在模板函数中使用常量字符串字面值(第二部分)

上一节，我们讨论了如何处理常量字符字面值的一些方法。在文中，可以得出，CONST_STR(CharType, string)可以扩展为const [signed/ unsigned] char *或者 const wchar_t *，但是不能同时扩展为两者。

然而，很多时候你只在一个常量字符字面值，但却把它当成多种字符串类型来访问。例如，假设一个常量字符串字面值(" f")，很多其他的函数都把它当成const char *或const unsigned char * 或const wchar_t *来访问。

把常量字符串字面值当成多种字符串类型来访问在很多时候非常有用，特别是以下的几种情况：

为了实现以上的要求，我们建立一个class (string_constant)类，这一个类内部包含了有关字符串的继承：

// DECLARATION """ // instead of something like // const char * HARD_ENTER = " "; // use the following // STRING_CONSTANT( HARD_ENTER, " ") // USAGE in code // if you're not using Visual C++, you can use any of the following // (even what Visual C++ users use) static const string_type strEnter = HARD_ENTER.value< CharType>(); static const std::string strEnter = HARD_ENTER.value< char>(); static const std::wstring strEnter = HARD_ENTER.value< wchar_t>(); // Visual C++ users static const string_type strEnter = const_str_val< CharType>( HARD_ENTER); static const std::string strEnter = const_str_val< char>( HARD_ENTER); static const std::wstring strEnter = const_str_val< wchar_t>( HARD_ENTER); // note: to maintain maximum portability, you should use the latter

以下的代码简单但又精确：

template< class CharType> inline const CharType * string_constant_value( const char * strNarrow, const wchar_t * strWide) { return ( const CharType *)strNarrow; } template<> inline const wchar_t * string_constant_value< wchar_t>( const char * strNarrow, const wchar_t * strWide) { return strWide; } // holds both 'const char *' and 'const wchar_t *' constants class string_constant { public: string_constant( const char * strNarrow, const wchar_t * strWide) : m_strNarrow( strNarrow), m_strWide( strWide) {} template< class CharType> const CharType * value() const { return string_constant_value< CharType>( GetNarrowStr(), GetWideStr()); } protected: const char * GetNarrowStr() const { return m_strNarrow; } const wchar_t * GetWideStr() const { return m_strWide; } private: const char * m_strNarrow; const wchar_t * m_strWide; }; #define STRING_CONSTANT( constant_name, constant_value) string_constant constant_name( constant_value, L##constant_value); // let's say we have: // STRING_CONSTANT( HARD_ENTER, " ") // (let's say we have 'CharType' as a templated type): // // // non VC-users can use the following: // // HARD_ENTER.value< CharType>(); // Example: HARD_ENTER.value< char>() // HARD_ENTER.value< wchar_t>() // VC has a bug; therefore we use a workaround // (the following code is used only for VC) // // const_str_val< CharType>( HARD_ENTER); // Example: const_str_val< char>( HARD_ENTER) // const_str_val< wchar_t>( HARD_ENTER) template< class CharType> class string_constant_t : public string_constant { public: static const CharType * value( const string_constant & str) { return (( const string_constant_t< CharType> &)str).str_value(); } private: const CharType * str_value() const { return string_constant_value< CharType>( GetNarrowStr(), GetWideStr()); } }; template< class CharType> const CharType * const_str_val( const string_constant & val) { return string_constant_t< CharType>::value( val); }

这里是remove_enters函数的范例：

// at beginning of file #include <string> #include <iostream> STRING_CONSTANT( HARD_ENTER, " ") // ... code template< class CharType> void remove_enters( std::basic_string< CharType> & strDest) { typedef std::basic_string< CharType> string_type; static const string_type strEnter = const_str_val< CharType>( HARD_ENTER); int idxFound = 0; while ( true) { int idxNext = strDest.find( strEnter, idxFound); if ( idxNext != std::basic_string< CharType>::npos) { strDest.erase( idxNext, strEnter.length()); idxFound = idxNext; } else { break; } } } int main(int argc, char* argv[]) { std::string strTest = "First line Second line Fourth (last) line"; std::cout << "Before removing enters" << std::endl; std::cout << "'" << strTest << "'" << std::endl; remove_enters( strTest); std::cout << "After removing enters" << std::endl; std::cout << "'" << strTest << "'" << std::endl; return 0; }