Question

I want to insert new element into a vector using a pointer I have the following sample code:

struct info {
    string Name;
    int places;  // i will use the binary value to identfy the visited places example 29 is 100101
                 // this means he visited three places (London,LA,Rome)  
    vector<int> times; // will represent the visiting time,e.g. 1,2,5 means london 1 time, LA
                       // twice and Rome five times
};

map<string,vector<info> *> log;

Peaple are coming from different cities, I will check if the city exists, just add the new person to the vector, else creat a new map object:

vector<info> tp;
info tmp;
if(log.size()==0|| log.count(city)==0) //empty or not exist 
{
    tp.push_back(tmp);
    vector<info>* ss = new vector<info>;
    ss=&(tp);
    // create a new object 
    log.insert(map<string,vector<info> * >::value_type(city,ss)); // new object 
}
else // city exist, just add the information to the vector
{
    map<string,vector<info> *>::iterator t;
    t=log.find(city);
    *(t->second).push_back(tmp);  //the problem  in this line
}

How can I insert the new tmp into the vector?

The information to be read as follows:

Paris,Juli,5,3,6
Paris,John,24,2
Canberra,John,4,3
London,Mary,29,4,1,2

Answer 1

There are a lot of mistakes here, and they all stem from misusing pointers. The line that is mentioned as the cause of the problem is a minor syntactic issue. There are bigger issues at hand.

All of them can be easily solved by not misusing pointers. There is no reason to use pointers here, so the ultimate fix is to make the map have this type map<string,vector<info>> log;.

Then the code becomes something like this:

info tmp;
log[city].push_back(tmp);
// the [] operator creates a new empty vector if it doesn't exist yet
// there's no point in doing the checks by hand

---

Now that we have a simple solution, I’ll mention the elephant in the room code.

vector<info>* ss = new vector<info>;
ss=&(tp);
// ...
log.insert(map<string,vector<info> * >::value_type(city,ss));

This sequence of operations will create a vector with dynamic storage duration, and immediately discard the sole pointer to it. That causes the vector that was just created to be lost, and the memory it uses is leaked; it cannot be recovered anymore.

To make matters worse, it sets ss to point to a local variable, and then saves that pointer to the local variable in the map. Because the local variable has automatic storage duration, it is gone once the function returns. That makes the pointer that was just stored in the map invalid, because it no longer has a vector to point to. After that, all kinds of havoc would be wreaked.