Conditionally compile function if `std::optional` exists

Question

I am having a mixed C++14/C++17 codebase and I want to enable a function isEmpty only if I am having std::optional at hand. So, I tried SFINAE:

template <typename T, typename int_<decltype(std::nullopt)>::type = 0>
inline bool isEmpty(const std::optional<T>& v) {
  return !v;
}

However, that doesn't work.

How can I conditionally compile isEmpty if std::optional is there?

Normally you wrap this in a check using the preprocessor for the presence of the C++17 language flag: `#ifdef c++17_flag your_code #endif` — NathanOliver, Jun 07 '22 at 15:09
You can find the flags here: https://stackoverflow.com/a/56483887/4342498 — NathanOliver, Jun 07 '22 at 15:17
Ah, the oft misunderstood sfinae. It is not for conditional compilation, but for pruning overloads in overload resolution. You can't just put a token soup there an expect it to work. — StoryTeller - Unslander Monica, Jun 07 '22 at 15:34
Related, for Visual Studio: https://stackoverflow.com/questions/52379233/is-has-cxx17-macro-usable-in-custom-project-headers-to-enable-c17-language — Spencer, Jun 07 '22 at 16:38

score 5 · Accepted Answer · edited Jun 07 '22 at 16:32

5

If your compiler is recent enough (e.g. GCC >= 9.1 or Clang >= 9.0.0), you may include header <version> and conditionally compile your function template if macro __cpp_lib_optional is defined:

#include <version> // provides, among others, __cpp_lib_optional

#ifdef __cpp_lib_optional
template <typename T>
inline bool isEmpty(const std::optional<T>& v) {
    return !v;
}
#endif

This way, isEmpty will only be compiled if you're in -std=c++17 mode (or higher).

edited Jun 07 '22 at 16:32

user3612643

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answered Jun 07 '22 at 16:11

paolo

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For Visual Studio this would be `#if _HAS_CXX17`. – Spencer Jun 07 '22 at 16:38
Don't forget to wrap `#include ` inside a conditional block, too. Either inside the shown `#ifdef __cpp_lib_optional` block, or inside of its own `#ifdef __has_header #if __has_header() #include #endif #endif` block – Remy Lebeau Jun 07 '22 at 17:37

Conditionally compile function if `std::optional` exists

1 Answers1