map中的lower_bound和upper_bound的意思其实很简单，就两句话：
map::lower_bound(key):返回map中第一个大于或等于key的迭代器指针
map::upper_bound(key):返回map中第一个大于key的迭代器指针
所以，理解这两个函数请不要按照字面意义思考太复杂，因为仅仅是不小于（lower_bound）和大于（upper_bound）这么简单。
看两个msdn里的例子
// map_upper_bound.cpp
// compile with: /EHsc
#include <map>
#include <iostream> int main( )
{
using namespace std;
map <int, int> m1;
map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
typedef pair <int, int> Int_Pair; m1.insert ( Int_Pair ( , ) );
m1.insert ( Int_Pair ( , ) );
m1.insert ( Int_Pair ( , ) );
// 返回m1中第一个key值大于2的元素的迭代器，当然是<3,30>
m1_RcIter = m1.upper_bound( );
cout << "The first element of map m1 with a key "
<< "greater than 2 is: "
<< m1_RcIter -> second << "." << endl; // If no match is found for the key, end is returned
m1_RcIter = m1. upper_bound ( ); // m1中key值并没有大于或等于4的
if ( m1_RcIter == m1.end( ) )
cout << "The map m1 doesn't have an element "
<< "with a key greater than 4." << endl;
else
cout << "The element of map m1 with a key > 4 is: "
<< m1_RcIter -> second << "." << endl; // The element at a specific location in the map can be found
// using a dereferenced iterator addressing the location
m1_AcIter = m1.begin( );
m1_RcIter = m1. upper_bound ( m1_AcIter -> first );
cout << "The 1st element of m1 with a key greater than\n"
<< "that of the initial element of m1 is: "
<< m1_RcIter -> second << "." << endl;
}
The first element of map m1 with a key greater than 2 is: 30.
The map m1 doesn't have an element with a key greater than 4.
The 1st element of m1 with a key greater than
that of the initial element of m1 is: 20.
// map_lower_bound.cpp
// compile with: /EHsc
#include <map>
#include <iostream> int main( )
{
using namespace std;
map <int, int> m1;
map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
typedef pair <int, int> Int_Pair; m1.insert ( Int_Pair ( , ) );
m1.insert ( Int_Pair ( , ) );
m1.insert ( Int_Pair ( , ) );
//key值大于等于2的是<2,20>
m1_RcIter = m1.lower_bound( );
cout << "The first element of map m1 with a key of 2 is: "
<< m1_RcIter -> second << "." << endl; // If no match is found for this key, end( ) is returned
m1_RcIter = m1. lower_bound ( ); if ( m1_RcIter == m1.end( ) )
cout << "The map m1 doesn't have an element "
<< "with a key of 4." << endl;
else
cout << "The element of map m1 with a key of 4 is: "
<< m1_RcIter -> second << "." << endl; // The element at a specific location in the map can be found
// using a dereferenced iterator addressing the location
m1_AcIter = m1.end( );
m1_AcIter--;
m1_RcIter = m1. lower_bound ( m1_AcIter -> first );
cout << "The element of m1 with a key matching "
<< "that of the last element is: "
<< m1_RcIter -> second << "." << endl;
}
The first element of map m1 with a key of 2 is: 20.
The map m1 doesn't have an element with a key of 4.
The element of m1 with a key matching that of the last element is: 30.