C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  const parameters to template class member functions Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Kenneth Massey Guest Posted: Mon Jul 19, 2004 9:08 pm    Post subject: const parameters to template class member functions I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. Kenneth #include <stdio.h> template<class T> class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } }; int main() { int k; myclass<int*> mc(&k); // T = int* const int* p = &k; // test.cpp: In function `int main()': // test.cpp:25: error: invalid conversion from `const int*' to `int*' // test.cpp:25: error: initializing argument 1 of `int myclass<T>::check(T) printf("%dn",mc.check(p)); return 0; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Tobias Güntner Guest Posted: Tue Jul 20, 2004 7:56 am    Post subject: Re: const parameters to template class member functions Kenneth Massey wrote: Quote: template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } }; I think the problem is the const T y. With T = int* this becomes int*const, i.e. a _constant pointer_ to a _non-const integer_. Since a pointer to a const int cannot be converted into a pointer to non-const (without a type cast, that is ), the compiler generates an error. For example, you could create a second template parameter (one for int* and another for const int*) or you could simply pass an int for T and declare your function as int check(const T*). Regards, Tobias [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Tue Jul 20, 2004 8:14 am    Post subject: Re: const parameters to template class member functions [email]eigenmaster (AT) yahoo (DOT) com[/email] (Kenneth Massey) writes: Quote: I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. If 'T' is a name for 'int *', 'const T' (and 'T const', for that matter) is a name for 'int * const'; i.e. the pointer is const, not the int pointed to. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Tue Jul 20, 2004 6:12 pm    Post subject: Re: const parameters to template class member functions Good morning Kenneth Massey, Kenneth Massey schrieb: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. Kenneth #include <stdio.h template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } }; Please note that any template member functions are not automatically instantiated. You have to distinguish the point of definition of the template (non-standard nomenclature) from the actual point of instantiation (aka POI, just before main in your example). Quote: int main() { int k; myclass const int* p = &k; // test.cpp: In function `int main()': // test.cpp:25: error: invalid conversion from `const int*' to `int*' // test.cpp:25: error: initializing argument 1 of `int myclass<T>::check(T) printf("%dn",mc.check(p)); return 0; } Since myclass<int*> does contain an int* member, you are effectivly trying to perform a code similar to: const int* p = &k; int* x = p; in your call of ms.check(). This obviously leads to a violation of constness contraints. Hope that helps, Daniel Krügler [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Maxim Yegorushkin Guest Posted: Tue Jul 20, 2004 6:26 pm    Post subject: Re: const parameters to template class member functions Thomas Maeder <maeder (AT) glue (DOT) ch> wrote: Quote: eigenmaster (AT) yahoo (DOT) com (Kenneth Massey) writes: I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. If 'T' is a name for 'int *', 'const T' (and 'T const', for that matter) is a name for 'int * const'; i.e. the pointer is const, not the int pointed to. I think putting cv-qualifier before the type is a common source of confusion. Had the OP put const after the type he would likely have not made that error. Is not it a good motivation for always putting cv-qualifiers after the type? -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Tue Jul 20, 2004 9:35 pm    Post subject: Re: const parameters to template class member functions [Not posted to comp.std.c++ as it is off-topic there.] Kenneth Massey wrote: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. snip When T is int *, const T is int * const and not const int *. See <http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.5>. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Tue Jul 20, 2004 9:57 pm    Post subject: Re: const parameters to template class member functions "Maxim Yegorushkin" <e-maxim (AT) yandex (DOT) ru> writes: Quote: If 'T' is a name for 'int *', 'const T' (and 'T const', for that matter) is a name for 'int * const'; i.e. the pointer is const, not the int pointed to. I think putting cv-qualifier before the type is a common source of confusion. Had the OP put const after the type he would likely have not made that error. Is not it a good motivation for always putting cv-qualifiers after the type? I agree. But as long as (otherwise) excellent C++ texts such as "The C++ Programming Language" and the ISO C++ Standard write const on the left, we have to be able to deal with that even if we ourselves write const on the right. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top dietmar_kuehl@yahoo.com Guest Posted: Wed Jul 21, 2004 11:55 am    Post subject: Re: const parameters to template class member functions Hi, Kenneth Massey wrote: Quote: However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. This is essentially a problem of 'const' placement - or, at least, the problem would have been immediately obvious if you had placed the 'const' to the right rather than to the left (obviously, there is just one right 'const' placement : 'const int*' is equivalent to 'int const*'. ... and 'const T' is equivalent to 'T const'. If you now replace 'T' by 'int*' you do not get 'const int*' but rather 'int* const': these two types are obviously not identical. I think there shall be warnings about the non-right 'const' placement (except in 'extern "C"' sections - it is my understanding that C only support the non-right placement). -- <mailto:dietmar_kuehl (AT) yahoo (DOT) com> <http://www.dietmar-kuehl.de/> <http://www.contendix.com> - Software Development & Consulting [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gareth Stockwell Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions [email]eigenmaster (AT) yahoo (DOT) com[/email] (Kenneth Massey) wrote in message news:<ac2b2aa1.0407190619.72a68f9d (AT) posting (DOT) google.com>... Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. template<class T class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } }; Kenneth, I think that what is happening here is that, given the signature int check(const T); with T=int*, upon instantiation the compiler is generating int check(T* const); rather than (as you want) int check(const T*); I can think of two solutions: 1. If you know that T will always be some kind of pointer, then re-write the template such that T is the value type, rather than the pointer type (so here, T=int). Then you can write the signature of check directly: int check(const T*); 2. More generally, I think you can get around the problem with a bit of template metaprogramming. Basically you need a structure which converts T to const T and gets the asterix in the correct place... template struct MakeConst { typedef const Type result; }; template<typename Type> struct MakeConst<Type*> { typedef const Type* result; }; Now you can write: int check(typename MakeConst<T>::result); Gareth [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Tokyo Tomy Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions [email]eigenmaster (AT) yahoo (DOT) com[/email] (Kenneth Massey) wrote in message news:<ac2b2aa1.0407190619.72a68f9d (AT) posting (DOT) google.com>... Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. Kenneth #include <stdio.h template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } }; int main() { int k; myclass const int* p = &k; // test.cpp: In function `int main()': // test.cpp:25: error: invalid conversion from `const int*' to `int*' // test.cpp:25: error: initializing argument 1 of `int myclass<T>::check(T) printf("%dn",mc.check(p)); return 0; } My MSVC++6 also issured the same error just as above. The interpretation of the error seems to be easy as follows. 1. "const T y" in the check argument is converted to "T const y", just like, for example,"const int i" is equivalent to "int const i". 2. T = int*, so the check argument becomes "int* const y" (const pointer to int). After I replaced "const int* p = &k;" with "int* const p = &k;", the code compiled with my MSVC++6. However, I just wander you can agree with this behavior. How can you do if you want the check argument to be "const int*" to accept the original p in the above code. Following is one solution, but T x becomes pointer to const int as an (probably) undesirable side effect. #include <stdio.h> template<class T> class myclass { T x; public: myclass(T x0)(x0) {} int check(T y) { return x==y; } }; int main() { int k; myclass<const int*> mc(&k); // T = const int* const int* p = &k; // same as in the orginal code printf("%dn",mc.check(p)); return 0; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Jozef Saniga Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Hi, Kenneth Massey wrote: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. The compiler does not ignore the const. The const is there just not where you expect it to be. Quote: Kenneth #include <stdio.h template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } When myclass is instantiated for int* check becomes: int check (int* const y) { return x == y; } since 'const T y' says that y is of type T and y is const. So y is of type int* and it is const. Note that y is const not the int y points to. Try to compile this piece of code that has a very similar problem: int main () { typedef int* T; const T y; y = 1; return 0; } Quote: }; int main() { int k; myclass const int* p = &k; // test.cpp: In function `int main()': // test.cpp:25: error: invalid conversion from `const int*' to `int*' // test.cpp:25: error: initializing argument 1 of `int myclass<T>::check(T) printf("%dn",mc.check(p)); return 0; } Jozef [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Ulrich Eckhardt Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Kenneth Massey wrote: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. #include template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } ^^^ *ouch* Don't do that in examples, your real code might have valid reasons to do so, but not examples. Quote: // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } Yes, but that's something different: assuming T='int*', you have one 'int const*' and one 'int * const'. If that makes no sense to you, just read the FAQ, it's explained there. Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top vze2hb3b@verizon.net Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions In comp.lang.c++.moderated Kenneth Massey <eigenmaster (AT) yahoo (DOT) com> wrote: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. Kenneth #include <stdio.h template class myclass { T x; public: myclass(T x0)(x0) {} int check(const T y) { return x==y; } // compiles fine with this line: T is explicitly replaced by int* // int check(const int* y) { return x==y; } }; int main() { int k; myclass const int* p = &k; Hi! The problem is that T is int *, so const T translates to int * const (constant pointer), not to const int * (pointer to constant integer). If you define p as int * const p = &k; then the compiler should be happy. Best, Alexander Quote: // test.cpp: In function `int main()': // test.cpp:25: error: invalid conversion from `const int*' to `int*' // test.cpp:25: error: initializing argument 1 of `int myclass<T>::check(T) printf("%dn",mc.check(p)); return 0; } -- Aleksandr Morgulis [email]aleksandr.morgulis (AT) verizon (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Jeffrey Lau Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions The problem is the placement of the const in the declaration of 'p': Quote: int k; myclass<int*> mc(&k); // T = int* const int* p = &k; If 'T' is 'int*', then 'const T' is 'int* const'; the constant version of 'int*'. This is perhaps why most template code I have seen will use 'T const' instead of 'const T'. I have also adopted this declaration style for all const variables in code: int const constantInt(5); char const* const constantPointerToConstantChar("Blah"); This allows you to read from right to left exactly the type of the variable. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Alberto Barbati Guest Posted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Kenneth Massey wrote: Quote: I have run into a peculiar problem, in which the following sample code does not compile (gcc 3.3). I have a template class with a member function that should take a const version of the template parameter. However, when I try to use the class with (T=int*), the compiler seems to ignore the const and gives an error if I pass a const int*. Any help would be much appreciated. If you write typedef int* T; Then "const T" is "int* const" (a constant pointer to an int) which is very different from "const int*" (a pointer to a constant int). If you analyse your code you will realize that it is a more complex case of this simple issue. Alberto [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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