C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  language question          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Chris Guest Posted: Wed Jul 20, 2005 9:53 am    Post subject: language question Hi, I found what looks like a bug, but it's more likely something I don't understand about C++. This code: const double OneEighty = 180.0; struct Degrees { double m_Degrees; Degrees( const double degrees ) : m_Degrees( degrees ) { } }; struct Angle { double m_Degrees; Angle(const Degrees & degree) : m_Degrees( degree.m_Degrees ) { } Angle & operator+=(const Angle & rhs) { m_Degrees += rhs.m_Degrees; return *this; } }; void test(void) { Degrees degrees180( OneEighty ); Angle angle1( Degrees(180.0 ));// an instance of Angle Angle angle2( Degrees(OneEighty));// declaration of a func?? // // The following line fails when uncommented // on the compilers on my system of the following versions: // gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-126) // gcc version 3.3.3 20040412 (Red Hat Linux 3.3.3-7) // gcc version 3.4.0 (Red Hat Linux 3.4.0-1) // gcc version 4.0.0 // gcc version 4.0.1 20050617 (prerelease) // angle1 += angle2; // won't compile because angle2 isn't an Angle //... } What's wrong with this? If I substitute "degrees180" for "Degrees(OneEighty)" the line below compiles OK. The difference in the declaration of angle1 and angle2 seems fairly minor -- a literal constant versus a constant value -- but the compiler thinks so. thanks Chris Arena Portsmout, RI [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tony_in_da_uk@yahoo.co.uk Guest Posted: Wed Jul 20, 2005 2:08 pm    Post subject: Re: language question Hi Chris, Your comment "declaration of a function" shows you already suspected the actual problem... angle2 is declared as a function "Angle(*)(Degrees)". If I remember coorrectly, the () around OneEighty are optional in naming the function parameter. It's silly, but that's life. There's a discussion of the problem in one of those common C++ books - probably one of Herb Sutter's. Cheers, Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Wed Jul 20, 2005 3:04 pm    Post subject: Re: language question Chris <ccarena (AT) cox (DOT) net> writes: Quote: I found what looks like a bug, but it's more likely something I don't understand about C++. Indeed. Rest assured that the issue is subtle enough to catch those who think they understand C++ well from time to time. Next time please copy&paste the actual error message(s) into your post. Quote: const double OneEighty = 180.0; struct Degrees { double m_Degrees; Degrees( const double degrees ) : m_Degrees( degrees ) { } }; [snip] Quote: void test(void) { Degrees degrees180( OneEighty ); Angle angle1( Degrees(180.0 ));// an instance of Angle Angle angle2( Degrees(OneEighty));// declaration of a func?? This line is syntactically equivalent to Angle angle2(Degrees OneEighty); and thus means the declaration of a function angle2 that takes a parameter of type Degrees and returns an Angle object. [Whenever a statement could in principle be both a definition and a declaration (that is not a definition), the compiler has to parse it as a declaration that is not a definition.] Quote: // // The following line fails when uncommented // on the compilers on my system of the following versions: // gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-126) // gcc version 3.3.3 20040412 (Red Hat Linux 3.3.3-7) // gcc version 3.4.0 (Red Hat Linux 3.4.0-1) // gcc version 4.0.0 // gcc version 4.0.1 20050617 (prerelease) // angle1 += angle2; // won't compile because angle2 isn't an Angle Since you probably haven't overloaded += for a lefthand side argument of type Angle and a righthand side arguemnt of type Angle(Degrees), the compiler won't accept this line. Quote: } What's wrong with this? If I substitute "degrees180" for "Degrees(OneEighty)" the line below compiles OK. Angle angle2(degrees180); can only be parsed as the definition of a local variable, so that's what it is. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Wed Jul 20, 2005 3:05 pm    Post subject: Re: language question Chris wrote: Quote: const double OneEighty = 180.0; struct Degrees [...] struct Angle [...] Degrees degrees180( OneEighty ); Angle angle1( Degrees(180.0 ));// an instance of Angle Angle angle2( Degrees(OneEighty));// declaration of a func?? The rule given in the standard is that when something can be parsed as a declaration, it is a declaration. Yes, countless others did and still do stumble across this. The above is equivalent to Angle angle2( Degrees OneEighty); The workaround is to add additional brackets Angle angle2( (Degrees(OneEighty))); or use static_cast. Quote: // gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-126) Just FYI: http://gcc.gnu.org/gcc-2.96.html Uli [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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