66. constexpr-i

Question.7

A developer uses if constexpr to select code at compile time:

template<typename T>
void send(T val) {
   if constexpr (sizeof(T) == 1)
       uart_send_byte(val);
   else if constexpr (sizeof(T) == 4)
       uart_send_word(val);
}

What does if constexpr do differently from a regular if?

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Nothing -- both evaluate at runtime
The false branch is completely discarded at compile time -- it is not compiled, not included in the binary, and cannot cause compilation errors
if constexpr is faster at runtime
if constexpr works only with integer types

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