Conditionally compiling classes and functions with enable_if

Template metaprogramming is a powerful feature of C++ that enables us to write generic classes and functions that work with any type. This is a problem sometimes because the language does not define any mechanism for specifying constraints on the types that can be substituted for the template parameters. However, we can still achieve this using metaprogramming tricks and by leveraging a rule called substitution failure is not an error, also known as SFINAE. This rule determines whether the compiler discards, from the overloaded set, a specialization when substituting the explicitly specified or deduced type for the template parameter when it fails, instead of generating an error. This recipe will focus on implementing type constraints for templates.

Getting ready

Developers have used a class template usually called enable_if for many years in conjunction with SFINAE to implement constraints on template types. The enable_if...