c++ - Usage of this next_combination code

Question

Currently I am trying to generate combinations from a vector that contains some integers. For now I want it to print out all of the combinations that are of length to_generate. I found this code at combination and permutation in C++ that uses std::next_permutation to generate combinations. The next_combination code is as follows:

template<class RandIt, class Compare>
bool next_combination(RandIt first, RandIt mid, RandIt last, Compare comp)
{
    std::sort(mid, last, std::tr1::bind(comp, std::tr1::placeholders::_2
                                        , std::tr1::placeholders::_1));
    return std::next_permutation(first, last, comp);
}

I tried to call it like this:

bool mycomp (int c1, int c2)
{ return (c1)<(c2); }


void test_combos(int to_generate){
    std::vector<int> combo(30);
    for(int i=0;i<30;++i){
        combo.push_back(i);
    }
    while(next_combination<std::vector<int>::iterator,   bool>(combo.begin(),combo.begin()+to_generate,combo.end(),mycomp)){
        for(std::vector<int>::iterator iter = combo.begin(); iter != combo.end() ; ++iter){
            std::cout << *iter << " ";
        }
    }
}

but I get a huge stl garbled error message. Is there anything really obvious going wrong here?

Which headers can I find std::placeholder?

As requested, here is the error message:

$ make
g++ -g -std=c++0x -I /usr/lib/ -c equity.cpp
In file included from /usr/include/c++/4.4/algorithm:62,
                 from misc.hpp:6,
                 from equity.cpp:3:
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘bool std::next_permutation(_BIter, _BIter, _Compare) [with _BIter = __gnu_cxx::__normal_iterator > >, _Compare = bool]’:
misc.hpp:15:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:3655: error: ‘__comp’ cannot be used as a function
/usr/include/c++/4.4/bits/stl_algo.h:3658: error: ‘__comp’ cannot be used as a function
In file included from /usr/include/c++/4.4/functional:70,
                 from /usr/include/c++/4.4/memory:70,
                 from /usr/include/boost/config/no_tr1/memory.hpp:21,
                 from /usr/include/boost/get_pointer.hpp:12,
                 from /usr/include/boost/bind/mem_fn.hpp:25,
                 from /usr/include/boost/mem_fn.hpp:22,
                 from /usr/include/boost/bind/bind.hpp:26,
                 from /usr/include/boost/bind.hpp:22,
                 from misc.hpp:4,
                 from equity.cpp:3:
/usr/include/c++/4.4/tr1_impl/functional: At global scope:
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:124:   instantiated from ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare) [with _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/tr1_impl/functional:195: error: ‘const volatile bool’ is not a class, struct, or union type
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:124:   instantiated from ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare) [with _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/tr1_impl/functional:195: error: ‘volatile bool’ is not a class, struct, or union type
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:124:   instantiated from ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare) [with _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/tr1_impl/functional:195: error: ‘const bool’ is not a class, struct, or union type
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:124:   instantiated from ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare) [with _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/tr1_impl/functional:195: error: ‘bool’ is not a class, struct, or union type
In file included from /usr/include/c++/4.4/algorithm:62,
                 from misc.hpp:6,
                 from equity.cpp:3:
/usr/include/c++/4.4/bits/stl_algo.h: In function ‘const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare) [with _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’:
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/bits/stl_algo.h:124: error: no match for call to ‘(std::_Bind, std::_Placeholder)>) (const int&, const int&)’
/usr/include/c++/4.4/bits/stl_algo.h:125: error: no match for call to ‘(std::_Bind, std::_Placeholder)>) (const int&, const int&)’
/usr/include/c++/4.4/bits/stl_algo.h:127: error: no match for call to ‘(std::_Bind, std::_Placeholder)>) (const int&, const int&)’
/usr/include/c++/4.4/bits/stl_algo.h:131: error: no match for call to ‘(std::_Bind, std::_Placeholder)>) (const int&, const int&)’
/usr/include/c++/4.4/bits/stl_algo.h:133: error: no match for call to ‘(std::_Bind, std::_Placeholder)>) (const int&, const int&)’
In file included from /usr/include/c++/4.4/functional:70,
                 from /usr/include/c++/4.4/memory:70,
                 from /usr/include/boost/config/no_tr1/memory.hpp:21,
                 from /usr/include/boost/get_pointer.hpp:12,
                 from /usr/include/boost/bind/mem_fn.hpp:25,
                 from /usr/include/boost/mem_fn.hpp:22,
                 from /usr/include/boost/bind/bind.hpp:26,
                 from /usr/include/boost/bind.hpp:22,
                 from misc.hpp:4,
                 from equity.cpp:3:
/usr/include/c++/4.4/tr1_impl/functional: At global scope:
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:2230:   instantiated from ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator > >, _Compare = std::_Bind, std::_Placeholder)>]’
misc.hpp:13:   instantiated from ‘bool next_combination(RandIt, RandIt, RandIt, Compare) [with RandIt = __gnu_cxx::__normal_iterator > >, Compare = bool]’
equity.cpp:28:   instantiated from here
/usr/include/c++/4.4/tr1_impl/functional:195: error: ‘const volatile bool’ is not a class, struct, or union type
/usr/include/c++/4.4/tr1_impl/functional: In instantiation of ‘std::_Result_of_impl’:
/usr/include/c++/4.4/tr1_impl/functional:149:   instantiated from ‘std::result_of’
/usr/include/c++/4.4/bits/stl_algo.h:2230:   instantiated from ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, _Tp, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Tp = int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:2301:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator > >, _Size = long int, _Compare = std::_Bind, std::_Placeholder)>]’
/usr/include/c++/4.4/bits/stl_algo.h:5258:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __g

Answer 1

The second template argument shouldn't be bool. You can allow compiller to handle types and simply write:

while(next_combination(combo.begin(), combo.begin() + to_generate, combo.end(), std::less<int>()))
    for(std::vector<int>::iterator iter = combo.begin(); iter != combo.end() ; ++iter)
        std::cout << *iter << " ";

And use spaces - it makes code look better.