Question

I have a map container. I want to copy all its keys with user specified first letters to a vector. How to do that in an efficient way? Thank you!

std::map(std:string, size_t) myMap;
myMap.insert(std:make_pair("Ace", 11);
myMap.insert(std:make_pair("Ape", 12);
myMap.insert(std:make_pair("Age", 13);
myMap.insert(std:make_pair("Beat", 21);
myMap.insert(std:make_pair("Boat", 22);
myMap.insert(std:make_pair("Boss", 23);
myMap.insert(std:make_pair("Coat", 31);
myMap.insert(std:make_pair("Cost", 32);
myMap.insert(std:make_pair("Cast", 33);

For example, if I want to extract all the element(s) with their keys starting with “Bo” from myMap and populate the satisfled results to myVec, myVec will have the following elements:

Boat
Boss

Answer 1

Declarations left out for brevity:

begin = myMap.lower_bound("Bo");
end = std::find_if(begin, myMap.end(), first_does_not_begin_with("Bo"));
for (i=begin; i!=end; ++i)
    myVec.push_back(i->first);

You can implement the first_does_not_begin_with functor class, right? It’s operator should have this signature:

bool operator()(std::pair<const std::string, size_t> const &);