C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  error not caught at link time: should it be?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message nzanella@cs.mun.ca Guest Posted: Wed Jul 27, 2005 6:36 pm    Post subject: error not caught at link time: should it be? Hello, I have two .cpp files. The first one contains: Foo *foo; and in the second one contains: extern Foo foo; I am using a modified version of gcc with Makefiles automatically created. The strange thing is that the tools go through the link phase without detecting the error. Isn't it to be expected that the error be caught by the linker??? I feel somewhat surprised that it didn't catch it. I guess when the linker looks for external symbols it doesn't care about matching their types (eg. pointer versus value). Thanks!!! Neil Back to top Victor Bazarov Guest Posted: Wed Jul 27, 2005 7:37 pm    Post subject: Re: error not caught at link time: should it be? [email]nzanella (AT) cs (DOT) mun.ca[/email] wrote: Quote: I have two .cpp files. The first one contains: Foo *foo; and in the second one contains: extern Foo foo; The two 'foo' are different. Essentially you have overloading of the symbol 'foo' here. Quote: I am using a modified version of gcc with Makefiles automatically created. Irrelevant here. Quote: The strange thing is that the tools go through the link phase without detecting the error. Isn't it to be expected that the error be caught by the linker??? Linking is not part of the language specification. Quote: I feel somewhat surprised that it didn't catch it. I guess when the linker looks for external symbols it doesn't care about matching their types (eg. pointer versus value). Since you didn't post _how_ your 'foo' symbols are used, there is nothing that can be said about it. V Back to top Old Wolf Guest Posted: Wed Jul 27, 2005 10:45 pm    Post subject: Re: error not caught at link time: should it be? Victor Bazarov wrote: Quote: nzanella (AT) cs (DOT) mun.ca wrote: I have two .cpp files. The first one contains: Foo *foo; and in the second one contains: extern Foo foo; The two 'foo' are different. Essentially you have overloading of the symbol 'foo' here. If the first is at file scope, then it is an ODR violation, causing undefined behaviour. A similar situation occurs in the FAQ: one TU contains char foo[9]; and the other contains extern char *foo; which causes undefined behaviour; the latter should have been extern char foo[]; Back to top Victor Bazarov Guest Posted: Thu Jul 28, 2005 1:32 pm    Post subject: Re: error not caught at link time: should it be? Old Wolf wrote: Quote: Victor Bazarov wrote: [email]nzanella (AT) cs (DOT) mun.ca[/email] wrote: I have two .cpp files. The first one contains: Foo *foo; and in the second one contains: extern Foo foo; The two 'foo' are different. Essentially you have overloading of the symbol 'foo' here. If the first is at file scope, then it is an ODR violation, causing undefined behaviour. I am still trying to understand (after an evening of thinking about it) why it would be an ODR violation. The first statement is a declaration and a *definition* of a pointer, if it's at the namespace level; it has static storage duration and as such is zero-initialised. So, it's a null pointer to Foo. Are you saying it's defined elsewhere again? The second statement is a declaration of a global object of type Foo defined elsewhere. If there is an ODR violation, it's the second one, the object, who isn't defined at all. And it only is an error if the object is _used_, which we haven't been able to see. Quote: A similar situation occurs in the FAQ: one TU contains char foo[9]; and the other contains extern char *foo; which causes undefined behaviour; the latter should have been extern char foo[]; Again, only if the name is used. V Back to top msalters Guest Posted: Thu Jul 28, 2005 4:56 pm    Post subject: Re: error not caught at link time: should it be? Victor Bazarov schreef: Quote: Linking is not part of the language specification. Of course it is. It's the part in which Translation Units are combined. Like any other part of the standard, there is no implementation detail mandated, but the phase must be there in some form. In particular, a _linker_ is not part of the specs, if the compiler can do it. Same as the preprocessor: doesn't have to be a standalone tool, but the action must be performed. HTH, Michiel Salters Back to top Old Wolf Guest Posted: Thu Jul 28, 2005 9:26 pm    Post subject: Re: error not caught at link time: should it be? Victor Bazarov wrote: Quote: Old Wolf wrote: Victor Bazarov wrote: [email]nzanella (AT) cs (DOT) mun.ca[/email] wrote: I have two .cpp files. The first one contains: Foo *foo; and in the second one contains: extern Foo foo; The two 'foo' are different. Essentially you have overloading of the symbol 'foo' here. If the first is at file scope, then it is an ODR violation, causing undefined behaviour. I am still trying to understand (after an evening of thinking about it) why it would be an ODR violation. The first statement is a declaration and a *definition* of a pointer, if it's at the namespace level; it has static storage duration and as such is zero-initialised. So, it's a null pointer to Foo. Are you saying it's defined elsewhere again? The second statement is a declaration of a global object of type Foo defined elsewhere. If there is an ODR violation, it's the second one, the object, who isn't defined at all. And it only is an error if the object is _used_, which we haven't been able to see. Yes, I think ODR violation would be the wrong term. I can't seem to find relevant text in the Standard that deals with this issue though. Quote: A similar situation occurs in the FAQ: one TU contains char foo[9]; and the other contains extern char *foo; which causes undefined behaviour; the latter should have been extern char foo[]; Again, only if the name is used. OK. 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