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Re: Interesting struggle with "Moveable<T>" usage in GCC [message #16943 is a reply to message #16855]

Tue, 22 July 2008 12:40

mrjt
Messages: 705
Registered: March 2007
Location: London

Contributor

The explanation for this is quite straightforward I think.

Structs with just public data members are essentially C aggregates, which can be initialised using the {,} style list.

As soon as you add anything that stops it being a 'pure' aggregate - such as a constructor, inheritance (even from an empty class as is the case with Moveable<T>) or virtual functions - the compiler starts treating the struct as a C++ class. As classes cannot be initialised like aggregates you get the error.

For example:

// This works with {,} initialisation
struct ST {
  Point Get() { return Point(x, y); }
  int x, y;
};

// These don't
struct BC { 	
};
struct ST : BC {
  int x, y;
};

struct ST {
  ST() : x(0), y(0) { }
  int x, y;
};

struct ST {
  virtual Point Get() { return Point(x, y); }
  int x, y;
};

As far as I can tell this is correct compiler behaviour, although it would be nice if it was a bit more forgiving.

I'm not sure about your problem with NTL_MOVEABLE though, I just tried it with MingW and MSC8 and had no problem with missing DeepCopyConstructs, but presumably you're using a more complex structure than my test.

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