Question

I insert a pair (key,value) to my hashmap but it seems like it doesn’t have the value when trying to print it

The thing is i don’t know why i can’t print the second element in the following code.

Let’s see some code :

.h

typedef hash_map<const string, string, strptrhash, strptrequal> StringHashMap;  
StringHashMap m_memberNameMap; 
typedef pair <const string,string> string_pair; 

.cpp

void Powerdomain::addMemberName(const string& name){  

if (m_memberNameMap[name]==""){
    m_memberNameMap.insert(string_pair(name,name));
    StringHashMap::iterator it = m_memberNameMap.begin();
    cout << it->first << endl;
    cout << it->second << endl;
    cout << m_memberNameMap["MD1"] << endl;
}

with name=”MD1″ this outputs :

MD1  
*blank*  
*blank*   

EDIT

concerning the answer of Alan :

ModuleType * moduletype4=new ModuleType("type4");
ModuleType * value = moduleTypeMap["type4"];

if (value==NULL) {
   ModuleType& value2=*moduleTypeMap["type4"];
   value2=*moduletype4;
   cout << "correctly inserted" << endl;
}
cout << moduleTypeMap["type4"]->getName() << endl;  

This doesn’t work. I Might be confused now !

Answer 1

What does m_memberNameMap[name] do in the if condition? If the key name doesn’t exist in the map, it creates a new slot with that key, and value equal to empty string.

That is, the expression m_memberNameMap[name] invokes operator[] which inserts an item equal to string_pair(name, "") to the map if the key name doesn’t exist,

The solution is to use find as:

if (m_memberNameMap.find(name) == m_memberNameMap.end())
{
    m_memberNameMap.insert(string_pair(name,name));
    //rest of the code...
}

Instead of insert function, you can also use :

   m_memberNameMap[name] = name; //simpler, cute!

After all, operator[] creates a new slot if the key doesn’t exist.