read_while.cpp

#include <chrono>
#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class stopwatch
{
    std::chrono::high_resolution_clock::time_point start;

public:
    stopwatch()
        : start(std::chrono::high_resolution_clock::now())
    {
    }

    ~stopwatch()
    {
        auto duration = std::chrono::high_resolution_clock::now() - start;
        cerr << std::chrono::duration_cast<std::chrono::milliseconds>(duration).count() << " ms" << endl;
    }
};

class ss
{
    const char *p_;

public:
    ss(const char *p)
        : p_(p)
    {
    }

    int get()
    {
        return *p_++;
    }

    void unget()
    {
        p_--;
    }
};

template<typename Pred, typename Stream>
static string read_while(Pred pred, Stream &is)
{
    char ch;
    string s;

    while ((ch = is.get()) && pred(ch))
        s += ch;

    is.unget();

    return s;
}

int main()
{
    istringstream is("hello world!");

    for (int k = 10; --k > 0; )
    {
        stopwatch sw;

        for (int i = 5000000; --i > 0; )
        {
            //is.seekg(is.beg);
            ss is2("hello world!");
            read_while(isalpha, is2);
        }
    }
}