What does `class =` inside template mean?

Question

I found this construct in C-code:

template<typename T, class = decltype(std::declval<T>() < std::declval<T>())>
struct check : std::true_type {};

Now I understand what it does but I don't understand how it works. It throws a compile error if type T doesn't support the < -operator. But, apparently, when changing class to something else, the whole thing won't compile and throws a Syntax Error.

What does class = sometypename mean?

Answer 1

class is the same as typename here. You could also do this:

template<typename T, typename = decltype(std::declval<T>() < std::declval<T>())>
struct check : std::true_type {};

You can specify default values for template arguments. For example

template<typename X = int> struct test { };

You can also leave off the name of the template arguments if you don't use them:

template<typename = int> struct test { };

So in your example, the second template parameter is just an unnamed parameter with a default argument.

Answer 2

使这个工作称为SFINAE概念（取代失败不是错误）和用于实现std::enable_if<>等http://en.cppreference.com/w/cpp/language/sfinae