C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  When is returning a pointer to a stack-allocated variable ok          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Kenneth Kasajian Guest Posted: Sun Mar 20, 2005 11:42 pm    Post subject: When is returning a pointer to a stack-allocated variable ok example code: char *r = get(); char* get() { char buf[16]; .... return buf } On the Microsoft compiler I'm using, it only warns me. Anyone know the reason why C does not make this a compiler error? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jack Klein Guest Posted: Mon Mar 21, 2005 7:02 am    Post subject: Re: When is returning a pointer to a stack-allocated variabl On 20 Mar 2005 18:42:19 -0500, "Kenneth Kasajian" <ken (AT) kasajian (DOT) com> wrote in comp.lang.c++.moderated: Quote: example code: char *r = get(); char* get() { char buf[16]; .... return buf } On the Microsoft compiler I'm using, it only warns me. Anyone know the reason why C does not make this a compiler error? If your question is about C, you should ask it in comp.lang.c or comp.lang.c moderated. As to why C++ does not make it an error, there is no problem unless the caller actually does something, pretty much anything at all, with the returned value. On a higher level, this is fairly straight forward example of undefined behavior. A compiler is never required to issue a diagnostic for undefined behavior. It is always easy to produce simple code snippets where undefined behavior is right out in the open and easy to see. But there are many, many more ways to produce undefined behavior in a program that are much more difficult for a human being to see, and can't actually be identified until the code is executed. Most compilers, probably including yours, have options to issue warning messages that are not required by the language standard. Have you tried this? -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Mar 21, 2005 9:53 pm    Post subject: Re: When is returning a pointer to a stack-allocated variabl Kenneth Kasajian wrote: Quote: example code: char *r = get(); char* get() { char buf[16]; .... return buf } On the Microsoft compiler I'm using, it only warns me. Anyone know the reason why C does not make this a compiler error? Because it's not easily detectable in the general case. Consider something like: char* get() { char buffer[ L_tmpnam ] ; return tmpnam( buffer ) ; } Compilers are free to warn in the cases they detect, but requiring an error would require compilers to be able to "look into" functions in separate compilation units, possibly even functions for which no source code is available. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Mon Mar 21, 2005 9:55 pm    Post subject: Re: When is returning a pointer to a stack-allocated variabl Kenneth Kasajian wrote: Quote: On the Microsoft compiler I'm using, it only warns me. Anyone know the reason why C does not make this a compiler error? Because the language designers didn't want to add the overhead to make the compiler keep track of what memory returned pointers are allocated from. It's fairly obvious here but if you were to take buf's address and stick it in a pointer and do some operations with it and return it, then you've probably had to add some special overhead to determine you're returning a pointer to memory that's about to become invalid. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Mon Mar 21, 2005 10:07 pm    Post subject: Re: When is returning a pointer to a stack-allocated variabl Kenneth Kasajian wrote: Quote: example code: char *r = get(); char* get() { char buf[16]; .... return buf } On the Microsoft compiler I'm using, it only warns me. Anyone know the reason why C does not make this a compiler error? Keep in mind that this is the easy case to recognize. There are harder ones: char *f(char *ptr); char *get() { char buf[16]; return f(ptr); } Does this version of get return a pointer to a local object? -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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