BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Newbie: calling a function with "var&" parameter          BorlandTalk.com Forum Index -> C++ Builder (Language C++) View previous topic :: View next topic   Author Message jonjon Guest Posted: Fri Dec 30, 2005 5:55 pm    Post subject: Newbie: calling a function with "var&" parameter Hi, I've a piece of code which is correctly compiling in MSVC but generates errors in BDS2006, and I think this is because of my lack of knowledge in the language. I hope somebody will be able to help me. - "vector3" is a class - I have a class aabb whose constructor is "aabb(vector3& a_Pos, vector3& a_Size) {};" - Somewhere in my code I call this constructor like this "return aabb( vector3(0,0,0), vector3(1,1,1));" Now BDS complains on that call that: E2285 Could not find a match for 'aabb::aabb(vector3,vector3)' I've succeeded by first creating the vectors like this "vector3 v1(0,0,0)..." and then calling the constructor with those parameters but isn't it a shorter way of doing this ? Why does MSVC likes it but BCC doesn't ? Thanks for any help. Regards, John. Back to top Alan Bellingham Guest Posted: Fri Dec 30, 2005 6:17 pm    Post subject: Re: Newbie: calling a function with "var&" parameter "jonjon" <none (AT) for (DOT) now> wrote: Quote: I've a piece of code which is correctly compiling in MSVC but generates errors in BDS2006, and I think this is because of my lack of knowledge in the language. I hope somebody will be able to help me. *Incorrectly* compiling in VC. *Correctly* failing in BCB. (And still compiling in Visual C++ 2005 Express, which does surprise me.) Quote: - "vector3" is a class - I have a class aabb whose constructor is "aabb(vector3& a_Pos, vector3& a_Size) {};" - Somewhere in my code I call this constructor like this "return aabb( vector3(0,0,0), vector3(1,1,1));" Now BDS complains on that call that: E2285 Could not find a match for 'aabb::aabb(vector3,vector3)' I've succeeded by first creating the vectors like this "vector3 v1(0,0,0)..." and then calling the constructor with those parameters but isn't it a shorter way of doing this ? Why does MSVC likes it but BCC doesn't ? Because BDS2006 is getting it right - it's refusing to bind a *non-const* reference to a temporary. The reason for this is that the C++ Standard Committee came to the conclusion that if you pass a non-const reference to a function, then the passed object is (presumably) to be updated. On the other hand, it makes no sense to modify a temporary variable that is going away once the function call is complete. Ergo, an attempt to pass a temporary to a non-const reference was almost guaranteed to be an error, yet a workaround for those few cases where it was meant was simplicity itself (you found it yourself). Question - why are you using a non-const reference? What do you expect to happen? Alan Bellingham -- ACCU Conference 2006 - 19-22 April, Randolph Hotel, Oxford, UK Back to top jonjon Guest Posted: Fri Dec 30, 2005 8:22 pm    Post subject: Re: Newbie: calling a function with "var&" parameter Quote: Ergo, an attempt to pass a temporary to a non-const reference was almost guaranteed to be an error, yet a workaround for those few cases where it was meant was simplicity itself (you found it yourself). Thank you for your clear explanation Alan. I guess what I didn't understand was the "vector3& v1" but I understand it's the same as "vector3 & v1" or "vector3 &v1" which means that the function is expecting a reference of vector3... a pointer parameter I guess in the delphi world. Quote: Question - why are you using a non-const reference? What do you expect to happen? The thing is, it's not originally my code and I was only trying to understand it and make it compile in BDS2006. I pretty much think now that it was rapidely coded in MSVC which didn't complain (or maybe only warned ?) about this call. Thanks for the clarification Alan. Regards, John. Back to top Chris Uzdavinis (TeamB) Guest Posted: Mon Jan 02, 2006 5:31 pm    Post subject: Re: Newbie: calling a function with "var&" parameter "jonjon" <none (AT) for (DOT) now> writes: Quote: Ergo, an attempt to pass a temporary to a non-const reference was almost guaranteed to be an error, yet a workaround for those few cases where it was meant was simplicity itself (you found it yourself). Thank you for your clear explanation Alan. I guess what I didn't understand was the "vector3& v1" but I understand it's the same as "vector3 & v1" or "vector3 &v1" which means that the function is expecting a reference of vector3... a pointer parameter I guess in the delphi world. The spacing of the reference is immaterial. vector3&v1 is yet another way to spell it. What Alan is saying, is that the reference is to a non-const object meaning your interface says "I plan to modify this value you are passing in." Since an unnamed temporary cannot be bound to a reference to non-const, the code should not compile. (The reasoning is that modifying a temporary is usually a programming mistake, and it's hard to track down without compiler support. If you really need to modify the value, simply declare it with a name, then pass in the named object to the function instead of creating one in the function invocation itself.) If you don't really need to modify the value, the function signature should be a reference to a const object, like this: void myfunc (vector3 const & v1) Now, you can pass an unnamed temporary object to the above function, because the reference is to a const vector3, and the interface says, "I won't change this". -- Chris (TeamB); Back to top jonjon Guest Posted: Tue Jan 03, 2006 2:19 pm    Post subject: Re: Newbie: calling a function with "var&" parameter Quote: If you don't really need to modify the value, the function signature should be a reference to a const object, like this: void myfunc (vector3 const & v1) Great and very good explanation. Thanks a lot to both of you. Regards, John. 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