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.
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.
---- EDIT ----
As @prapin observed in a comment (thanks!), if constexpr can be necessary for an auto function.
I propose another trivial (and silly) example
Without if constexpr, the following bar() function
template <typename T>
auto bar (T const &)
{
if ( true == std::is_same_v<T, std::string>> )
return 42;
else
return 42L;
}
doesn't compile, because the first return return a int value, the second return a long; so, given that without if constexpr the compiler must instantiate both return's, so the compiler can't conciliate the returns types and can't determine the return type of the function.
With if constexpr,
if constexpr ( true == std::is_same_v<T, std::string>> )
return 42;
else
return 42L;
the compiler instantiate the first return or the second one; never both. So the compiler ever determine the type returned from the function (int when called with a std::string, long otherwise).
if constexpris only really useful with properelseor anotherelse if constexpr. Otherwise the fallback instruction is always instantiated which is normally not wanted.