充分利用universal_string类的特点

在程序开发过程中，程序员通常要处理很多不同的字符串类型(如std::string，std::wstring等)，这些过程会涉及到很多字符串的转换问题。然而，这里提供了一种简单而又有效的方法，当碰到下面情况时可以避免调用很多字符串的转换函数：

universal_string类

convert_string 函数(能够把一个字符类型转换为其他类型)使用很方便，但是universal_string类使用起来更加简单。universal_string 类在内部上存放了两种字符类型：通用字符(const char *)以及广义字符(const wchar_t *)。

universal_string 类提供了以下的优点：

• 构造函数中有指向字符指针的字符串(e.g., const char *, const signed char *, const wchar_t *, etc.)。
• 构造函数中有所有字符转换的函数(e.g., std::basic_string< char>, std::basic_string< wchar_t>, etc.)。
• 使用=操作符可以很方便地重新更新字符串。

下面是universal_string 类的代码：

#include <string> #include <assert.h> template< class FromCharType, class ToCharType> std::basic_string< ToCharType> convert_string_helper( const std::basic_string< FromCharType> & strSource) { std::basic_string< ToCharType> strDest; int nSourceLen = strSource.length(); // make sure we hold memory for the string strDest.resize( nSourceLen); for ( int idxChar = 0; idxChar < nSourceLen; idxChar++) { strDest[ idxChar] = ( ToCharType)strSource[ idxChar]; } return strDest; } // helpers template< class CharType> inline const std::basic_string< CharType> & universal_string_value_helper( const std::string & strNarrow, const std::wstring & strWide) { return ( const std::basic_string< CharType> &)strNarrow; } template<> inline const std::wstring & universal_string_value_helper< wchar_t>( const std::string & strNarrow, const std::wstring & strWide) { return strWide; } // universal string - once initialized from either // a 'const [signed/ unsigned] char *' / 'const wchar_t *' // string, you can ask for any string class universal_string { public: // construction: // - from any std::basic_strings // - from any 'const char_type *' strings template< class CharType> explicit universal_string( const std::basic_string< CharType> & str) { Init( str); } template< class CharType> explicit universal_string( const CharType * str) { Init( str); } universal_string() {} // operator=: // - from any std::basic_strings // - from any 'const char_type *' strings // - from universal_string template< class CharType> void operator=( const CharType * str) { Init( str); } template< class CharType> void operator=( const std::basic_string< CharType> & str) { Init( str); } void operator=( const universal_string & val) { m_strNarrow = val.m_strNarrow; m_strWide = val.m_strWide; } template< class CharType> const std::basic_string< CharType> & value() const { return universal_string_value_helper< CharType>( GetNarrow(), GetWide()); } private: template< class CharType> void InitFromPtr( const CharType * wstr, const wchar_t * ignored) { m_strWide = wstr; m_strNarrow = convert_string_helper< wchar_t, char>( m_strWide); } template< class CharType> void InitFromPtr( const CharType * str, ...) { m_strNarrow = ( const char *)str; m_strWide = convert_string_helper< char, wchar_t>( m_strNarrow); } template< class CharType> void InitFromString( const std::basic_string< CharType> & wstr, const wchar_t * ignored) { m_strWide = wstr; m_strNarrow = convert_string_helper< wchar_t, char>( m_strWide); } template< class CharType> void InitFromString( const std::basic_string< CharType> & str, ...) { m_strNarrow = ( const std::basic_string< char> &)str; m_strWide = convert_string_helper< char, wchar_t>( m_strNarrow); } template< class CharType> void Init( const CharType * str) { InitFromPtr( str, ( const CharType *)NULL); } template< class CharType> void Init( const std::basic_string< CharType> & str) { InitFromString( str, ( const CharType *) NULL); } protected: const std::string & GetNarrowStr() const { return m_strNarrow; } const std::wstring & GetWideStr() const { return m_strWide; } private: std::string m_strNarrow; std::wstring m_strWide; }; // let's say we have: // universal_string str( "c:temp.txt") // // // non VC-users can use the following: // // std::cout << str.value< char>(); // std::wcout << str.value< wchar_t>(); // VC has a bug; therefore we use a workaround // (the following code is used only for VC // // std::cout << universal_string_val< char>( str); // std::wcout << universal_string_val< wchar_t>( str); template< class CharType> class universal_string_t : public universal_string { public: static const std::basic_string< CharType> & value( const universal_string & str) { return (( const universal_string_t< CharType> &)str).str_value(); } private: const std::basic_string< CharType> & str_value() const { return universal_string_value_helper< CharType>( GetNarrowStr(), GetWideStr()); } }; template< class CharType> const std::basic_string< CharType> & universal_string_val( const universal_string & val) { return universal_string_t< CharType>::value( val); }

下面是个例子:

#include <iostream> #include <string> #include <fstream> std::wstring read_file_name() { std::wcout << L"Enter a file name: "; std::wstring strFileName; std::wcin >> strFileName; return strFileName; } int main(int argc, char* argv[]) { // Let's say we're working with wchar_t characters // and we're trying to open a file, to read it universal_string strFileName; strFileName = read_file_name(); // there is no .open() function taking a wchar_t string std::wfstream file; file.open( universal_string_val< char>( strFileName).c_str()); // ... etc. file.close(); // printing universal_string str( "from const char *"); std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = ( unsigned char *)"from unsigned char *"; std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = ( signed char *)"from signed char *"; std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = L"from wchar_t *"; std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = std::basic_string< char>( "from char string"); std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = std::basic_string< unsigned char>( ( unsigned char *)"from unsigned char string"); std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = std::basic_string< signed char>( ( signed char *)"from signed char string"); std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; str = std::basic_string< wchar_t>( L"from wchar_t string"); std::cout << universal_string_val< char>( str) << std::endl; std::wcout << universal_string_val< wchar_t>( str) << std::endl; return 0; }