Usage of if constexpr in template

January 2, 2023

I am trying to understand the utility of if constexpr and want to know if there is any utility in using it in this way.

template<bool B>
int fun()
{
    if constexpr (B)
        return 1;
    return 0;
}

Is this function changed at all by using if constexpr instead of a regular if? I assume the performance would be the same. My understanding of templates is that the outcome of the if statement is already known at compile time so there is no difference.

>Solution :

Utility of constexpr?

A trivial example… if you write the following function

template <typename T>
auto foo (T const & val)
{
   if ( true == std::is_same_v<T, std::string>> )
      return val.size()
   else
      return val;
}

and call it with an integer

foo(42);

you get a compilation error, because the instruction

val.size();

has to be instantiated also when val is an int but, unfortunately, int isn’t a class with a size() method

But if you add constexpr after the if

// VVVVVVVVV
if constexpr ( true == std::is_same_v<T, std::string>> )
   return val.size()

now the return val.size(); instruction is instantiated only when T is std::string, so you can call foo() also with arguments without a size() method.