std::map 用迭代器查找距离，程序不会终止

Question

As I compile ( g++ -std=c++14 map.cpp ) and run this program, it doesn't seem to terminate. Can any one explain Why? However as I do find('a') instead of 'c' it gives a zero.

#include <iostream>
#include <string>
#include <vector>
#include <map> 
#include <algorithm>
using namespace std; 



int main()
{
    map<char, float> m;
    m['a'] = 3.4;
    m['b'] = 5.3;
    m['c'] = 33.3;
    m['d'] = 43.;

    auto it = m.find( 'c' );
    cout << "distance : " << std::distance( it , m.begin() ) << endl;

}

Answer 1

Use

std::distance( m.begin(), it  )

Otherwise the call

std::distance( it , m.begin() )

has undefined behavior because there is used an invalid range. Ranges in C++ are specified like [first, last ) where first precedes or equal to last. In the last case when first is equal to last the range is empty.

From the C++ Standard (27.4.3 Iterator operations)

4 Effects: If InputIterator meets the requirements of random access iterator, returns (last - first); otherwise, returns the number of increments needed to get from first to last .

Answer 2

std::distance(first,last) starts running from first and advances the iterator until it reaches the last . In your case this will never happen because it is most likely found after m.begin() so it'll loop forever. change the order of parameters given to std::distance

std::distance reference:

The behavior is undefined if last is not reachable from first by (possibly repeatedly) incrementing first.