The body of constexpr function not a return-statement

Question

In the following program, I have added an explicit return statement in func(), but the compiler gives me the following error:

m.cpp: In         


        

Answer 1

Prior to C++14, the body of a constexpr function must consist solely of a return statement: it cannot have any other statements inside it. This works in C++11 :

constexpr int func (int x) 
{
  return x < 0 ? -x : x;
}

In C++14 and up, what you wrote is legal, as are most other statements.

Source.

Answer 2

C++11's constexpr functions are more restrictive than that.

From cppreference:

the function body must be either deleted or defaulted or contain only the following:

• null statements (plain semicolons)
• static_assert declarations
• typedef declarations and alias declarations that do not define classes or enumerations
• using declarations
• using directives
• exactly one return statement.

So you can say this instead:

constexpr int func (int x) { return x < 0 ? -x : x; }

static_assert(func(42) == 42, "");
static_assert(func(-42) == 42, "");

int main() {}

Note that this restriction was lifted in C++14.