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

user_4190567 0
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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
templates c++
3 Answers

Use static_assert

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

int main() {
  MyClass<50> t; //< Compile time failure
}
user_29908960

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.

user_43424980

You can use std::enable_if if you are using C++11.

template<std::size_t k, typename unused = typename std::enable_if<k <= 10>::type>
class MyClass {};

You can use std::enable_if_t if you are able to use C++14 or higher.

template<std::size_t k, typename unused = std::enable_if_t<k <= 10 >>
class MyClass {};
user_4345510
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