为什么 boost::locale::to_title 没有返回预期的输出？

Question

So I've already found How to capitalize a word in a C++ string? , but I've tried similar code as has been suggested, including what's provided in the examples for Boost::locale. I'll also include what my code is currently and what the expected and real output are. So I'm trying to understand why I'm not getting the expected output.

Code

#include <iostream>
#include <string>
#include <boost/locale.hpp>
#include <boost/algorithm/string/case_conv.hpp>

int main() {
    using namespace std;
    using namespace boost::locale;

    generator gen;
    auto loc = gen("");
    locale::global(loc);
    cout.imbue(loc);

    ios_base::sync_with_stdio(false);

    cout << to_upper("hello!") << " " << boost::to_upper_copy("hello!"s) << endl;
    cout << to_lower("HELLO!") << " " << boost::to_lower_copy("HELLO!"s) << endl;
    cout << to_title("hELLO!") << endl;
    cout << fold_case("HELLO!") << endl;

    return 0;
}

Expected Output

HELLO! HELLO!
hello! hello!
Hello!
hello!

Real Output

HELLO! HELLO!
hello! hello!
hELLO!
hello!

Additional Information

• OS: Windows 10 Home 64-bit
• Compiler: Microsoft Visual Studio 15.8.0
• Platform: x64
• Non-default Compilation Options: /std:c++latest
• BOOST_VERSION: 106700

EDIT #1

It seems that the Boost that is installed by vcpkg doesn't get compiled with ICU, which is apparently required for boost::locale::to_title to function correctly.

Answer 1

vcpkg ( https://github.com/Microsoft/vcpkg ) by default installs Boost without depending on ICU for the Boost::locale and Boost::regex libraries.

So, instead of installing those like this:

vcpkg install boost-locale:x64-windows boost-regex:x64-windows

I had to do the following:

vcpkg install boost-locale[icu]:x64-windows boost-regex[icu]:x64-windows

This automatically fetches and builds the ICU library, and (since I had already installed Boost without ICU) it automatically rebuilt all of the Boost libraries.

I wish the Boost documentation for those libraries made it clear that you needed ICU to use the functionality that requires it.

Answer 2

title_case is handled only for ICU as per source code of boost locale , while for other platforms like for ex win32 it returns input value as it is.

So in order to use to_title function you have to make sure to use boost locale for ICU

virtual string_type convert(converter_base::conversion_type how,char_type const *begin,char_type const *end,int flags = 0) const 
{
    icu_std_converter<char_type> cvt(encoding_);
    icu::UnicodeString str=cvt.icu(begin,end);
    switch(how) {
        ...
        case converter_base::title_case:
            str.toTitle(0,locale_);
            break;
        case converter_base::case_folding:
            str.foldCase();
            break;
        default:
            ;
    }
    return cvt.std(str);
}