I test timings:

Test code:

#include <Core/Core.h>

using namespace Upp;

CONSOLE_APP_MAIN
{
	double a;
	String s = "269.96336996337";
	#define LOOPS 1000000
	for(int i = 0; i < LOOPS; i++){
		RTIMING("Using atof");
		a = atof(s);
	}
	for(int i = 0; i < LOOPS; i++){
		RTIMING("Using ScanDouble");
		a = ScanDouble(s);
	}
}

TIMING results:

TIMING Using ScanDouble:  1.12 s  -  1.12 us ( 1.22 s  / 1000000 ), min:  0.00 ns, max:  1.00 ms, nesting: 1 - 1000000
TIMING Using atof     : 337.45 ms - 337.45 ns (439.00 ms / 1000000 ), min:  0.00 ns, max:  1.00 ms, nesting: 1 - 1000000

It's not dramatically slower! But standard settings (on various environments) show right results!

TIMING Using ScanDouble: 120.72 ms - 120.72 ns (148.00 ms / 1000000 ), min:  0.00 ns, max:  2.00 ms, nesting: 1 - 1000000
TIMING Using atof     : 281.72 ms - 281.72 ns (309.00 ms / 1000000 ), min:  0.00 ns, max:  1.00 ms, nesting: 1 - 1000000