映射：即使找到了密钥，Ptr Ptr也很差

Question

I have a map inside struct:

struct amountOfDist
{
        int time;
        vector<int> distVector;
        map<string,int> pairsMap;
};
amountOfDist m_tempDistStruct;

when the code runs, it crushes when I try to find a value:

if(m_tempDistStruct.pairsMap.find("(1,2)")->second != 1)
{
       ...
}

I tried to isolate the command by:

map<string,int>::iterator it;
it = m_tempDistStruct.pairsMap.find("(1,2)");

and I get padptr . But when I put break point on the line:

it = m_tempDistStruct.pairsMap.find("(1,2)");

I can see that the map hols all the keys and values (the correct ones) and the key (1,2) exists.

Why does the find command returns badptr ?

I will be happy for guidance.

Thanks.

Answer 1

If the map really does contain the key you're looking for, then your code should work; something else has gone horribly wrong if it doesn't. However, the code is rather fragile, since it will give undefined behaviour if the key is missing.

You have to check whether find succeeded before dereferencing it; if it fails, then it returns a past-the-end iterator which isn't dereferencable. Alternatively, use [] which will insert a new element if it was missing.

So safer versions are:

// use find and check it exists
auto found = map.find(key);
if (found != map.end() && found->second != 1)

// use [] to insert if it doesn't exist
if (map[key] != 1)

If neither of these work, or you're absolutely sure that the key must exist, then we'll need to see a complete test case to figure out what's going wrong with the code you haven't shown us.

Answer 2

Using your code, I get totally correct behavior. So I am pretty sure that the value you're looking for is not in the map .

amountOfDist m_tempDistStruct;

m_tempDistStruct.pairsMap["(1,2)"] = 10;

if  (m_tempDistStruct.pairsMap.find("(1,2)")->second == 10)
{
    cout << "GOT HERE!" << endl;
}

The above code snippet adds "(1,2)" => 10 to the map, then finds it just fine. I suspect the key in your map is subtly different to what you think it is.