C++ std :: transform of pairs-> first to new vector

template<class InputIt, class OutputIt, class UnaryOperation>
OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first, UnaryOperation unary_op);

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<std::pair<int, std::string>> pairs = {{1, "one"}, {2, "two"}, {3, "three"}};
    std::vector<int> keys;

    std::transform(pairs.begin(), pairs.end(), std::back_inserter(keys),
                   [](const std::pair<int, std::string>& pair) {
                       return pair.first;
                   });

    // 打印提取出的键
    for (const auto& key : keys) {
        std::cout << key << " ";
    }
    std::cout << std::endl;

    return 0;
}

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