Discussion on: Simplify code with 'if constexpr' in C++17

[Ireneusz Szcześniak]

Hi Bartek,

Thank you for an interesting article. I wonder whether you could use the constexpr if statement to optionally provide to a function at compile-time some extra code to execute at run-time. Here's the motivating example:

#include <iostream>

struct empty
{
  void operator()()
  {
  }
};

template<typename T = empty>
void
doit(T t = empty())
{
  if constexpr (!std::is_same_v<T, empty>)
    t();
}

void
foo()
{
  std::cout << "Hello!\n";
}

int
main()
{
  doit();
  doit(foo);
}

I think this implementation is going to perform well, because:

• if I don't pass a callable, no object of class empty will be created (will be optimized out), and so there will be no performance hit,

• if I pass a callable, it will be called, without checking anything at run-time.

However, I wonder if you could share your expertise on whether this problem could be solved better, maybe without defining the empty type, or maybe replacing it with some standard type.

Thanks & best,
Irek