The Trailing Requires Clause

How to constrain classes with the trailing requires clause

When we learn that we can use the trailing requires clause with class templates, we might at first think of an implementation like this:

template <typename T>
class WrappedNumber {
public:
    WrappedNumber(T num) : m_num(num) {}
private:
    T m_num;
} requires Number<T>;

However, this doesn’t have any benefits and doesn’t compile. Imagine if we had to scroll down to the end of some long classes just to see these constraints. We can make this more efficient.

Note: The trailing requires clause can be used with class templates on functions.

How to constrain a function of a class template

So far we’ve seen examples of constraining the template parameter for the whole class. But what if we wanted to constrain the usage of a certain function of a class template? Or what if we wanted to provide different implementations based on certain traits of the template parameter type? It’s possible without using the difficult to pronounce and difficult to understand write-only SFINAESFINAE" stands for "substitution failure is not an error.

let’s see how we can restrict the usage of a function based on the ...