Copy std::map into std::vector of pairs

Question

I\'m trying to copy a map into a vector of pair, so I can then sort the vector by the second data member of the pairs. I have resolved this doing like this:

Answer 1

Just use std::vector's assign member function.

//no need to call reserve, bidirectional iterators or better will compute the size and reserve internally.
vectorWordsList.assign(mappedWordsList.begin(), mappedWordsList.end());

If you have existing values in the vector that you don't want overwritten then use insert instead like

vectorWordsList.reserve(vectorWordsList.size() + mappedWordsList.size()); // make sure we only have a single memory allocation
vectorWordsList.insert(vectorWordsList.end(), mappedWordsList.begin(), mappedWordsList.end());

Answer 2

It's worth noting that if you are creating a vector for this purpose, you may use the vector's constructor directly:

std::vector<std::pair<FirstType,SecondType>> vectorWordsList( mappedWordsList.begin(), mappedWordsList.end() );

In C++17, you may also omit the vector's template arguments to have the compiler deduce them:

std::vector vectorWordsList( mappedWordsList.begin(), mappedWordsList.end() );

Answer 3

You can use std::copy and std::back_inserter:

std::copy(mappedWordsList.begin(), 
          mappedWordsList.end(), 
          std::back_inserter(vectorWordsList));

Honestly, I think that a range-for loop is clearer:

for(const auto& kv : mappedWordsList) 
     vectorWordsList.emplace_back(kv);

Regardless, you can use std::vector::reserve to preallocate memory on your target vector, avoiding unnecessary reallocations.