I got an error while compiling the following code in gcc3.3.2, what's matter? //source comp.cpp //////////////////////////////////////////////// 01 #include <iostream> 02 #include <map> 03 #include <ext/functional> 04 using namespace std; 05 using namespace __gnu_cxx; 06 07 class CPrint 08 { 09 public: 10 CPrint(){}; 11 void operator()(int tvalue) 12 { 13 cout<< tvalue<<endl; 14 } 15 }; 16 17 int main() 18 { 19 map<int, int> mtest; 20 mtest[1]=2; 21 mtest[3]=4; 22 for_each(mtest.begin(), mtest.end(), 23 compose1(CPrint(), select2nd<map<int, int>::value_type>())); //the error line 24 } //////////////////////////////////////////////////////////////// //error msg://////////////////////////////////////////////////// comp.cpp: In instantiation of `__gnu_cxx::unary_compose<CPrint, __gnu_cxx::select2nd<std::pair<const int, int> > >': comp.cpp:23: instantiated from here comp.cpp:23: error: no type named `result_type' in `class CPrint' /usr/local/include/c++/3.3.2/ext/functional:135: error: no type named ` result_type' in `class CPrint' /usr/local/include/c++/3.3.2/bits/stl_algo.h: In function `_Function std::for_each(_InputIter, _InputIter, _Function) [with _InputIter = std::_Rb_tree_iterator<std::pair<const int, int>, std::pair<const int, int>&, std::pair<const int, int>*>, _Function = __gnu_cxx::unary_compose<CPrint, __gnu_cxx::select2nd<std::pair<const int, int> > >]': comp.cpp:23: instantiated from here /usr/local/include/c++/3.3.2/bits/stl_algo.h:157: error: no match for call to `(__gnu_cxx::unary_compose<CPrint, __gnu_cxx::select2nd<std::pair<const int, int> > >) (std::pair<const int, int>&)' why?

You need to provide a couple of typedefs in your functor class for the adaptors to work. The easiest way of doing this is deriving the class from std::unary_function<void, int> (declared in <functional>; the first template argument is the result type and the second the argument type of your operation). Please check out SGI's docs for more info.

Quote:Original post by Sharlin
You need to provide a couple of typedefs in your functor class for the adaptors to work. The easiest way of doing this is deriving the class from std::unary_function<void, int> (declared in <functional>; the first template argument is the result type and the second the argument type of your operation). Please check out SGI's docs for more info.

Thank you. but when I derive class unary_function as following
class CPrint : public unary_function<int, void>

I got another compiling error:

...
comp.cpp:23: instantiated from here
/usr/local/include/c++/3.2.2/ext/functional:136: passing `const CPrint' as
`this' argument of `void CPrint::operator()(int)' discards qualifiers

how to resolve it?

Quote:Original post by sprime

Quote:Original post by Sharlin
You need to provide a couple of typedefs in your functor class for the adaptors to work. The easiest way of doing this is deriving the class from std::unary_function<void, int> (declared in <functional>; the first template argument is the result type and the second the argument type of your operation). Please check out SGI's docs for more info.

Thank you. but when I derive class unary_function as following
class CPrint : public unary_function<int, void>

I got another compiling error:

...
comp.cpp:23: instantiated from here
/usr/local/include/c++/3.2.2/ext/functional:136: passing `const CPrint' as
`this' argument of `void CPrint::operator()(int)' discards qualifiers

how to resolve it?

OK, I find the solution:)
the operation should be declared as const, that's all right!