Lazy instantiation of C++ templates

2018-03-09

One (tiny) piece of code that I often find useful when meta-programming:

template
<
  template<class...> class T,
  typename... Args
>
struct lazy_template {
  using instantiated = T<Args...>;
};

Where does this help? For example when there’s some condition based on which I want different types (so std::conditional) and these types are templates which can only be instantiated if the condition is true respective false (and otherwise result in a compiler error).

Extremely simplified example:

#include <type_traits>

template<typename>
struct good {
  static const bool value = true;
};

template<typename>
struct bad; // declaration only, cannot be instantiated

int main() {
  // std::conditional<good<char>, bad<int>>::type fails
  using tmpl = std::conditional<
    true,
    lazy_template<good, char>,
    lazy_template<bad, int>>::type;
  // Instantiation happens only after this point, and only for the
  // good template.
  return tmpl::instantiated::value;
}