Precision is lost when using lexical_cast(string)

Question

When using boost::lexical_cast (I'm using boost version 1.58 on VS2013), I can't get the exact value specified in the string, even though it is represent-able in float:

std::wstring t = L"91.25";
float r;
r = boost::lexical_cast<float>(t);

r is 91.249992 (0x42B67FFF) instead of 91.250000 (0x42b68000)

Prior versions of boost behaved the expected way. Is there a precision setting I'm missing?

Cannot reproduce: http://melpon.org/wandbox/permlink/CZY6eLdu0QdDagOV — NathanOliver, Jun 23 '16 at 15:11
Rather than editing your question, you should create your own answer. Although you may leave in the part about which compiler version you're using, since it's relevant. — Mark Ransom, Aug 02 '16 at 15:13

score 2 · Answer 1 · answered Aug 02 '16 at 17:12

It turns out this is unrelated to boost. It seems to be a problem with Visual Studio and only VS2013.

#include <iostream>
#include <sstream>
#include <iomanip>
int main()
{
    float a;
    std::stringstream s;
    s.str("91.25");
    s >> a;
    std::wcout << std::setprecision(20) << a << std::endl;
    // displays "91.249992370605469" when compiled with 
    // VS2013 and "91.25" when compiled with VS2010 or VS2015
    return 0;
}

Precision is lost when using lexical_cast(string)

1 Answers1