deleted functions with a compile time message when they are used

Question

So I'm trying to restrict operations on a type that has a conversion operator to bool - for example:

template <typename R> Result operator&&(const R&) { STATIC_ASSERT(false, MY_MESSAGE); return Result(); }

STATIC_ASSERT is my wrapper macro around c++11 static_assert and a macro-ish c++98 static assert.

I want a somewhat useful message as an error for users that try to use this so making it private or deleting it in c++11 aren't an option.

This however works only for MSVC because of the difference between msvc and g++/clang - with g++/clang the static assert always fires - even when the "deleted" function is not used.

The only thing I've seen that would do the trick is to use an undefined type with it's name as the message as the return type of the template - like this:

template<typename R> STATIC_ASSERT_Expression_Too_Complex_Please_Rewrite_As_Binary_Comparison& operator&&(const R&);

I first saw this here

Is there any other way to do this in c++98 - a deleted function with a custom message when the user tries to use it?

@HansPassant thanks. (this was regarding > this < question)

onqtam
– onqtam

2016-09-14 10:36:17 +00:00
Commented Sep 14, 2016 at 10:36 — onqtam
– onqtam, Commented Sep 14, 2016 at 10:36

Jarod42 · Accepted Answer · 2024-04-14 14:28:28Z

2

in static_assert(false, message), false is non dependent of template.

You have to make your condition depending of template.

as static_assert(!std::is_same<T, T>::value, message)

Since p2593r1 (C++23, but retro-actif), static_assert(false) is possible.

edited Apr 14, 2024 at 14:28

answered Sep 13, 2016 at 11:51

Jarod42

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3 Comments

onqtam Over a year ago

Thanks! Actually I just realized the same thing and came up with a solution of my own - template <typename T> struct deferred_false { static const bool value = false; }; and I pass deferred_false<R>::value to the static assert!

Fabio says Reinstate Monica Over a year ago

Can you clarify what it means that it's C++23, but retro-active?

Jarod42 Over a year ago

Recent compilers (which have C++23 support) should allow static_assert(false), even in C++11 mode.

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