Lazy (call once)

cpp_call_once.cpp

#include <iostream>
#include <memory>
#include <functional>
#include <mutex>

template <typename T>
struct lazy {
public:
  template <typename Fun>
  explicit lazy(Fun&& fun) : fun(std::forward<Fun>(fun)) {}

  T& get() const {
    std::call_once(flag, [this] { ptr.reset(fun()); });
    return *ptr;
  }

private:
  std::once_flag flag;
  std::unique_ptr<T> ptr;
  std::function<T*()> fun;
};

class foo {};

int main() {

  lazy<foo> x([] { return new foo; });

  return 0;
}