a.cc

#include <string>
#include <map>

void test() {
    std::map<std::string, std::string> m;
    m["1"] = "foo";
    m["2"] = "bar";
    m["3"] = "baz";
    m["4"] = "asdf";
    m["5"] = "zxcv";
    m["6"] = "abcd";
    m["7"] = "efgh";
    m["8"] = "qweqwe";
    m["9"] = "werwer";
    m["10"] = "hfjfhfhff";

    m["1"];
    m["2"];
    m["3"];
    m["4"];
    m["5"];
    m["6"];
    m["7"];
    m["8"];
    m["9"];
    m["10"];
}

int main(int argc, char *argv[]) {
    for (unsigned i = 0; i < 1000000; i++) {
        test();
    }
    return 0;
}

b.cc

#include <string>
#include <vector>

class SimpleMap_t {

    std::vector<std::pair<std::string, std::string> > v;

public:

    SimpleMap_t() {
        v.reserve(10);
    }

    std::string& operator[](const std::string &key) {
        for (unsigned i = 0; i < v.size(); i++) {
            if (v[i].first == key) {
                return v[i].second;
            }
        }
        v.push_back(std::pair<std::string,std::string>(key, ""));
        return v[v.size()-1].second;
    }
    
};

void test() {
    SimpleMap_t m;
    m["1"] = "foo";
    m["2"] = "bar";
    m["3"] = "baz";
    m["4"] = "asdf";
    m["5"] = "zxcv";
    m["6"] = "abcd";
    m["7"] = "efgh";
    m["8"] = "qweqwe";
    m["9"] = "werwer";
    m["10"] = "hfjfhfhff";

    m["1"];
    m["2"];
    m["3"];
    m["4"];
    m["5"];
    m["6"];
    m["7"];
    m["8"];
    m["9"];
    m["10"];
}

int main(int argc, char *argv[]) {
    for (unsigned i = 0; i < 1000000; i++) {
        test();
    }
    return 0;
}

Makefile

all: a b
	echo a
	time ./a
	time ./a
	echo b
	time ./b
	time ./b

a: a.cc
	g++ -O2 -o a a.cc

b: b.cc
	g++ -O2 -o b b.cc