Re: Can you please help me explain constexpr?

DeMarcus <>
Wed, 6 Jul 2011 20:23:12 CST
On 2011-07-06 20:19, Daniel Kr?gler wrote:

Am 06.07.2011 01:03, schrieb DeMarcus:


Why doesn't this compile?

class MyClass
MyClass( int i ) : i_(i) { /* non-const ops */ }

constexpr int f() const { return i_; }


const int i_;

In gcc4.6.1 I get
error: enclosing class of ?int MyClass::f() const? is not a literal type

To me f() only uses a constant, so what can go wrong?

For a literal type it is irrelevant whether a member is constant or not,
the problem with your type is, that there is no constexpr constructor
that initializes the member i_. Interpreting your comment correctly,
MyClass::MyClass(int i) could not be marked as constexpr constructor
(because of further operations within the constructor body), but without
this guarantee there is no way to possibly initialize an object of
MyClass in a way that the aforementioned member.

Test case:

constexpr int i = MyClass(42).f();

Somehow I thought that the compiler would see that it's actually f()
that is used (which is constexpr) and that it uses a const int which
must have been initialized by the time it's used in f(). Therefore the
compiler could ignore that the constructor is non-constexpr. But I'm not
familiar with constexpr and I guess it's more complicated than that.

In this example the sub expression MyClass(42) does not satisfy the
criteria for a constant expression, therefore MyClass can never be part
of a constant expression. Either

a) remove the constexpr from MyClass::f(), or

b) make MyClass::f() a constexpr *static* member function returning the
result of some constant expression (e.g. it could return the value of a
static member that can be used in constant expressions), or

c) provide another constexpr constructor for MyClass that initializes
the member i_.

I was just so keen on the following but I guess I can forget it.

class MyClass
   MyClass( int id, const std::string& name )
      : id_(id), name_(name) { /* non-const ops */ }

   // Conversion operator to be used in switch.
   constexpr operator int() const { return id_; }

   std::string getName() const { return name_; }

   const int id_;
   const std::string name_;

const MyClass FLYING_CAR( 42, "Flying car" );
const MyClass DRINKIN_BOAT( 4711, "Drinking boat" );

int main()
   int i = randomVehicle();

   switch( i )
      case FLYING_CAR:
         std::cout << "Lycky you!" << std::endl;

      case DRINKING_BOAT:
         std::cout << "Bon voyage!" << std::endl;

         std::cout << "Mi casa es tu casa!" << std::endl;

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