[gtkmm] Returning objects from member functions and operators

[Michael Johnson <michaelj maine rr com>]

At Murray's suggestion, I'd like to bring up a topic that I think would 
be useful to address in gtkmm/glibmm 2.4, which is how objects are 
returned from functions and operators.

There are a number of places in glibmm/gtkmm where objects are returned 
on the stack from functions. These are invariably returned as non-const 
objects. This results in behavior which is inconsistent with the 
behaviors of C++ built-in types. Scott Meyers presents a detailed 
discussion of this in Effective C++ item 21:

   
http://www.awprofessional.com/content/images/0201924889/items/item21.html

For example in glib/glibmm/main.h there is a get_default() function 
defined for MainContext:

   static Glib::RefPtr<MainContext> get_default();

This function returns a non-const object. So it is possible to do 
something like this:

   return MainContext::get_default() = 0;

This clearly is not correct code, but it is something that could easily 
happen by accident. If one were to do something similar with integers, 
the compiler would flag it, because the built-in operators return const 
values. However, since the function is returning a non-const object on 
the stack, the assignment is legal. Wrong, but legal.

Basically, so long as there is an appropriate copy constructor for the 
class being returned, there's no reason why any function or operator 
should ever return a non-const object on the stack. Doing so increases 
the risk of incorrect code, whereas making sure to return objects on the 
stack as const carries no penalty.

I suggest that going forward, all functions and operators in 
glibmm/gtkmm that return an object on the stack should return that 
object as const, e.g.:

   static const Glib::RefPtr<MainContext> get_default()

Doing so should not impact existing programs in any way, unless they 
contain incorrect code.

..mj