Should explicit instantiation of templates be used if I want to limit what types a class template can accept?

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I have a templated class, and I want to only allow certain types to be used. I believe one way to do this is to use explicit instantiation at the end of the source file -- I believe this will cause linker errors if the developer tried to instantiate a type that is not one of the explicitly instantiated types.

However, when I searched on SO, e.g., in the 2 links below, I did not see any mentioning of using explicit instantiation, so I am questioning if explicit instantiation shoudl be used for this.

Could someone advise?

restrict a template function, to only allow certain types

Restricting templates to only certain classes?

explicit-instantiation templates c++
1 Answers

C++ has evolved over the last decade, and what you described was a common hack that is no longer necessary. This (and several alternative approaches) was used with older C++ standards by using a combination of two kinds of sub-optimal practices: defining the actual template in a standalone translation unit and explicitly instantiating the template methods (including its constructor) in that translation unit, and by relying on the fact that, otherwise, templates can only be effectively implemented in a header file. This effectively achieves that goal by producing a cryptic link failure.

Fortunately, that hack is not needed in modern C++ any more. As I mentioned, C++ has evolved to the point where something like that can be specified cleanly, explicitly, and in a crystal clear way using C++20 constraints:

#include <type_traits>

template<typename T>
    requires std::is_same_v<T, int> || std::is_same_v<T, char>
struct my_template {};

my_template<int> a;      // OK
my_template<char> b;     // OK
my_template<char *> c;   // ERROR

Your actual template can be defined and implemented normally, in a header file, and the constraint clearly imposes a restriction that your template be instantiated only with specific types.

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