C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  const          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Massimiliano Alberti Guest Posted: Wed Feb 25, 2004 6:25 pm    Post subject: const Sometimes I've seen something like: const char* ReturnNull() { return ""; } Is this legal? Why? Is this because the string is a constant? And... Is this legal: const int* ReturnZero() { const int i = 0; return &i; } Back to top Massimiliano Alberti Guest Posted: Wed Feb 25, 2004 6:29 pm    Post subject: Re: const Quote: const char* ReturnNull() { return ""; } Wrong example: const char* ReturnNull() { const char *p = ""; return p; } Back to top Ron Natalie Guest Posted: Wed Feb 25, 2004 6:42 pm    Post subject: Re: const "Massimiliano Alberti" <xanatos (AT) geocities (DOT) com> wrote Quote: Sometimes I've seen something like: const char* ReturnNull() { return ""; } This is legal because a string literal is persistant. It's a bunch of const chars at an undetermined location. They are not local to the block. Quote: const int* ReturnZero() { const int i = 0; return &i; } This will compile, but any attempt to use the returned pointer will be problematic as in this case the value is a pointer to an object that doesn't exist anymore. Back to top Rolf Magnus Guest Posted: Wed Feb 25, 2004 8:05 pm    Post subject: Re: const Massimiliano Alberti wrote: Quote: Sometimes I've seen something like: const char* ReturnNull() { return ""; } Is this legal? Yes. Quote: Why? What makes you think it's not? Quote: Is this because the string is a constant? No, it's because it's a literal, and string literals exist through the whole life time of the program. Quote: And... Is this legal: const int* ReturnZero() { const int i = 0; return &i; } No. You're returning a pointer to a local variable that gets destroyed when the function returns. Back to top Leor Zolman Guest Posted: Wed Feb 25, 2004 10:01 pm    Post subject: Re: const On Wed, 25 Feb 2004 13:42:35 -0500, "Ron Natalie" <ron (AT) sensor (DOT) com> wrote: Quote: const int* ReturnZero() { const int i = 0; return &i; } This will compile, but any attempt to use the returned pointer will be problematic as in this case the value is a pointer to an object that doesn't exist anymore. It will compile, but every self-respecting compiler will at least issue a warning about it. If you don't see a warning, consider getting a newer compiler (or tightening up the warning level options). -leor Leor Zolman BD Software [email]leor (AT) bdsoft (DOT) com[/email] www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix C++ users: Download BD Software's free STL Error Message Decryptor at www.bdsoft.com/tools/stlfilt.html Back to top Nick Hounsome Guest Posted: Thu Feb 26, 2004 7:38 am    Post subject: Re: const "Massimiliano Alberti" <xanatos (AT) geocities (DOT) com> wrote Quote: const char* ReturnNull() { return ""; } Wrong example: const char* ReturnNull() { const char *p = ""; return p; } No - his example is fine - yours just wastes a bit of stack space to acheive exactly the same thing. Actually according to the principle of calling it what it is it should be const char* ReturnNull() { return NULL; } or const char* ReturnEmptyString() { return ""; } Quote: Back to top Rolf Magnus Guest Posted: Thu Feb 26, 2004 10:48 am    Post subject: Re: const Nick Hounsome wrote: Quote: "Massimiliano Alberti" <xanatos (AT) geocities (DOT) com> wrote in message news:8Q5%b.21112$gk.900093 (AT) news3 (DOT) tin.it... const char* ReturnNull() { return ""; } Wrong example: const char* ReturnNull() { const char *p = ""; return p; } No - his example is fine - yours just wastes a bit of stack space to acheive exactly the same thing. The "his" and the "yours" you are talking about are the same person. Back to top Roger Leigh Guest Posted: Wed Mar 03, 2004 7:26 pm    Post subject: Re: const "Massimiliano Alberti" <xanatos (AT) geocities (DOT) com> writes: Quote: Sometimes I've seen something like: const char* ReturnNull() { return ""; } Is this legal? Why? Is this because the string is a constant? Yes. [OT: On a UNIX-like OS (e.g. Linux/ELF), this will probably be put in the .rodata (read-only data) section of the executable. This part is mapped into memory with read-only permissions, so if you actually attempt to modify it, you'll get a SEGV.] Quote: And... Is this legal: const int* ReturnZero() { const int i = 0; return &i; } It's "legal" in that it will compile. However, it's wrong because i will be destroyed when it goes out of scope when the function returns. This will leave you with a pointer to a nonexistent object, and probably a crash. -- Roger Leigh Printing on GNU/Linux? http://gimp-print.sourceforge.net/ GPG Public Key: 0x25BFB848. 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