On Apr 15, 2004, at 11:08 AM, Dill, John wrote:
While having pass-by-value may be more convenient and occur more often, the counter-intuitiveness of the semantics will lead to subtle bugs in people's code who misunderstand or forget bind's behavior in this area.
A bold prediction in the face of contradictory evidence.
Can you elaborate? You may have no problems understanding the library, but the first time I tried to bind with a reference, I ran into the same confusion. I am speaking from my own limited experience with the library. I've read the documentation:
// Start of docs The arguments that bind takes are copied and held internally by the returned function object. For example, in the following code:
int i = 5;
bind(f, i, _1);
a copy of the value of i is stored into the function object. boost::ref and boost::cref can be used to make the function object store a reference to an object, rather than a copy:
int i = 5;
bind(f, ref(i), _1); // End of docs
I know this makes it explicit that you need a ref or cref to have bind store a reference to an object, but there is no reasoning behind this design decision in the documentation, or that I can find in the implementation and I've not stumbled across it in the mail archives.
Hello John, There's a big technical reason: If bind would have reference semantics, you could not do this: bind(f, 1, _1) Bind would have to take its arguments as template <class F, class A, class B, ...> bind( F, A&, B&, ...) 1 is a non-const rvalue, which cannot be bound to non-const reference. Cheers, Jaakko