Re: double_ parser is less precise than atof

I think the "raw" tags in Nabble stripped the code from the email. Trying
again.

{
	using namespace boost::spirit;
	using namespace boost::spirit::qi;

	const double expected = -0.9022464718061531;
	const char* input = "-0.9022464718061531";

	// Both atof and lexical_cast give the same result
	double fromAtof = atof(input);   // -0.90224647180615314
	double fromLexCast = boost::lexical_cast<double>(input);  //
-0.90224647180615314

	// qi::parse gives a slightly different answer
	char const* f(input);
	char const* l(f + strlen(f));
	double fromSpirit = 0.0;
	parse(f, l, double_, fromSpirit); // -0.90224647180615325

	// The cause is the calculation in boost::spirit::traits::scale:
	//    n = T(acc_n) / pow10<T>(-exp);
	// I've expanded the code a bit here:
	unsigned long long acc_n = 9022464718061531ul;  // 9022464718061531
	double d_acc_n = double(acc_n);                  // 9022464718061532.0  !
	double fromInternalCalc = d_acc_n / -10000000000000000.; //
-0.90224647180615325
}



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