How to delete an entry from std::map by value?

[WhatIf]

What is the best way to delete (key, value) from std::map based on value?

I'm trying to delete an entry in std::map based on the value, not the key. Here is the code I have:

for(std::map<int, std::string>::iterator it = map.begin(); it != map.end(); it++)
{
	if((it->second) == itemToRemove)
	{
		map.erase(it);
	}
}

I have another code exactaly the same as above with only map.erase(it); replaced with map.erase(it->first);

Is there any difference between the 2 codes? I have been searching online and can't find a standard way to delete an entry by value.

[Peter87]

Using map.erase(it); is more efficient because it doesn't have to do a lookup with the key to find the node that should be deleted.

Your code has a problem because the iterator that you pass to erase will be invalidated so you can't keep using it as if nothing has happened. The erase function returns an iterator to the element after the removed element so you could use this to update your iterator it = map.erase(it); but then you need to make sure not to increment the iterator it++ after you removed an element because otherwise you will miss to check the element after the removed element.

for(std::map<int, std::string>::iterator it = map.begin(); it != map.end();)
{
	if((it->second) == itemToRemove)
	{
		it = map.erase(it);
	}
	else
	{
		it++;
	}
}

Note that if you know that the values are unique (so only one element can be removed) you could instead just break out of the loop after you have removed the element.

for(std::map<int, std::string>::iterator it = map.begin(); it != map.end(); it++)
{
	if((it->second) == itemToRemove)
	{
		map.erase(it);
		break;
	}
}

[jlb]

Both should accomplish your goals, the first removes by the iterator, the second removes by the key.

Have you looked at this page? http://www.cplusplus.com/reference/map/map/erase/