User blog comment:Deedlit11/Bignum Bakeoff appetizer/@comment-5529393-20140922175020/@comment-5529393-20140923051500

Perhaps

i = 9 << 99999, b = 2; B(y,x=0) {return y ? y % b * pow(b+1, B(x)) + B(y/b, x+1) : 0;} main {while(i = B(i)-1) b++; return b;}

will work for C++, getting down to 99 characters, unless we used a shortcut that doesn't work for C++.