How to write code to do conditional template instantiation in C++

Question

I am writing a class template that is parametrized on size_t,

template<size_t k>
class MyClass {...}

The parameter k should really be less than 10, in this case and I would like it to fail to compile if it goes beyond that. How can I do that in C++11 and above?

MyClass<1> instance1; // ok
MyClass<2> instance2; // ok
MyClass<100> instance100; // fail to compile

Answer 1

Use static_assert

template<size_t k>
class MyClass {
    static_assert(k < 10, "Illegal k");
};

int main() {
  MyClass<50> t; //< Compile time failure
}

Answer 2

You could add a check in the template parameter like

template<size_t k, std::enable_if_t<k <= 10, bool> = true>
class MyClass {};

This will allow MyClass<1> instance1; but MyClass<100> instance100; will fail to compile.