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.NET Forum / Languages / Managed C++ / June 2004namespace not needed
namespace MyNS { struct A { int m; }; template<typename T> T plus (const T& lhs, const T& rhs) { T ret(lhs); ret.m += rhs.m; return ret; } ///< + rhs } //============================================================================== // M A I N //------------------------------------------------------------------------------ void main() { MyNS::A a, b; plus(a, b); //why not 'MyNS::' needed ??? } > namespace MyNS { > struct A { [quoted text clipped - 3 lines] > template<typename T> T plus (const T& lhs, const T& rhs) { T > ret(lhs); ret.m += rhs.m; return ret; } ///< + rhs } //========================================================================== ==== > // M A I N //-------------------------------------------------------------------------- ---- > void main() > { > MyNS::A a, b; > plus(a, b); //why not 'MyNS::' needed ??? > } It's called "argument dependent lookup". Because a and b are instances of a class in namespace MyNS, the compiler searches MyNS (in addition to the global namespace) for the definition of plus(). -cd >namespace MyNS { > struct A { [quoted text clipped - 12 lines] > plus(a, b); //why not 'MyNS::' needed ??? >} When you call an unqualified function (no namespace:: at the start), the name is searched for first of all in the scope that the call is in (in this case both "main" and the global scope), and also in any namespaces that the parameters reside in (A is from MyNS, so MyNS is also searched). This is called argument dependent lookup. I think MSVC didn't properly support it until VC7.0. Tom  SignatureC++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html > This is called argument dependent lookup. I think MSVC > didn't properly support it until VC7.0. I think it was 7.1 actually. It absolutely was not supported in VC6 and before. -cd >> This is called argument dependent lookup. I think MSVC >> didn't properly support it until VC7.0. > >I think it was 7.1 actually. It absolutely was not supported in VC6 and >before. Except, slightly bizarrely, for operator overloads. e.g. namespace ns { struct A{}; bool operator==(A a, A b) { return true; } } int main() { ns::A a1, a2; a1 == a2; return 0; } compiles fine on MSVC6. Tom SignatureC++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html >>> This is called argument dependent lookup. I think MSVC >>> didn't properly support it until VC7.0. [quoted text clipped - 3 lines] > > Except, slightly bizarrely, for operator overloads. e.g. Truly bizarre! -cd >>>This is called argument dependent lookup. I think MSVC >>>didn't properly support it until VC7.0. [quoted text clipped - 3 lines] > > Except, slightly bizarrely, for operator overloads. e.g. I don't think that's bizarre. If you are going to implement argument dependent lookup, the first place to do it is when resolving overloaded operators, since that is the one place where adding a qualifier to the function name isn't really possible. When I was working on a C++ compiler many years ago, that's exactly how I did it. SignatureDavid Olsen qg4h9ykc5m@yahoo.com >>>>This is called argument dependent lookup. I think MSVC >>>>didn't properly support it until VC7.0. [quoted text clipped - 9 lines] >function name isn't really possible. When I was working on a C++ >compiler many years ago, that's exactly how I did it. I agree that that was the rationale, but I still find it "slightly bizarre"; if you implement the ADL code for operators, and the standard says it should work for other functions too, why wouldn't you just reuse the ADL code for functions too? My only idea is that ADL for non-operator functions was a fairly late change to the standard. Certainly Andrew Koenig's original ADL proposal only mentions operators: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/1995/N0645.pdf Tom SignatureC++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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