C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How to pass various structures to function?          C++Talk.NET Forum Index -> C Language View previous topic :: View next topic   Author Message Fred Guest Posted: Thu May 03, 2007 8:04 am    Post subject: How to pass various structures to function? Is it possible to create a function which will take any number of structures as an argument? For example, what if I wanted depending on the circumstances to pass a structure named astruct, or bstruct, or cstruct, to the function? Sort of like this: myfunc(char * str, struct astruct *as) From googling, it always appears that one specific structure is passed to the function. -TIA Back to top Dave Vandervies Guest Posted: Thu May 03, 2007 8:04 am    Post subject: Re: How to pass various structures to function? In article <rv2dnaKFYbbIzKTbnZ2dnUVZ_uuqnZ2d (AT) comcast (DOT) com>, Fred <itfred (AT) cdw (DOT) com> wrote: Quote: Is it possible to create a function which will take any number of structures as an argument? For example, what if I wanted depending on the circumstances to pass a structure named astruct, or bstruct, or cstruct, to the function? What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". dave -- Dave Vandervies dj3vande (AT) csclub (DOT) uwaterloo.ca There was a twit named Ross St John, who everyone wished would be gone. He called himself Cass, We plonked him en-masse, And let out a c-l-c yawn. --Dann Corbit in comp.lang.c Back to top Martin Ambuhl Guest Posted: Thu May 03, 2007 8:12 am    Post subject: Re: How to pass various structures to function? Fred wrote: Quote: Is it possible to create a function which will take any number of structures as an argument? For example, what if I wanted depending on the circumstances to pass a structure named astruct, or bstruct, or cstruct, to the function? Sort of like this: myfunc(char * str, struct astruct *as) From googling, it always appears that one specific structure is passed to the function. Use the facilities of <stdarg.h>. Remember that you need at least one named argument after which the variable argument list occurs. In your case the poorly-named char *str could be used to pass information about the number and types of arguments, as does the format string for the printf() and scanf() families. Your example uses pointers-to-structs which is a good idea if you are mixing differently defined struct arguments as well as changing their number. I would, however, suggest that you start by learning how to use variable argument lists with the arguments being drawn from the fundamental integer and floating types before moving on to more complex arguments and argument lists. Back to top Fred Guest Posted: Thu May 03, 2007 8:31 am    Post subject: Re: How to pass various structures to function? On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: Quote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". dave I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. Back to top Chris Johnson Guest Posted: Thu May 03, 2007 8:56 am    Post subject: Re: How to pass various structures to function? On May 2, 10:04 pm, Fred <itf...@cdw.com> wrote: Quote: Is it possible to create a function which will take any number of structures as an argument? For example, what if I wanted depending on the circumstances to pass a structure named astruct, or bstruct, or cstruct, to the function? Sort of like this: myfunc(char * str, struct astruct *as) From googling, it always appears that one specific structure is passed to the function. -TIA A void * argument will accept a pointer to any data type. However, it could just as easily be passed a char[] or an int as a struct, so if you want to catch those possibilities, this wouldn't work for you. And there's no way to retrieve the original type directly -- it could be done with an extra parameter and selection logic. Back to top Eric Sosman Guest Posted: Thu May 03, 2007 9:11 am    Post subject: Re: How to pass various structures to function? Fred wrote: Quote: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. If the function doesn't know what the struct looks like, how will it know where to search? For example, let's say you have two structs struct a { int whiz; char *name; }; struct b { char *this, *that; double whiz; } .... and pretend there's a magical way to get the function to accept either of them[*]: int myfunc(const *key, struct a_or_b *sptr) { ... } Knowing only that its second argument points to a struct a or to a struct b, but not knowing which, what exactly are you planning to have myfunc() do? [*] Actually, it's not so magical: There's a way to pass a "pointer to anything at all" to a function, by using a function parameter of type `void*'. But this leaves you in exactly the same hole: You know that the argument points at something, but you don't know what kind of a something it points at. Unless you can deduce the nature of the pointed-at thing from some other piece of information, the things you can do with an "opaque" type are fairly limited. -- Eric Sosman esosman@acm-dot-org.invalid Back to top Flash Gordon Guest Posted: Thu May 03, 2007 9:11 am    Post subject: Re: How to pass various structures to function? Richard Heathfield wrote, On 03/05/07 05:33: Quote: Fred said: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. How will the function know what kind of structure you're passing it? It will need this information, in order to know how to conduct the search. Unless the key to be searched on is a common initial sequence to all the structure types of interest. Without knowing more we can't say what the best solution is, that much is certain. -- Flash Gordon Back to top Richard Heathfield Guest Posted: Thu May 03, 2007 9:11 am    Post subject: Re: How to pass various structures to function? Fred said: Quote: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. How will the function know what kind of structure you're passing it? It will need this information, in order to know how to conduct the search. -- Richard Heathfield "Usenet is a strange place" - dmr 29/7/1999 http://www.cpax.org.uk email: rjh at the above domain, - www. Back to top Nick Keighley Guest Posted: Thu May 03, 2007 9:11 am    Post subject: Re: How to pass various structures to function? On 3 May, 04:31, Fred <itf...