Boost :: tuple - присвоение get <>()

Question

Следующие команды не скомпилируются при назначении t.get < 1>().

struct pull_from_memory { 
    pull_from_memory(MemoryBank &m) : m_map(m) {}; 
    void operator()(boost::tuple<string&, string&> &t) { 
     MemoryBank::iterator i; 
     if ((i = m_map.find(t.get<0>())) != m_map.end()) { 
      t.get<1>() = *i; // compilation error 
     } 
    } 
private: 
    MemoryBank &m_map; 
}; 

сообщение об ошибке, FWIW, является:

tom.cpp: In member function 'void pull_from_memory::operator()(boost::tuples::tuple<std::string&, std::string&, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>&)': 
tom.cpp:424: error: no match for 'operator=' in '((boost::tuples::tuple<std::string&, std::string&, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>*)t)->boost::tuples::tuple<std::string&, std::string&, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>::<anonymous>.boost::tuples::cons<HT, TT>::get [with int N = 1, HT = std::string&, TT = boost::tuples::cons<std::string&, boost::tuples::null_type>]() = i.boost::unordered_detail::hash_iterator<A, G>::operator* [with A = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, G = boost::unordered_detail::ungrouped]()' 
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/basic_string.h:510: note: candidates are: std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/basic_string.h:518: note:     std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/basic_string.h:529: note:     std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] 

UPDATE 1: после коррекции для того, что map.find возвращает пару, теперь я получаю сообщение об ошибке связи, по-видимому, связано с инстанцировании шаблон

/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h: In function '_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = boost::zip_iterator<boost::tuples::tuple<__gnu_cxx::__normal_iterator<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >*, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, __gnu_cxx::__normal_iterator<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >*, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> >, _Funct = bind_from_memory]': 
/mxhome/charrison/git/sdcore/libs/CommUtils/test/../include/Block.h:160: instantiated from 'void Block::traverse_record(const D_RecordType&, Function) [with Function = bind_from_memory]' 
tom.cpp:473: instantiated from here 
/usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/bits/stl_algo.h:4200: error: no match for call to '(bind_from_memory) (boost::tuples::cons<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, boost::tuples::cons<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, boost::tuples::null_type> >)' 
tom.cpp:460: note: candidates are: void bind_from_memory::operator()(boost::tuples::tuple<const std::string&, std::string&, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>&) 

Answer 1

Ваша проблема в том, что вы ожидаете *i быть std::string, но это не так. Это (ссылка) std::pair<std::string,std::string>.

Вот как работают итераторы для карт.

Попробуйте вместо этого:

t.get<1>() = i->second; //if you want the value corresponding to the key 

Answer 2

судя по ней, вы пытаетесь присвоить std::pair<std::string const, std::string> к std::string. Это не сработает. Вы хотели это написать?

t.get<1>() = i->first;