Comparing 3 modern c++ ways to convert integral values to strings

Question

I was trying to pick a standard way to convert integrals to strings, so I went on and did a small performance evaluation by measuring the execution time of

Answer 1

Question 1. Why is the string stream method consistently the worst?

The classical mistake: creating a new stringstream every single time

template<typename T> // 1. Using stringstream
string StringFromIntegral_SS(T const &value) {
    thread_local stringstream ss;
    ss.str("");
    ss.clear();
    ss << value;
    return ss.str();
}

Question 2. Why is lexical cast consistently the best? Can we assume that this is the fastest implementation ?

Because it's most specialized; and, no, faster implementations exist. FastFormat and Boost Spirit have competitive offerings, as far as I know.

Update Boost Spirit Karma still easily beats the bunch:

template<typename T> // 4. Karma to string
std::string StringFromIntegral_K(T const &value) {
    thread_local auto const gen = boost::spirit::traits::create_generator<T>::call();
    thread_local char buf[20];
    char* it = buf;
    boost::spirit::karma::generate(it, gen, value);
    return std::string(buf, it);
}

Timings:

C++ 11 method 111
String stream method 103
Lexical cast method 57
Spirit Karma method 36
Spirit Karma method with string_ref 13

See it Live On Coliru Clang or GCC

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BONUS

Just to goof off, a version using boost::string_ref is much faster still due the reduced allocations:

template<typename T> // 5. Karma to string_ref
boost::string_ref StringFromIntegral_KSR(T const &value) {
    thread_local auto const gen = boost::spirit::traits::create_generator<T>::call();
    thread_local char buf[20];
    char* it = buf;
    boost::spirit::karma::generate(it, gen, value);
    return boost::string_ref(buf, it-buf);
}

I've tested all modified methods for correctness using an asserting test loop:

return measure<>::execution(
    //[&]() { for (auto const &i : v1) { func(i); }});
    [&]() { for (auto const &i : v1) { assert(func(i) == StringFromIntegral_LC(i)); }});