Is declaring variables as const redundant after constexpr was added to the language?

Question

As the keyword constexpr implies const and it can also be calculated at compile time, does it mean that now declaring variables as const doesn't make sense and we should always declare them as constexpr?

Answer 1

and it can also be calculated at compile time, does it mean that now declaring variables as const doesn't make sense and we should always declare them as constexpr?

And must be calculated at compile time (ignoring the as-if rule).

So you can't declare constexpr a variable initialized with a run-time known value. But you can declare it const.

For example: you can't declare bar constexpr

int foo;

std::cin >> foo;

constexpr int bar = foo;  // compilation error

but you can declare it const

int foo;

std::cin >> foo;

const int bar = foo;  // compile

Answer 2

No, not at all.

constexpr means "constant expression", as in [possibly] statically-known, as in "[possibly] known at compile time".

const means "cannot be changed after initialisation".

These are completely separate concepts. A const object can be initialised with a runtime value, for example.

constexpr can imply const, but const certainly does not imply constexpr.

(I think constexpr is a very confusing name, due to this.)

Answer 3

Adding to @max66 answer: constexpr can only replace a top-level const. It can never replace pointer-to-const or const reference. So, sometimes constexpr and const can be used in the same declaration. E.g.

const char* const s = "Hello";

can be replaced with:

constexpr const char* s = "Hello";