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Constant Expressions

Whereas const marks a runtime constant (its value can only be determined at runtime, and cannot be changed after it is assigned), constexpr marks a compile-time constant.

Using constexpr enforces a variable or function to be evaluable during compile-time (otherwise it's a compile error). The compiler can then substitute their occurences with the evaluated value for optimization.

constexpr with variables

Variables with constexpr will be evaluated during compile-time. (Note that constexpr implies const, as the variable's value cannot be changed after initialization.)

int main() {
  // These are OK because the values
  // can be evaluated during compile-time
  constexpr double gravity = 9.8;
  constexpr int sum = 4 + 5;

  int age{};
  scanf("%d", &age);
  // The following causes compile-error
  // constexpr int const_age = age;
}

constexpr with functions

Functions with constexpr will be evaluated during compile-time.

// You can now use this function in constexpr expressions
constexpr int max(int a, int b) {
  return a > b ? a : b;
}

int main() {
  // If you don't specify `constexpr` for `max`,
  // the line below will cause compilation error.
  constexpr int value = max(2, 3);
}

References