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 versus normal const.

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

The above code returns an error "invalid initialization of reference of type 'int&' from expression of type 'const int'", why?

Also, the next code has an error:

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

If I replaced the constexpr keyword with const, all above worked correctly.

Answer 1

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

The second line is a problem because the pointer does not point to a const object. The pointer itself is const.

Using

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

solves that problem. However, that will be still a problem if the variables are defined in a function scope.

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

int main() {}

is a valid program.

void foo()
{
   constexpr int i = 0;
   constexpr int const* ri = &i;
}

int main() {}

is not a valid program.

Here's what the C++11 standard has to say about address constant expression:

5.19 Constant expressions

3 .. An address constant expression is a prvalue core constant expression of pointer type that evaluates to the address of an object with static storage duration, to the address of a function, or to a null pointer value, or a prvalue core constant expression of type std::nullptr_t.