问题
考虑以下向量：
std::vector<std::string> extensions;
extensions.push_back(".cpp");
extensions.push_back(".CPP");
extensions.push_back(".h");
extensions.push_back(".H");
std::vector<std::string> caselessUniqueExtensions;
将扩展内容复制到caselessUniqueExtensions中，但丢弃所有唯一扩展名，忽略大小写。 （即，结果应该包含两个元素 - .cpp或.CPP之一，以及.h和.H之一。）
问题
显然有很多方法可以做到这一点，有些方法比其他方式更有效，有些方式比其他方式更具可读性。我认为可能有用的一种方法如下：
#include <boost/algorithm/string/compare.hpp>
...
std::unique_copy(
extensions.begin(), extensions.end(),
caselessUniqueExts.begin(),
boost::is_iequal());
但是，它无法使用MSVS2010进行编译：
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(626): error C2440: 'type cast' : cannot convert from 'unsigned char' to 'std::basic_string<_Elem,_Traits,_Ax>'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(2211) : see reference to function template instantiation '_Elem std::_Maklocchr<_Elem>(char,_Elem *,const std::_Locinfo::_Cvtvec &)' being compiled
1> with
1> [
1> _Elem=std::basic_string<char,std::char_traits<char>,std::allocator<char>>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(2210) : while compiling class template member function 'std::basic_string<_Elem,_Traits,_Ax> std::ctype<std::basic_string<_Elem,_Traits,_Ax>>::do_widen(char) const'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\locale(261) : see reference to class template instantiation 'std::ctype<_Elem>' being compiled
1> with
1> [
1> _Elem=std::basic_string<char,std::char_traits<char>,std::allocator<char>>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\locale(259) : see reference to function template instantiation '_Elem std::toupper<T1>(_Elem,const std::locale &)' being compiled
1> with
1> [
1> _Elem=std::basic_string<char,std::char_traits<char>,std::allocator<char>>,
1> T1=std::basic_string<char,std::char_traits<char>,std::allocator<char>>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(2042) : see reference to function template instantiation 'bool boost::algorithm::is_iequal::operator ()<std::basic_string<_Elem,_Traits,_Ax>,std::basic_string<_Elem,_Traits,_Ax>>(const T1 &,const T2 &) const' being compiled
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>,
1> T1=std::basic_string<char,std::char_traits<char>,std::allocator<char>>,
1> T2=std::basic_string<char,std::char_traits<char>,std::allocator<char>>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(2070) : see reference to function template instantiation '_OutIt std::_Unique_copy<_InIt,_OutIt,_Pr>(_FwdIt,_FwdIt,_OutIt,_Pr,std::forward_iterator_tag)' being compiled
1> with
1> [
1> _OutIt=std::_Vector_iterator<std::_Vector_val<std::string,std::allocator<std::string>>>,
1> _InIt=std::basic_string<char,std::char_traits<char>,std::allocator<char>> *,
1> _Pr=boost::algorithm::is_iequal,
1> _FwdIt=std::basic_string<char,std::char_traits<char>,std::allocator<char>> *
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(2094) : see reference to function template instantiation '_OutIt std::_Unique_copy1<std::basic_string<_Elem,_Traits,_Ax>*,_OutIt,_Pr>(_InIt,_InIt,_OutIt,_Pr,std::tr1::true_type)' being compiled
1> with
1> [
1> _OutIt=std::_Vector_iterator<std::_Vector_val<std::string,std::allocator<std::string>>>,
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>,
1> _Pr=boost::algorithm::is_iequal,
1> _InIt=std::basic_string<char,std::char_traits<char>,std::allocator<char>> *
1> ]
1> c:\users\stv04463\documents\visual studio 2010\projects\pgp sandbox\pgp sandbox\main.cpp(64) : see reference to function template instantiation '_OutIt std::unique_copy<std::_Vector_iterator<_Myvec>,std::_Vector_iterator<_Myvec>,boost::algorithm::is_iequal>(_InIt,_InIt,_OutIt,_Pr)' being compiled
1> with
1> [
1> _OutIt=std::_Vector_iterator<std::_Vector_val<std::string,std::allocator<std::string>>>,
1> _Myvec=std::_Vector_val<std::string,std::allocator<std::string>>,
1> _InIt=std::_Vector_iterator<std::_Vector_val<std::string,std::allocator<std::string>>>,
1> _Pr=boost::algorithm::is_iequal
1> ]
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(593): error C2440: 'type cast' : cannot convert from 'std::basic_string<_Elem,_Traits,_Ax>' to 'unsigned char'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(2229) : see reference to function template instantiation 'char std::_Maklocbyte<std::basic_string<_Elem,_Traits,_Ax>>(std::basic_string<_Elem,_Traits,_Ax>,const std::_Locinfo::_Cvtvec &)' being compiled
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\xlocale(2225) : while compiling class template member function 'char std::ctype<_Elem>::_Donarrow(_Elem,char) const'
1> with
1> [
1> _Elem=std::basic_string<char,std::char_traits<char>,std::allocator<char>>
1> ]
我是否遗漏了一些基本的理解，或者是否无法将std::unique_copy与boost::is_iequal合并？
我知道其他选择 - 我想知道为什么我得到这个编译错误。
答案 0 :(得分：2)
boost::is_iequal谓词用于单个字符，而不是像你一样在整个字符串上使用。
不幸的是，Boost字符串算法文档不是最好的，但你可以看到这个指定的here：“定义元素比较谓词”，其中“element”指的是字符串字符。
在内部，boost::is_iequal在其参数上调用std::toupper，这只能用于字符。