二进制搜索C ++ STL

Question

我有一个unordered_map的向量，它根据我定义的比较器函数进行排序。我想使用二进制搜索来使用比较器函数查找其中一个值。但是，二进制搜索只返回bool，我需要结果的索引/迭代器。我该怎么办？

Answer 1

#include <algorithm>
using namespace std;

//!!!!!   a must be sorted using cmp. Question indicates that it is.        
it = lower_bound(a.begin, a.end(), value, cmp);

//Check that we have actually found the value. 
//If the requested value is missing
//then we will have the value before where the requested value 
//would be inserted.
if(it == a.end() || !cmp(*it, value))
{
   //element not found
} 
else
{
   //element found
}

Answer 2

#include <algorithm>
using namespace std;

it = lower_bound(a.begin, a.end(), value, cmp);