的std :: for_each的用的boost :: zip_iterator編譯失敗

Question

下面的代碼編譯失敗對我來說：從live example

#include <iostream> 
#include <vector> 

#include <boost/iterator/zip_iterator.hpp> 

typedef boost::tuple<int&, float&> EntryTuple; 

struct zip_func : 
    public std::unary_function<EntryTuple&, void> 
{ 
    void operator()(EntryTuple& t) const 
    { 
    std::cout << t.get<0>() << " " << t.get<1>() << std::endl; 
    } 
}; 


int main() 
{ 

const int N = 5; 
std::vector<int> intVec(N,2); 
std::vector<float> valueVec(N,5.5); 

std::for_each(
boost::make_zip_iterator(
    boost::make_tuple(intVec.begin(), valueVec.begin()) 
), 
boost::make_zip_iterator(
    boost::make_tuple(intVec.end(), valueVec.end()) 
), 
zip_func() 
); 

return 0; 
} 

錯誤消息：

In file included from /usr/include/c++/4.9/algorithm:62:0, from /usr/include/boost/utility/swap.hpp:24, from /usr/include/boost/tuple/detail/tuple_basic.hpp:40, from /usr/include/boost/tuple/tuple.hpp:33, from /usr/include/boost/iterator/zip_iterator.hpp:19, from prog.cpp:4: /usr/include/c++/4.9/bits/stl_algo.h: In instantiation of '_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = boost::zip_iterator >, __gnu_cxx::__normal_iterator >, 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 = zip_func]': prog.cpp:34:1:
required from here /usr/include/c++/4.9/bits/stl_algo.h:3755:14: error: no match for call to '(zip_func) (boost::iterator_facade >, __gnu_cxx::__normal_iterator >, 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> >, boost::tuples::cons >, boost::random_access_traversal_tag, boost::tuples::cons >, int>::reference)' __f(*__first); ^prog.cpp:9:8: note: candidate is: struct zip_func : ^prog.cpp:12:8: note: void zip_func::operator()(EntryTuple&) const void operator()(EntryTuple& t) const ^prog.cpp:12:8: note: no known conversion for argument 1 from 'boost::iterator_facade >, __gnu_cxx::__normal_iterator >, 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> >, boost::tuples::cons >, boost::random_access_traversal_tag, boost::tuples::cons >, int>::reference {aka boost::tuples::cons >}' to 'EntryTuple& {aka boost::tuples::tuple&}'

如果我加入一些const S，它compiles：

typedef boost::tuple<const int&, const float&> EntryTuple; 

struct zip_func : 
    public std::unary_function<const EntryTuple&, void> 
{ 
    void operator()(const EntryTuple& t) const 
    { 
    std::cout << t.get<0>() << " " << t.get<1>() << std::endl; 
    } 
}; 

是什麼原因？

Answer 1

GCC 5.1 produces a more understandable error message:

/usr/local/include/c++/5.1.0/bits/stl_algo.h:3767:5: error: invalid initialization of non-const reference of type 'EntryTuple& {aka boost::tuples::tuple&}' from an rvalue of type 'EntryTuple {aka boost::tuples::tuple}'

所以const是運營商的說法只是必要的：

typedef boost::tuple<int&, float&> EntryTuple; 

struct zip_func : 
    public std::unary_function<const EntryTuple&, void> 
{ 
    void operator()(const EntryTuple& t) const 
    { 
    t.get<0>() = 10; 
    std::cout << t.get<0>() << " " << t.get<1>() << std::endl; 
    } 
}; 

活生生的例子：https://ideone.com/hwC3bb