在C ++中插入以映射类型

Question

I dont understand what does that piece of code does

static TwoWayHostPair hostpair;
map <TwoWayHostPair, Traffic> mymap;
//here some map element inserted to mymap and hostpair initialized

map <TwoWayHostPair, Traffic>::iterator iter = mymap.begin();
iter = mymap.find(hostpair);
if (iter == mymap.end()) { 
    iter = mymap.insert(make_pair(hostPair, Traffic())).first; //line8
}

My question is what happens in line8? I didnt get it. Isnt it supposed to be type map<...>:iterator and after this insertion does it stay same type?

Answer 1

std::map::insert return std::pair< iterator,bool > , below statement is correct. second bool return value indicates whether the insertion took place

iter = mymap.insert(make_pair(hostPair, Traffic())).first; //line8

see reference here

Answer 2

As used here,

iter = mymap.insert(make_pair(hostPair, Traffic())).first; //line8

mymap.insert returns a pair<iterator,bool> . first , then, accesses the iterator .

Answer 3

insert(make_pair......

make_pair is used to insert pair value to a map.You have iterated for all the element in map having hostpair.

EDIT Refer cplusplus map to know more.

Answer 4

iter = mymap.find(hostpair);
if (iter == mymap.end()) { 
    iter = mymap.insert(make_pair(hostPair, Traffic())).first; //line8
}

The first line looks for key hostPair in the map, and if it is not found, then it enters the if block where it inserts the key along with its value, and .first returns the iterator to the inserted item.

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Improvement

But you can make an improvement to this. You could just write this:

iter = mymap.insert(make_pair(hostPair, Traffic())).first; 

which is exactly equivalent to your code. No need to use find and then insert . The result is a performance gain.

If the key already exists, the insert function will NOT insert any item to the map and .first will return you the iterator to the found item. If the key doesn't exist, only then it will insert and .first will return you the iterator to the newly inserted item.

If you want to know whether the key already existed or not, then you can do this:

auto pair = mymap.insert(make_pair(hostPair, Traffic())); //NO .first!
if (pair.second)
{
     //a newly created item is inserted 
     auto iter = pair.first; //iterator to the newly inserted item
}
else
{
    //an item with key `hostPair` already exists in the map
     auto iter = pair.first; //iterator to the found item
}

Hope that helps.