What is constexpr in C++?

Question

I am really confused about a constexpr concept, as I have read constexpr is evaluated at compile time, so it is useful for performance optimization ver

Answer 1

As you said, constexpr is evaluated at compile time. So the value must be evaluable when compiling.

For example:

constexpr int i = 0;
constexpr int& ri = i;

For first line, 0 is evaluable when compiling, its value is 0.

But for second line, compiler needs address of i to do the assignment, which is determined at runtime. So this line will fail.