benchmarking stl string stream - https://quick-bench.com/q/c5iiDBpM4ZpZGtnEK_bGBe9mMbg

benchmark-stringstream.cpp

struct stringstream {
    std::string stream;
    stringstream() = default;
    stringstream(std::string_view&& sv) : stream { std::move(sv) }
    {}

    std::string str() {
        return stream;
    }
    
    std::string str(std::string&& s) {
        this->stream = s;
        return stream;
    }
    
    friend stringstream& operator<<(stringstream& ss, std::string_view&& s) {
        ss.stream += s.data();
        return ss;
    }
};

static void MyStringStream(benchmark::State& state) {
  for (auto _ : state) {
    stringstream ss {};
    ss << "hello" << "world";
    benchmark::DoNotOptimize(ss);
  }
}
BENCHMARK(MyStringStream);

#include <sstream>
static void STLStringStream(benchmark::State& state) {
  for (auto _ : state) {
    std::stringstream ss {};
    ss << "hello" << "world";
    
    benchmark::DoNotOptimize(ss);
  }
}
BENCHMARK(STLStringStream);