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c++non-type-template-parameter

Is there a way to give a specification about the allowed values of a non-type template parameter?

For example, is there a way of saying "n must be larger than 2" in this code?

template<int n>
class MyClass
{
    ...
}
Solution

  • Use requires from C++20:

    template <int n> requires(n > 2) // The parentheses are often optional, but not with this condition.
class MyClass
{
    // ...
};

    Alternatively you can use a static_assert:

    template <int n>
class MyClass
{
    static_assert(n > 2, "N must be less or equal to 2."); // The description is optional.
    // ...
};

    The first option doesn't let you specify a custom message, but is "SFINAE-friendly", i.e. it's possible to check the condition from a different template without triggering the error on failure. But it doesn't let you specify a custom message. The first option is usually preferred.

    The first option can be replicated pre-C++20 with std::enable_if_t, see @SamVarshavchik's answer.