Locale invariant guarantee of boost::lexical_cast<>

Question

I'm using boost::lexical_cast<std::string>(double) for converting doubles to string, generating JSON serialized byte stream, that is (on remote side) parsed by .NET.

I'm able to force the .NET to use InvariantCulture for parsing, thereby returning predictable result on every possible language.

However, I was not able to find this guarantee in boost::lexical_cast documentation. I tried it a little bit, and it works the same way for different locales set. But, I cannot be sure only from few tests, am I missing something in the documentation, or this cannot be guaranted at all, and I have to use something else?

EDIT: I've found an issue.

std::locale::global(std::locale("Czech")); 
std::cout << boost::lexical_cast<std::string>(0.15784465) << std::endl;

returns 0,15784465, and that is undesired. Can I force the boost::lexical_cast<> not to be aware of locales?

Answer 1

Can I force the boost::lexical_cast<> not to be aware of locales?

No, I don't think that is possible. The best you can do is call

std::locale::global(std::locale::classic());

to set the global locale to the "C" locale as boost::lexical_cast relies on the global locale. However, the problem is if somewhere else in the code the global locale is set to something else before calling boost::lexical_cast, then you still have the same problem. Therefore, a robust solution would be imbue a stringstream like so, and you can be always sure that this works:

std::ostringstream oss;
oss.imbue(std::locale::classic());
oss.precision(std::numeric_limits<double>::digits10);
oss << 0.15784465;

Answer 2

A better solution to this problem is to use a boost::locale instead of a std::locale as the globale locale. From the documentation:

Setting the global locale has bad side effects... it affects even printf and libraries like boost::lexical_cast giving incorrect or unexpected formatting. In fact many third-party libraries are broken in such a situation. Unlike the standard localization library, Boost.Locale never changes the basic number formatting, even when it uses std based localization backends, so by default, numbers are always formatted using C-style locale. Localized number formatting requires specific flags.

Boost locale requires you to specify explicitly when you want numeric formatting to be locale aware, which is more consistent with recent library decisions like std::money_put.