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C++ Template Metaprogramming

Constraining Nontemplate Member Functions

Explore how to apply constraints to nontemplate member functions within class templates to restrict functionality to specific types. Understand the advantages over traditional SFINAE techniques and see practical examples that improve error handling and code clarity in modern C++.

We'll cover the following...

Understanding nontemplate member functions

Nontemplate functions that are members of class templates can be constrained in a similar way to what we have seen so far. This enables template classes to define member functions only for types that satisfy some requirements. In the following example, the equality operator is constrained:

template<typename T>
struct wrapper
{
   T value;
   bool operator==(std::string_view str)
   requires std::is_convertible_v<T, std::string_view>
   {
      return value == str;
   }
};
Constraining the equality operator of the wrapper class template

The wrapper class holds a value of a T type and defines the operator== member only for types that are convertible to std::string_view. Let’s see how this can be used:

C++ 17
wrapper<int>         a{ 42 };
wrapper<char const*> b{ "42" };
// if(a == 42)   {}             // error
if(b == "42") {}                // OK

Note: Uncommenting line 3 in the code above will generate an error.

We have two instantiations of the wrapper class here, one for int and one for char const*. The attempt to compare the a ...