Re: Opening large text file makes program unhappy.

On Sat, 24 Jun 2006 19:57:18 -0700, hazmaz wrote:


Rather than periodically returning control to the OS, it might be simpler
to change your design so that your function does a small amount of work
each time it's called, rather than all the work at once. Like:

class FileReader
{
    std::vector<std::string> lines_;
    std::ifstream file_;

    void on_complete();
    void on_error();

    bool read_line();
public:
    FileReader(const char* name)
        : file_(name)
    { }

    static bool callback(void* user_data);
};

// Read a line, return true if the function
// should be called again to read more lines,
// false otherwise. (You probably really
// want to read more than one line at a time,
// but this is easier for the purposes of the
// example).
bool FileReader::read_line()
{
    if( file_.fail() )
    {
        if( file_.eof() )
            on_complete();
        else
            on_error();
        return false;
    }
    std::string line_;
    std::getline(file_, line);
  lines_.push_back(line);
    return true;
}

// This is a static function so that you can pass
// a pointer to it to whatever API your system
// offers to do delayed execution.
bool FileReader::callback(void* user_data)
{
    FileReader* r = static_cast<FileReader*>user_data;

    return r->read_some_lines();
}

And implement on_complete and on_error to do whatever you need to do to
handle the completion of the read, or the error condition. Depending on
how your system works, you may have to have the callback function request
that it be invoked again, something like:

bool FileReader::callback(void* user_data)
{
    FileReader* r = static_cast<FileReader*>user_data;

    if( r->read_some_lines() )
    {
        call_me_later(FileReader::callback, user_data);
        return true;
    }
    else
    {
        return false;
    }
}

Replacing "call_me_later" with whatever is the appropriate function on
your platform.

BTW,

Do you need to read into the vector and _then_ add to the listbox? Or
could you just add the data to the listbox as you read it in?

People on windows programming groups will probably be able to help you
more - they'll be familiar with event-based programming, and with the
specific API functions you'll need to call.