C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  const function?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message franco ziade Guest Posted: Fri Feb 25, 2005 2:12 pm    Post subject: const function? what does this declaration mean in C++ void func_name() const { ... } is this a const function ? what are the implications? thanks Back to top Tim Love Guest Posted: Fri Feb 25, 2005 2:20 pm    Post subject: Re: const function? "franco ziade" <fziade (AT) videotron (DOT) ca> writes: Quote: what does this declaration mean in C++ void func_name() const { ... } is this a const function ? what are the implications? http://www-h.eng.cam.ac.uk/help/tpl/languages/C++/argsC++.html may help. Back to top Peter Koch Larsen Guest Posted: Fri Feb 25, 2005 2:34 pm    Post subject: Re: const function? "franco ziade" <fziade (AT) videotron (DOT) ca> skrev i en meddelelse news:6qGTd.48193$6U2.948612 (AT) weber (DOT) videotron.net... Quote: what does this declaration mean in C++ void func_name() const { ... } is this a const function ? what are the implications? thanks This does not compile. Probably from a textbook where "..." intends to mean that the body is left out. /Peter Back to top Gianni Mariani Guest Posted: Fri Feb 25, 2005 3:55 pm    Post subject: Re: const function? franco ziade wrote: Quote: what does this declaration mean in C++ void func_name() const { ... } is this a const function ? It means that func_name() does not change *this; You can think of the "this" pointer being an invisible paramter to the function. void func_name( T * this ) (no const) void func_name( const T * this ) (const) Quote: what are the implications? You can use this to discriminate between a const function call and a non-const function call. i.e. struct A { void f () const; void f(); }; int main() { A a; const A& ca = a; a.f(); // calls void f() ac.f();// calls void f () const; } Quote: thanks Back to top Chris Jefferson Guest Posted: Fri Feb 25, 2005 4:57 pm    Post subject: Re: const function? franco ziade wrote: Quote: what does this declaration mean in C++ void func_name() const { ... } The const only really makes sense in member functions of classes. It means that this class won't alter the state of the class. You should try to mark all functions const which are const :) For example, consider: class S { int i; public: int get() const { return i; } void put(int j) { i=j; } }; notice that I made get() const as it doesn't effect the state of the class. Put is not const (and if you tried making it const it wouldn't compile) Now consider this function void print(const S& s) { std::cout << s.get(); } If get() wasn't const, this wouldn't compile, you you would be calling a non-const member function on a const object. One particular point of note is that you should be sure to make ==, !=, <, and the other relational operators on classes const (assuming of course they are), as many of the implementations of the standard algorithms assume (and I believe they are allowed to assume) they can put a const on things without effecting anything. One final point. You can do: public S { int i; public: int get() const {return i;} int get() {i=0; return i;} }; ie you can define the const and non-const version of a function differently, and give them different meanings. You DON'T have to do this just so non-const objects have a function (the const version can still be used on non-const Ss). Now different things will happen depending on is some instance of S is const or not. Of course I've only ever seen someone write this once when they weren't just messing about, and I was forced to kill them and bury them under the patio so no other such code could ever escape. Don't make me do the same thing to you! Chris Back to top Gianni Mariani Guest Posted: Fri Feb 25, 2005 5:13 pm    Post subject: Re: const function? Chris Jefferson wrote: .... Of course I've only ever seen someone write this once when Quote: they weren't just messing about, and I was forced to kill them and bury them under the patio so no other such code could ever escape. Don't make me do the same thing to you! I'm running... :) How about this: class Buffer { char * buf; public: Buffer( char * buf ) : buf( buf ) { } char * Get() { return buf; } const char * Get() const { return buf; } }; Sometimes (this is a simple case) you do want to do somthing different. const containers usually want to do somthing very different with accessors. Back to top Shezan Baig Guest Posted: Sat Feb 26, 2005 3:20 am    Post subject: Re: const function? Peter Koch Larsen wrote: Quote: "franco ziade" <fziade (AT) videotron (DOT) ca> skrev i en meddelelse news:6qGTd.48193$6U2.948612 (AT) weber (DOT) videotron.net... what does this declaration mean in C++ void func_name() const { ... } is this a const function ? what are the implications? thanks This does not compile. Probably from a textbook where "..." intends to mean that the body is left out. /Peter Thanks for pointing that out, Peter. :) Back to top Chris Jefferson Guest Posted: Sat Feb 26, 2005 5:07 pm    Post subject: Re: const function? Gianni Mariani wrote: Quote: Chris Jefferson wrote: ... Of course I've only ever seen someone write this once when they weren't just messing about, and I was forced to kill them and bury them under the patio so no other such code could ever escape. Don't make me do the same thing to you! I'm running... :) How about this: class Buffer { char * buf; public: Buffer( char * buf ) : buf( buf ) { } char * Get() { return buf; } const char * Get() const { return buf; } }; Sometimes (this is a simple case) you do want to do somthing different. const containers usually want to do somthing very different with accessors. *slaps forehead*.. thats what I get for ever saying "Don't do X in C++" for any X :) Yes of course, I should have said (thanks!) that you can overload functions differently to preserve const correctness, and in fact all the standard contaners do that... 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