你如何创建地图的地图？

Question

我试图做到以下几点：

.h 
map<int, map<int,int> > forwardingTable; 

.cpp 
int 
UpdateForwardingTable(int dest, int hop, int cost) 
{ 
    if(forwardingTable.at(dest) != forwardingTable.end()) 
     forwardingTable.at(dest) = make_pair(hop, cost); 
    else 
     forwardingTable.insert(dest, make_pair(hop, cost)); 
} 

，但我得到一百万编译器错误，类似于：

In file included from /usr/include/c++/4.8/map:60:0, 
       from globals.h:25, 
       from rtngnode.h:2, 
       from rtngnode.cpp:1: 
/usr/include/c++/4.8/bits/stl_tree.h:316:5: note: template<class _Val> bool std::operator!=(const std::_Rb_tree_iterator<_Tp>&, const std::_Rb_tree_const_iterator<_Val>&) 
    operator!=(const _Rb_tree_iterator<_Val>& __x, 
    ^
/usr/include/c++/4.8/bits/stl_tree.h:316:5: note: template argument deduction/substitution failed: 
rtngnode.cpp:205:53: note: ‘std::map<int, std::map<int, int, std::less<int> > >::mapped_type {aka std::map<int, int, std::less<int> >}’ is not derived from ‘const std::_Rb_tree_iterator<_Tp>’ 
    if(forwardingTable.at(dest) != forwardingTable.end()) 

难道我做错了什么？这种类型的东西有更好的容器吗？

Answer 1

有拖的问题：

1，make_pair回报pair，不map。

2，at(dest)可能会抛出一个out_of_range异常，请参阅map::at

它应该是：

int 
UpdateForwardingTable(int dest, int hop, int cost) 
{ 
    map<int, map<int,int> >::iterator itr = forwardingTable.find(dest); 
    if(itr != forwardingTable.end()) 
    { 
     itr->second.insert(hop, cost); 
     // forwardingTable.at(dest) = make_pair(hop, cost); 
    } 
    else 
    { 
     map<int, int> obj; 
     obj.insert(hop, const); 
     forwardingTable.insert(dest, obj); 
     // forwardingTable.insert(dest, make_pair(hop, cost)); 
    } 
}