STL映射，使用类的迭代器和指针

Question

I am having trouble accessing data inside a map using an iterator. I want to return all of the values inserted into the map by using an iterator. However, when I use the iterator, it doesn't acknowledge any of the members in the instance of the class it has been past into.

int main()
{
    ifstream inputFile;
    int numberOfVertices;
    char filename[30];
    string tmp;

    //the class the holds the graph
    map<string, MapVertex*> mapGraph;

    //input the filename
    cout << "Input the filename of the graph: ";
    cin >> filename;
    inputFile.open(filename);

    if (inputFile.good())
    {
        inputFile >> numberOfVertices;
        inputFile.ignore();

        for (int count = 0; count < numberOfVertices; count++)
        {
            getline(inputFile, tmp);
            cout << "COUNT: " << count << "  VALUE: " << tmp << endl;

            MapVertex tmpVert;
            tmpVert.setText(tmp);
            mapGraph[tmp]=&tmpVert;
        }

        string a;
        string connectTo[2];

        while (!inputFile.eof())
        {

            //connectTo[0] and connectTo[1] are two strings that are behaving as keys

            MapVertex* pointTo;
            pointTo = mapGraph[connectTo[0]];
            pointTo->addNeighbor(mapGraph[connectTo[1]]);
            //map.find(connectTo[0]).addNeighbor(map.find(connectTo[1]));
            //cout << connectTo[0] << "," << connectTo[1] << endl;
        }

        map<string,MapVertex*>::iterator it;
        for (it=mapGraph.begin(); it!=mapGraph.end(); it++)
        {
            cout << it->getText() << endl;

        }
    }

    return 0;
}

Compiler output:

\lab12\main.cpp||In function `int main()':|
\lab12\main.cpp|69|error: 'struct std::pair<const std::string, MapVertex*>'
                           has no member named 'getText'|
||=== Build finished: 1 errors, 0 warnings ===|

There is an access member in my MapVertex class called getText() which returns the data that is inside it.

Answer 1

To fix the compiler error, you need to do it->second->getText() as the *iterator is a pair<string, MapVertex*> . But there are other problems in your code. While inserting into the map, you are inserting the address of a local variable into it. This address will be invalid by the time you try to iterate the map using for loop. I would suggest you to declare the map as std::map<string, MyVertex> so that when you insert into the map a copy of the MyVertex is inserted into map.

Answer 2

tmpVert is the problem. Look, you create it on the stack. It is destroyed at the end of each for loop.

It Is Destroyed.

So, your mapGraph is holding pointers to objects that don't exist.

Answer 3

'struct std::pair' has no member named 'getText'

means that what the iterator returns is std::pair, not your object directly; the first element of the pair is the key, the second the value, so you need to get the value, and then call the method: it->second->method() .