C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  cannot convert char** to const char**          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Brad Moore Guest Posted: Wed Oct 13, 2004 5:13 am    Post subject: cannot convert char** to const char** Hey all, I'm getting the following compiler error from my code. I was wondering if anyone could help me understand the concept behind it (I actually did try and compile this degenerate example). int foo(const char* argv[]) { return 0; } int main(int argc, char* argv[]) { foo(argv); return 0; } Error: cannot convert parameter 1 from char** to const char** I tried using an explicit type cast, and it worked. However, I'd like to know the difference between the above code and the following (which did compile): int goo(const int x) { return 0; } int main(int argc, char* argv[]) { int x; goo(x); } I've been reading that using const in your args list is a useful way of noting that an argument isn't changed. If foo(const char* argv[]) is bad form in some way, please let me know. To reiterate, I did get the above code to work. I was just wondering what the good programming practice is (and the concept behind it). Thanks for your expertise and time, -Brad Back to top Gianni Mariani Guest Posted: Wed Oct 13, 2004 6:07 am    Post subject: Re: cannot convert char** to const char** Brad Moore wrote: Quote: Hey all, I'm getting the following compiler error from my code. I was wondering if anyone could help me understand the concept behind it (I actually did try and compile this degenerate example). int foo(const char* argv[]) { return 0; } int main(int argc, char* argv[]) { foo(argv); return 0; } Error: cannot convert parameter 1 from char** to const char** You need to consider the meaning of "**" - in this case it means a pointer to a pointer to a [const] char. This is a double level of indirection which allows the pointer to be modified in one place and the object to be modified in another. If the conversion from char** to const char** was allowed, then you could write this code. const char * A[2] = { "A", "B" }; char * B[2]; int main() { char ** b = B; const char ** a = b; // illegal //const char ** a = ( const char ** ) b; // BAD work-around (1) //const char * const * a = b; // better option (2) a[0] = A[0]; // illegal if you use (2) ok if you use (1) b[0][0] = 'X'; // const violation - big probs if you use (1) } This would implicity allow const objects to be accessed in a non-const way. Quote: I tried using an explicit type cast, and it worked. However, I'd like to know the difference between the above code and the following (which did compile): int goo(const int x) { return 0; } int main(int argc, char* argv[]) { int x; goo(x); } This is a very different issue, the earlier one had a level of indirection, this one has none. In this case x is "copied" to a const value parameter. Quote: I've been reading that using const in your args list is a useful way of noting that an argument isn't changed. If foo(const char* argv[]) is bad form in some way, please let me know. To reiterate, I did get the above code to work. I was just wondering what the good programming practice is (and the concept behind it). What you did was probably not what you wanted to do. Back to top John Harrison Guest Posted: Wed Oct 13, 2004 6:20 am    Post subject: Re: cannot convert char** to const char** "Brad Moore" <mooreb (AT) cis (DOT) ohio-state.edu> wrote Quote: Hey all, I'm getting the following compiler error from my code. I was wondering if anyone could help me understand the concept behind it (I actually did try and compile this degenerate example). int foo(const char* argv[]) { return 0; } int main(int argc, char* argv[]) { foo(argv); return 0; } Error: cannot convert parameter 1 from char** to const char** I tried using an explicit type cast, and it worked. However, I'd like to know the difference between the above code and the following (which did compile): int goo(const int x) { return 0; } int main(int argc, char* argv[]) { int x; goo(x); } I've been reading that using const in your args list is a useful way of noting that an argument isn't changed. If foo(const char* argv[]) is bad form in some way, please let me know. OK, first thing to note is that foo(char* argv[]) is just another way of writing foo(char** argv). C and C++ allows you to use array notation in function parameters but its a lie. The compiler always converts to array to a pointer. So its better to use the pointer notation, its more truthful. C++ says its ok to convert char* to const char* because this it harmless. char* is a pointer to char, and const char* is a pointer to constant char (note its the char that is constant not the pointer). Adding a const can't do any harm. So sometimes people think that char** should be convertible to const char** but this conversion is most definitely not harmless. Here's why const char a = 'a'; // this is a constant it should never change char* x; const char** y = &x; // in reality this is illegal *y = &a; // now x points to a *x = 'b'; // now we have modified a By allowing a conversion from char** to const char** we've managed to write a sequence of statements which have modified a constant. This is obviously a bad thing john Back to top Brad Moore Guest Posted: Wed Oct 13, 2004 3:33 pm    Post subject: Re: cannot convert char** to const char** I want to thank everyone who responded. I understand the error in my reasoning. Thanks, -Brad Back to top Default User Guest Posted: Wed Oct 13, 2004 4:21 pm    Post subject: Re: cannot convert char** to const char** John Harrison wrote: Quote: OK, first thing to note is that foo(char* argv[]) is just another way of writing foo(char** argv). This is true. Quote: C and C++ allows you to use array notation in function parameters but its a lie. The compiler always converts to array to a pointer. So its better to use the pointer notation, its more truthful. This is your personal opinion, not an objective fact. Many people prefer the other notation, and their programs are none the less for it. Brian Rodenborn Back to top John Harrison Guest Posted: Wed Oct 13, 2004 6:51 pm    Post subject: Re: cannot convert char** to const char** "Default User" <first.last (AT) boeing (DOT) com.invalid> wrote Quote: John Harrison wrote: OK, first thing to note is that foo(char* argv[]) is just another way of writing foo(char** argv). This is true. C and C++ allows you to use array notation in function parameters but its a lie. The compiler always converts to array to a pointer. So its better to use the pointer notation, its more truthful. This is your personal opinion, not an objective fact. Many people prefer the other notation, and their programs are none the less for it. OK, well perhaps we can at least agree that people should be aware of the real meaning of the array notation in a function parameter. Its my experience in this group that some newbies are not. John Back to top kaikai Guest Posted: Wed Dec 28, 2005 2:43 pm    Post subject: Re: cannot convert char** to const char** I know this post is old enough.