使用 cout 打印 unordered_map 迭代器

Question

我正在尝试使用迭代器在多层 unordered_map 中输出整数，但遇到了问题，错误位于代码下方。

#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;
}

错误如下：

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;

您必须重载 << 运算符来支持您的类 _map。
<< 旨在使用标准数据类型，而不是像您的类那样的自定义数据类型。
<< 不理解自定义类类型 _map，因此出现错误。

因此，要使用您提到的语句执行此操作，您必须重载 << 以接受 _map 作为参数。

或者，您可以使用它可以理解的参数及其实现的参数来调用 <<，

cout<<map_it->second<<endl;

这确保传递给 << 的参数是它可以理解的数据类型，因此它可以工作。

Answer 2

boost::unordered_map 的迭代器是一个 std::pair。由于您有两个级别，因此您必须为每个级别获取正确的迭代器，如下所示：

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 指向一对值，第一个是键，第二个是值。因此，这一行

cout << *map_it << endl;

尝试打印一个 std::pair。

如果要打印 MAP[0][0] 的内容，请使用

cout << map_it->second << endl;