’t complicate things with maybe-this-way-maybe-that-way. – Supine

IMHO the question is if you want to return something (an object) that is on the heap or if you want to retrun a value (not created on the heap - here optional helps). If you only need to support heap objects you could mimic the optional API with a wrapper that uses std::unique_ptr. (Note: my company used a std::optional backport to C++11, so it is possible to use it with C++11.) – Marasmus

@Mas "Not that complex" 1500 lines – Toastmaster

Answer 1

There is no standard way of using std::optional in C++11. You can either depend on C++17, or you cannot use std::optional.

Should I provide my own std::optional implementation?

You can write your own optional implementation, but you cannot call it std::optional. Alternatively, you can use a pre-existing implementation such as the one in Boost.

All that said, if you're returning a pointer anyway, then there is probably not much point in using optional since pointers already have a representation for "empty" value: the null pointer. If however you need to distinguish null from empty, then optional may be useful.

Twentyone

Answer 2

use this header: https://github.com/TartanLlama/optional

It is the equivalent of std::optional. But it works on C++11 also. When you upgrade to C++17, switch your code to #include <optional>.

Apeak

Answer 3

You should not make return type dependent on C++ Standard version. Standard version switches are meant to be able to compile different parts of a program with different values. If you behave differently based on this, you'll break ODR.

Analyse