constexpr int and constexpr double in c++

Question

I have encountered a strange case.

// this does not work, and reports:
// constexpr variable 'b' must be initialized by a constant expression
int main() {
  const double a = 1.0;
  constexpr double b = a;
  std::cout << b << std::endl;
}

// this works...
int main() {
  const int a = 1;
  constexpr int b = a;
  std::cout << b << std::endl;
}

anything special about double so it cannot make constexpr work?

Answer 1

anything special about double so it cannot make constexpr work?

ints and doubles behave differently in this case.

For core constant expression, a in the 2nd case (with type int) is usable in constant expressions , but a in the 1st case (with type double) isn't.

A core constant expression is any expression whose evaluation would not evaluate any one of the following:

• 9. an lvalue-to-rvalue implicit conversion unless....

     • a. applied to a non-volatile glvalue that designates an object that is usable in constant expressions (see below),

       int main() {
           const std::size_t tabsize = 50;
           int tab[tabsize]; // OK: tabsize is a constant expression
                             // because tabsize is usable in constant expressions
                             // because it has const-qualified integral type, and
                             // its initializer is a constant initializer
       
           std::size_t n = 50;
           const std::size_t sz = n;
           int tab2[sz]; // error: sz is not a constant expression
                         // because sz is not usable in constant expressions
                         // because its initializer was not a constant initializer
       }
       

and

(emphasis mine)

Usable in constant expressions In the list above, a variable is usable in constant expressions if it is

• a constexpr variable, or
• it is a constant-initialized variable
  • of reference type or
  • of const-qualified integral or enumeration type.

You can declare a as constexpr to make it usable in constant expression. e.g.

constexpr double a = 1.0;
constexpr double b = a;   // works fine now