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How to c++ check in noexcept that the template parameter T defined through template will throw or not throw an exception during the operation+. In fact, you need to check the addition T + T for throwing a possible exception. I wrote the following code: noexcept(T + T), but, unfortunately, such code does not compile.

Tippa Toppa
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  • Which version of C++ are you asking about? – Nicol Bolas Jan 26 '22 at 15:54
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    What you did in your previous question (`noexcept(std::declval() + std::declval())`) should compile. There might be issues with it for testing only rvalue or only lvalue operands, but I don't see why you changed it? – user17732522 Jan 26 '22 at 15:58
  • ```noexcept(std::declval() + std::declval())``` is correct code? I'm in training on how to do this check. But, unfortunately, the system does not accept my answer, so I doubted this code. – Tippa Toppa Jan 26 '22 at 16:00
  • @TippaToppa Yes, it is valid code and you also used it in a correct way in your previous question. What is "the system"? Do you mean you got a compilation error with it? – user17732522 Jan 26 '22 at 16:02

2 Answers2

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T + T is not a valid expression. T is a type, and types do not have operators. What you want are objects of type T and to add them together to see if that expression is noexcept or not. You can do that like

noexcept(std::declval<T>() + std::declval<T>())
NathanOliver
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  • Note that `noexcept` is applied to functions. Which typically have parameters. Which typically have names that `noexcept` can already access. And typically, these parameters will involve at least one which is of type `T`. So `declval` is almost always unnecessary. – Nicol Bolas Jan 26 '22 at 16:00
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If you're using C++20, you can write a concept to check this:

template<class T>
concept noexcept_addable = requires(T a)
{
    requires noexcept(a + a);
};

Note that it's requires noexcept(a + a);, not just noexcept(a + a);. The latter would just check that noexcept(a + a); is valid to compile, whereas the former actually requires that it evaluates to true.

You can see a demo of this concept here.

Nathan Pierson
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