How can I get a value from a map?

c++ dictionary stdmap
Aneesh Narayanan picture Aneesh Narayanan · May 22, 2012 · Viewed 145.8k times · Source

I have a map named valueMap as follows:

typedef std::map<std::string, std::string>MAP;
MAP valueMap;
...
// Entering data.

Then I am passing this map to a function by reference:

void function(const MAP &map)
{
  std::string value = map["string"];
  // By doing so I am getting an error.
}

How can I get the value from the map, which is passed as a reference to a function?

Answer

Steve Jessop picture Steve Jessop · May 22, 2012

Unfortunately std::map::operator[] is a non-const member function, and you have a const reference.

You either need to change the signature of function or do:

MAP::const_iterator pos = map.find("string");
if (pos == map.end()) {
    //handle the error
} else {
    std::string value = pos->second;
    ...
}

operator[] handles the error by adding a default-constructed value to the map and returning a reference to it. This is no use when all you have is a const reference, so you will need to do something different.

You could ignore the possibility and write string value = map.find("string")->second;, if your program logic somehow guarantees that "string" is already a key. The obvious problem is that if you're wrong then you get undefined behavior.

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