How can I get a value from a map?

Question

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 1

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.

Answer 2

map.at("key") throws exception if missing key.

If k does not match the key of any element in the container, the function throws an out_of_range exception.

http://www.cplusplus.com/reference/map/map/at/