c++ Function to format time_t as std::string: buffer length?

Question

I want a function that will take a time_t parameter and an arbitrary format string and format it. I want something like this:

std::string GetTimeAsString(std::string formatString, time_t theTime)
{
    struct tm *timeinfo;
    timeinfo = localtime( &theTime);

    char buffer[100];
    strftime(buffer, 100, formatString.c_str(), timeinfo);
    std::string result(buffer);
    return result;
}

However one problem I'm running into is the buffer length. I was thinking of doing something like formatString * 4 as the buffer length. But I guess you can't dynamically set the buffer length? Maybe I could pick an arbitrarily large buffer? I'm a little stuck as to how to make it generic.

How can I write a function to achieve this?

Answer 1

If you have C++11:

std::string GetTimeAsString(std::string formatString, time_t theTime)
{
    struct tm *timeinfo;
    timeinfo = localtime( &theTime);

    formatString += '\a'; //force at least one character in the result
    std::string buffer;
    buffer.resize(formatstring.size());
    int len = strftime(&buffer[0], buffer.size(), formatString.c_str(), timeinfo);
    while (len == 0) {
        buffer.resize(buffer.size()*2);
        len = strftime(&buffer[0], buffer.size(), formatString.c_str(), timeinfo);
    } 
    buffer.resize(len-1); //remove that trailing '\a'
    return buffer;
}

Note I take formatString as a const reference, (for speed and safety), and use the result string as the buffer, which is faster than doing an extra copy later. I also start at the same size as the formatstring, and double the size with each attempt, but that's easily changable to something more appropriate for the results of strftime.