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?

See this [question](https://stackoverflow.com/questions/13346879/const-vs-constexpr-on-variables). — 1201ProgramAlarm, Oct 23 '19 at 03:23

songyuanyao · Answer 1 · 2019-10-23T06:53:58.087

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:
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

constexpr int and constexpr double in c++

1 Answers1