c++ - What if I write return statement in constructor?

Question

What if I write return statement in constructor? Is it standard conformant?

struct A
{ 
     A() { return; } 
};

The above code compiles fine, without any error at ideone. But the following code doesn't:

struct A
{ 
   A() { return 100; } 
};

It gives this error at ideone:

error: returning a value from a constructor

I understand that returning value from constructor doesn't make sense at all, because it doesn't explicitly mention return type, and we cannot store the returned value after all. But I'm curious to know :

• Which statement from the C++ Standard allows the first example but forbids the second one? Is there any explicit statement?
• Is the return type in the first example void?
• Is there any implicit return type at all?

Answer 1

Yes, using return statements in constructors is perfectly standard.

Constructors are functions that do not return a value. The family of functions that do not return a value consists of: void functions, constructors and destructors. It is stated in 6.6.3/2 in the C++ standard. The very same 6.6.3/2 states that it is illegal to use return with an argument in a function that does not return a value.

6.6.3 The return statement

2 A return statement without an expression can be used only in functions that do not return a value, that is, a function with the return type void, a constructor (12.1), or a destructor (12.4). A return statement with an expression of non-void type can be used only in functions returning a value; the value of the expression is returned to the caller of the function.

Additionally, 12.1/12 states that

12.1 Constructors

12 No return type (not even void) shall be specified for a constructor. A return statement in the body of a constructor shall not specify a return value.

Note, BTW, that in C++ it is legal to use return with an argument in a void function, as long as the argument of return has type void

void foo() {
  return (void) 0; // Legal in C++ (but not in C)
}

This is not allowed in constructors though, since constructors are not void functions.

There's also one relatively obscure restriction relevant to the usage of return with constructors: it is illegal to use return in function-try-block of a constructor (with other functions it is OK)

15.3 Handling an exception

15 If a return statement appears in a handler of the function-try-block of a constructor, the program is ill formed.