Weird bug while inserting into C++ std::map
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16-09-2019 - |
Question
I'm trying to insert some value pairs into a std::map. In the first case, I receive a pointer to the map, dereference it and use the subscript operator to assign a value. i.e.
(*foo)[index] = bar;
Later, when I try to iterate over the collection, I'm returned key/value pairs which contain null for the value attribute in all cases except for the first (map.begin()) item. The weird thing is, if I do the insertion via the map's insert function, everything is well, i.e:
foo->insert(std::pair<KeyType,ValueType>(myKey, myValue));
Why would this be? Aren't the two methods functionally equivalent? I've pasted some snippets of actual code below for context
...
typedef std::map<int, SCNode*> SCNodeMap;
...
void StemAndCycle::getCycleNodes(SCNodeMap* cycleNodes)
{
(*cycleNodes)[root->getId()] = root;
SCNode* tmp = root->getSucc();
while(tmp->getId() != root->getId())
{
// (*cycleNodes)[tmp->getId()] == tmp; // crashes (in loop below)
cycleNodes->insert(std::pair<int, SCNode*>(tmp->getId(), tmp));//OK
std::pair<int, SCNode*> it = *(cycleNodes->find(tmp->getId()));
tmp = tmp->getSucc();
}
// debugging; print ids of all the SCNode objects in the collection
std::map<int, SCNode*>::iterator it = cycleNodes->begin();
while(it != cycleNodes->end())
{
std::pair<int, SCNode*> p = (*it);
SCNode* tmp = (*it).second; // null except for it = cycleNodes->begin()
std::cout << "tmp node id: "<<tmp->getId()<<std::endl;
it++;
}
}
I'm all out of ideas. Does anyone have a suggestion please?
Solution
In your actual code you have:
(*cycleNodes)[tmp->getId()] == tmp;
This will not assign tmp into the map, but will instead reference into the map creating an empty value (see @Neil Butterworth) - you have == instead of =. What you want is:
(*cycleNodes)[tmp->getId()] = tmp;
OTHER TIPS
You should be aware that operator[] for std::map will insert a value into the map if one does not exist when used in expressions like this:
if ( amap[x] == 42 ) {
...
}
If the value x does not exist, one will be created and assigned the value created by the value types default constructor, or zero for the built-in types. This is almost never what you want, and you should generally avoid the use of operator[] with maps.
Does your value type have an assignment operator?
Take a look at this reference. The [] operator returns a non-const reference to the value. If your assignment is wrong or somehow works in an unexpected way this might be the cause.
The insert method on the other hand takes a value and stuffs it into the map. The [] operator constructs an object with the default constructor, then let's you assign stuff to it using it's assignment operator.