08-22-10 - AutoPrintf v1

Well autoprintf v1 appears to be all working. The core element is a bunch of functions like this :

template < typename T1, typename T2, typename T3, typename T4 >
inline String autoToString( T1 arg1, T2 arg2, T3 arg3, T4 arg4 )
{
    return ToString(arg1) + autoToString( arg2,arg3,arg4);
}


template < typename T2, typename T3 >
inline String autoToString( const char *fmt, T2 arg2, T3 arg3 )
{
    autoFormatInfo fmtInfo = GetAutoFormatInfo(fmt);
    if ( fmtInfo.autoArgI )
    {
        String newFmt = ChangeAtoS(fmt,fmtInfo);
        if ( 0 ) ;
        else if ( fmtInfo.autoArgI == 1 ) return autoToString(newFmt.CStr(), ToString(arg2).CStr(),arg3);
        else if ( fmtInfo.autoArgI == 2 ) return autoToString(newFmt.CStr(), arg2,ToString(arg3).CStr());
        else return autoPrintf_BadAutoArgI(fmt,fmtInfo);
    }

         if ( fmtInfo.numPercents == 0 )    return ToString(fmt) + autoToString(arg2,arg3);
    else if ( fmtInfo.numPercents == 1 )    return StringPrintf(fmt,arg2) + autoToString(arg3);
    else if ( fmtInfo.numPercents == 2 )    return StringPrintf(fmt,arg2,arg3);
    else return autoPrintf_TooManyPercents(fmt,fmtInfo);
};

you have an autoToString that takes various numbers of template args. If the first arg is NOT a char *, it calls ToString on it then repeats on the remaning args. Any time the first arg is a char *, it uses the other specialization which looks in fmt to see if it's a printf format string, then splits the args based on how many percents they are. I also added the ability to use "%a" to mean auto-typed args, which is what the first part of the function is doing.

That's all dandy, but you should be able to see that for large numbers of args, it generates a massive amount of code.

The real problem is that even though the format string is usually a compile-time constant, I can't parse it at compile time, so I generate code for each arg being %a or not being %a, and for each possible number of percents. The result is something like 2^N codegens for N args. That's bad.

So, I know how to fix this, so I don't think I'll publish v1. I have a method for v2 that moves most of the work out of the template. It's much simpler actually, and it's a very obvious idea, all you have to do is make a template like :

autoprintf(T1 a1, T2 a2, T3 a3)
{
    autoPrintfSub( autoType(a1), autoArg(a1) ,autoType(a2), autoArg(a2) , .. )
}

where autoType is a template that gives you the type info of the arg, and autoArg does conversions on non-basic types for you, and then autoPrintfSub can be a normal varargs non-template function and take care of all the hard work.

... yep new style looks like it will work. It requires a lot more fudging with varargs, the old style didn't need any of that. And I'm now using undefined behavior, though I think it always works in all real-world cases. In particular, in v2 I'm now relying on the fact that I can do :

  va_start(vl)
  va_arg(vl) .. a few types to grab some args from vl
  vsnprintf(  vl);

that is, I rely on the fact that va_arg advances me one step in the va_list, and that I then still have a valid va_list for remaining args which I can pass on. This is not allowed by the standard technically but I've never seen a case where it doesn't work (unless GCC decided to get pedantic and forceably make it fail for no good reason).