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我正在尝试使用该boost::locale库在我的代码（版本 1.71）中执行字符串的大写和小写转换。

我对“ß”的大写有疑问。为了符合我的代码库中已经存在的单元测试，我希望将字母“ß”大写为“SS”。这应该不是问题，因为据我了解这是预期的行为（https://www.boost.org/doc/libs/1_71_0/libs/locale/doc/html/conversions.html）。

这是此页面上提供的示例的副本以供参考：

Upper GRÜSSEN

Lower grüßen

Title Grüßen

Fold grüssen

但是，当我在代码中使用该方法时，情况并非如此。应用大写方法时，“ß”保持为“ß”。

boost::locale我很困惑，在库源中找到了以下示例：

//
//  Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//
#include <boost/locale.hpp>
#include <boost/algorithm/string/case_conv.hpp>
#include <iostream>

#include <ctime>



int main()
{
    using namespace boost::locale;
    using namespace std;
    // Create system default locale
    generator gen;
    locale loc=gen(""); 
    locale::global(loc); 
    cout.imbue(loc);

    
    cout<<"Correct case conversion can't be done by simple, character by character conversion"<<endl;
    cout<<"because case conversion is context sensitive and not 1-to-1 conversion"<<endl;
    cout<<"For example:"<<endl;
    cout<<"   German grüßen correctly converted to "<<to_upper("grüßen")<<", instead of incorrect "
                    <<boost::to_upper_copy(std::string("grüßen"))<<endl;
    cout<<"     where ß is replaced with SS"<<endl;
    cout<<"   Greek ὈΔΥΣΣΕΎΣ is correctly converted to "<<to_lower("ὈΔΥΣΣΕΎΣ&quot;)<<", instead of incorrect "
                    <<boost::to_lower_copy(std::string("ὈΔΥΣΣΕΎΣ&quot;))<<endl;
    cout<<"     where Σ is converted to σ or to ς, according to position in the word"<<endl;
    cout<<"Such type of conversion just can't be done using std::toupper that work on character base, also std::toupper is "<<endl;
    cout<<"not even applicable when working with variable character length like in UTF-8 or UTF-16 limiting the correct "<<endl;
    cout<<"behavior to unicode subset BMP or ASCII only"<<endl;
   
}

// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4

// boostinspect:noascii

我尝试编译它，这是我得到的结果：

Correct case conversion can't be done by simple, character by character conversion
because case conversion is context sensitive and not 1-to-1 conversion
For example:
   German grüßen correctly converted to GRÜßEN, instead of incorrect GRüßEN
     where ß is replaced with SS
   Greek ὈΔΥΣΣΕΎΣ is correctly converted to ὀδυσσεύσ, instead of incorrect ὈΔΥΣΣΕΎΣ
     where Σ is converted to σ or to ς, according to position in the word
Such type of conversion just can't be done using std::toupper that work on character base, also std::toupper is 
not even applicable when working with variable character length like in UTF-8 or UTF-16 limiting the correct 
behavior to unicode subset BMP or ASCII only

强调部分：

   German grüßen correctly converted to GRÜßEN, instead of incorrect GRüßEN
     where ß is replaced with SS

我真的不明白这句话是怎么回事。实际的预期行为是什么？

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