From 921472429052796926 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,41c422257f67d844 X-Google-Attributes: gidf78e5,public From: James Kanze Subject: Re: Difference in constructor invokation Date: 1997/07/04 Message-ID: #1/1 X-Deja-AN: 254541866 References: <33BA74FF.4128@stgl.sel.alcatel.de> X-Original-Date: 04 Jul 1997 13:38:48 +0200 Organization: - X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUBM70DX+EDnX0m9pzZAQFERwGAnSIgVXEeE+niNhivopfd1ODPq/4r3JDU nqsycg277GHPUMf3vRHm1Kx9N1dg6KgA =Mmpv Newsgroups: comp.std.c++ Originator: fjh@murlibobo.cs.mu.OZ.AU Martin Lang writes: |> Is there a difference if the object in the following example is |> created by copy construction or by initialization construction? Maybe. |> class A { |> public: |> A(int v); |> A(const A& rhs); |> private: |> int value; |> }; |> |> |> class B { |> public: |> B(int v); |> operator int() const; |> private: |> int value; |> }; |> |> |> void f(const B& x) |> { |> A a1= x; // ??? Illegal. Formally, the rule for this is convert the expression on the right hand side of the equal sign to an A, then copy construct the object using the results of this conversion. The compiler is explicitly allowed to elide the call to the copy constructor if it wishes, but it must still analyse the program as if it called the copy constructor. In this case, you need an A, or something convertible to an A, on the right side of the equals sign. But x is a B, which is not convertible to an A. (The conversion would involve two user defined conversions.) |> A a2(x); // ??? This is legal. The rule is to convert x to something which can be used as an argument to a constructor for A (not necessarily the copy constructor). A has a constructor taking an int, and x (a B) can be converted to an int. |> } -- James Kanze home: kanze@gabi-soft.fr +33 (0)1 39 55 85 62 office: kanze@vx.cit.alcatel.fr +33 (0)1 69 63 14 54 GABI Software, Sarl., 22 rue Jacques-Lemercier, F-78000 Versailles France -- Conseils en informatique industrielle -- --- [ comp.std.c++ is moderated. To submit articles: try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ FAQ: http://reality.sgi.com/employees/austern_mti/std-c++/faq.html ] [ Policy: http://reality.sgi.com/employees/austern_mti/std-c++/policy.html ] [ Comments? mailto:std-c++-request@ncar.ucar.edu ]