unordered_map迭代器打印使用cout

Question

我想使用迭代器输出一个多层unordered_map中的整数，我遇到了麻烦，错误是在代码下面。下面

#include <boost/unordered_map.hpp> 
#include <iostream> 

using namespace std; 

int main() 
{ 
    typedef boost::unordered_map <int, boost::unordered_map<int, int> > _map; 
    _map MAP; 

    boost::unordered_map<int, int>::iterator map_it; 

    MAP[0][0] = 10; 
    map_it = MAP[0].begin(); 

    cout<<*map_it<<endl; 

    return 0; 
} 

错误：

unordered_map_iterator\unordered_map_iterator.cpp||In function 'int main()':| 
unordered_map_iterator\unordered_map_iterator.cpp|16|error: no match for 'operator<<' in 'std::cout << map_it.boost::unordered_detail::hash_iterator<A, G>::operator* [with A = std::allocator<std::pair<const int, int> >, G = boost::unordered_detail::ungrouped]()'| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|108|note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>& (*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|117|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>& (*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|127|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|165|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|169|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|173|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|91|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|180|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|105|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|191|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|200|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|204|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|209|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|213|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|221|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\ostream|225|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>]| 
c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\bits\ostream.tcc|119|note:     std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>]| 
||=== Build finished: 1 errors, 0 warnings ===| 

Answer 1

为了能够做到这一点：

cout<<*map_it<<endl; 

你将不得不超载<<运营商来支持你_map类。
<<设计用于处理标准数据类型，而不是类似您的类的自定义数据类型。
<<不理解定制类的类型_map，因此错误。

所以要用你提到的陈述来做，你必须重载<<接受_map作为参数。

或者，你可以打电话<<带参数也可以理解，它是为实现的，

cout<<map_it->second<<endl; 

这将确保参数传递给<<是能够理解的数据类型，因此它会工作。

Answer 2

boost::unordered_map的迭代器是一个std :: pair。既然你有两个层次，你必须得到适当的迭代这样每个级别：

typedef boost::unordered_map <int, boost::unordered_map<int, int> > _map; 
_map MAP; 

MAP[0][0] = 10; 
_map::iterator map_it = MAP.begin(); 

// map_it is an iterator, where first is the key (0), and 
// second is the value (boost::unordered_map<int, int>) 

boost::unordered_map<int, int>::iterator internal_map_it = map_it->second.begin(); 

// internal_map_it is an iterator, where first is the key (also 0) 
// and second is the value (10) 

cout<< internal_map_it->second <<endl; 

Answer 3

boost::unordered_map::iterator点，一对值，其中第一项是关键，第二个值。所以，这条线

cout << *map_it << endl; 

正试图打印std::pair。

如果你要打印的MAP[0][0]内容使用

cout << map_it->second << endl;