@cdw.com> wrote: Quote: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". dave I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. how will your function know where to look for the data? What do the structs look like? What does the data look like? -- Nick Keighley Back to top Joe Estock Guest Posted: Fri May 04, 2007 5:39 am    Post subject: Re: How to pass various structures to function? Eric Sosman wrote: Quote: Fred wrote: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. If the function doesn't know what the struct looks like, how will it know where to search? For example, let's say you have two structs #define TYPE_A 1 #define TYPE_B 2 .... #define TYPE_Z 26 Quote: struct a { int whiz; char *name; }; struct b { char *this, *that; double whiz; } struct a { int type; int whiz; char *name; }; struct b { int type; char *this, *that; double whiz; }; .... struct z { int type; float p; char *bar; int foo; }; Quote: ... and pretend there's a magical way to get the function to accept either of them[*]: int myfunc(const *key, struct a_or_b *sptr) { ... } int myfunc(void *key, void *sptr) { switch (sptr->type) { case TYPE_A: /* logic here */ break; ... } ... } Be forewarned, however, that I'm not sure if "sptr->type" is legal. It may be better to pass the structure type as an additional parameter instead of making it a member of each structure. The function would be (obviously) huge depending on the number of structure's however it may be more maintainable than several functions which do (mostly) the same thing. Again this all depends on the implementation and personally I would prefer the different function approach whenever possible. Quote: Knowing only that its second argument points to a struct a or to a struct b, but not knowing which, what exactly are you planning to have myfunc() do? [*] Actually, it's not so magical: There's a way to pass a "pointer to anything at all" to a function, by using a function parameter of type `void*'. But this leaves you in exactly the same hole: You know that the argument points at something, but you don't know what kind of a something it points at. Unless you can deduce the nature of the pointed-at thing from some other piece of information, the things you can do with an "opaque" type are fairly limited. Back to top Jack Klein Guest Posted: Fri May 04, 2007 7:32 am    Post subject: Re: How to pass various structures to function? On Thu, 03 May 2007 19:39:51 -0500, Joe Estock <jestock (AT) NOSPAMnutextonline (DOT) com> wrote in comp.lang.c: Quote: Eric Sosman wrote: Fred wrote: On Thu, 03 May 2007 03:11:04 +0000, Dave Vandervies wrote: What are you really trying to do? I'm having trouble coming up with a case where the Right Answer is something other than "write a different function for each struct", possibly in the degenerate case of "use a single struct for all of the options". I'm trying to search for data in a structure, and want to have 1 function that I can pass multiple structures to. If the function doesn't know what the struct looks like, how will it know where to search? For example, let's say you have two structs #define TYPE_A 1 #define TYPE_B 2 ... #define TYPE_Z 26 Ugh, no, that's hideous and hard to maintain. Instead: enum data_type { TYPE_A, TYPE_B, /* etc. */ TYPE_Z }; Quote: struct a { int whiz; char *name; }; struct b { char *this, *that; double whiz; } struct a { int type; int whiz; char *name; }; struct b { int type; char *this, *that; double whiz; }; ... struct z { int type; float p; char *bar; int foo; }; No, let's keep it legal, defined C. Do this with Eric's structure definitions: struct data { enum data_type type; union { struct a a; struct b b; /* etc. */ } }; Quote: ... and pretend there's a magical way to get the function to accept either of them[*]: int myfunc(const *key, struct a_or_b *sptr) { ... } int myfunc(void *key, void *sptr) { switch (sptr->type) { case TYPE_A: /* logic here */ break; ... } ... } Be forewarned, however, that I'm not sure if "sptr->type" is legal. It No, it's not. But with the union approach: int myfunc(struct data *data) { switch (data->type) { case TYPE_A: /* yada, yada, yada */ Quote: may be better to pass the structure type as an additional parameter instead of making it a member of each structure. The function would be (obviously) huge depending on the number of structure's however it may be more maintainable than several functions which do (mostly) the same thing. Again this all depends on the implementation and personally I would prefer the different function approach whenever possible. Knowing only that its second argument points to a struct a or to a struct b, but not knowing which, what exactly are you planning to have myfunc() do? [*] Actually, it's not so magical: There's a way to pass a "pointer to anything at all" to a function, by using a function parameter of type `void*'. But this leaves you in exactly the same hole: You know that the argument points at something, but you don't know what kind of a something it points at. Unless you can deduce the nature of the pointed-at thing from some other piece of information, the things you can do with an "opaque" type are fairly limited. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://c-faq.com/ comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C Language All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.