(Over one year...) But I do think I have just realize the concept behind it. 'Type' could be converted to 'const Type', as we all know. But to the situation, 'char **' and 'const char**'. NOTICE, the Type here is not just 'char**'. 'const char**' is NOT 'const Type' with Type be 'char**'. Its 'Type *' with Type be 'const char*'. So the problem is actully 'Type1 *' convert to 'Type2 *' where Type1 is 'char*' and Type2 is 'const char*'. We known that pointer types can not convert from each other unless one of them is pointer to void (void*). So it is now clear why 'char **' can not convert to 'const char **'. Was is a tongue twister ? -_-||| kaikai Brad Moore wrotes: Quote: Hey all, I'm getting the following compiler error from my code. I was wondering if anyone could help me understand the concept behind it (I actually did try and compile this degenerate example). int foo(const char* argv[]) { return 0; } int main(int argc, char* argv[]) { foo(argv); return 0; } Error: cannot convert parameter 1 from char** to const char** I tried using an explicit type cast, and it worked. However, I'd like to know the difference between the above code and the following (which did compile): int goo(const int x) { return 0; } int main(int argc, char* argv[]) { int x; goo(x); } I've been reading that using const in your args list is a useful way of noting that an argument isn't changed. If foo(const char* argv[]) is bad form in some way, please let me know. To reiterate, I did get the above code to work. I was just wondering what the good programming practice is (and the concept behind it). Thanks for your expertise and time, -Brad Back to top deane_gavin@hotmail.com Guest Posted: Wed Dec 28, 2005 2:51 pm    Post subject: Re: cannot convert char** to const char** kaikai wrote: Quote: I know this post is old enough.(Over one year...) But I do think I have just realize the concept behind it. 'Type' could be converted to 'const Type', as we all know. But to the situation, 'char **' and 'const char**'. NOTICE, the Type here is not just 'char**'. 'const char**' is NOT 'const Type' with Type be 'char**'. Its 'Type *' with Type be 'const char*'. So the problem is actully 'Type1 *' convert to 'Type2 *' where Type1 is 'char*' and Type2 is 'const char*'. We known that pointer types can not convert from each other unless one of them is pointer to void (void*). So it is now clear why 'char **' can not convert to 'const char **'. Was is a tongue twister ? -_-||| kaikai Brad Moore wrotes: Hey all, I'm getting the following compiler error from my code. I was wondering if anyone could help me understand the concept behind it (I actually did try and compile this degenerate example). int foo(const char* argv[]) { return 0; } int main(int argc, char* argv[]) { foo(argv); return 0; } Error: cannot convert parameter 1 from char** to const char** I tried using an explicit type cast, and it worked. However, I'd like to know the difference between the above code and the following (which did compile): int goo(const int x) { return 0; } int main(int argc, char* argv[]) { int x; goo(x); } I've been reading that using const in your args list is a useful way of noting that an argument isn't changed. If foo(const char* argv[]) is bad form in some way, please let me know. To reiterate, I did get the above code to work. I was just wondering what the good programming practice is (and the concept behind it). Thanks for your expertise and time, -Brad http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.17 Gavin Deane Back to top kaikai Guest Posted: Wed Dec 28, 2005 3:03 pm    Post subject: Re: cannot convert char** to const char** Em..., that example gives us the reason why the convertion is dangerous and I have read it before. To be honest, I did not understand it until I have read the example. But I am trying to explain the concept behind it. kaikai <deane_gavin (AT) hotmail (DOT) com> wrote:1135781494.205578.277190 (AT) g43g2000cwa (DOT) googlegroups.com... Quote: [snip] http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.17 Gavin Deane Back to top kaikai Guest Posted: Wed Dec 28, 2005 3:26 pm    Post subject: Re: cannot convert char** to const char** <deane_gavin (AT) hotmail (DOT) com> ??????:1135781494.205578.277190 (AT) g43g2000cwa (DOT) googlegroups.com... Quote: kaikai wrote: I know this post is old enough.(Over one year...) But I do think I have just realize the concept behind it. [snip] Thanks for your expertise and time, -Brad http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.17 Gavin Deane OMG! I am sorry for my rudeness top-posting.(2 times...) kaikai Back to top Tsubasa Guest Posted: Thu Dec 29, 2005 4:06 am    Post subject: Re: cannot convert char** to const char** /*************************************************************** We known that pointer types can not convert from each other unless one of them is pointer to void (void*) ***************************************************************/ in fact, we could use reinterpret_cast to convert type1* to type2*, for instance, char* ch = new char(''); int** i = NULL; *i = reinterpret_cast<int*>(ch); I could compile this code with VC7.1, BCB5.5 maybe C conversion couldn't convert type1* to type2*, but reinterpret_cast may convert many strange conversion all reply will be approciated 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 (comp.lang.c++) 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.