Re: Repetitive indexing a std::map with same index

I have a case where I must access a std::map using operator[] where
there is a high probability that the same index is used repetitively
for a number of times. Will I improve performance when I remember the
last index used and compare each time I access the map, or are map
implementations usually optimized for this case?

I am thinking of something like this (much simplified):

class Debug {
    pthread_t last_tid;
    struct LineBuffer *last_buf, *line_buf;
    std::map<pthread_t, LineBuffer> map;

    template<class T>
    Debug&operator<<(const T&a) {
        pthread_t tid = pthread_self();
        if (tid == last_tid) {
            line_buf = last_buf;
        } else {
            line_buf =&map[tid];
            last_tid = tid;
        }

        // write 'a' to the line_buf,
        // flush to cerr when complete.
        ;

        return *this;
    }
};

so multiple threads can write

    Debug()<< "foo value is "<< foo<< " yadda yadda"<< std::endl;

without having their output mangled with other debug messages.