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Documentation and Summary for std::wstring Constructor and MultiByteToWideChar

std::wstring(size_t count, wchar_t ch)

Summary

The std::wstring(size_t count, wchar_t ch) constructor is a member of the C++ Standard Library's std::basic_string class, specialized for wide characters (wchar_t) as std::wstring. It creates a wide string of length count, where each character is initialized to the specified wide character ch. In the provided code, it is used to allocate a std::wstring of size size - 1 filled with null characters (0) to prepare a buffer for the MultiByteToWideChar conversion.

Documentation

• Function Signature: std::wstring::basic_string(size_t count, wchar_t ch, const Allocator& alloc = Allocator())
• Parameters:
  • count: The number of characters in the resulting string.
  • ch: The wide character (wchar_t) to initialize each position in the string.
  • alloc: (Optional) The allocator object used for memory management (defaults to the default allocator).
• Return Value: A std::wstring object of length count, with each character initialized to ch.
• Behavior:
  • Constructs a wide string with count copies of the character ch.
  • Allocates sufficient storage for the string using the provided or default allocator.
  • If count is zero, an empty string is constructed.
• Exceptions:
  • May throw std::length_error if count exceeds the maximum size of the string (max_size()).
  • May throw exceptions from the allocator if memory allocation fails.
• Reference: cppreference.com - std::basic_string constructor (see constructor (2)).
• Example Usage in Code:

  std::wstring result(size - 1, 0); // Creates a wstring of size-1 characters, all initialized to 0

MultiByteToWideChar

Summary

MultiByteToWideChar is a Windows API function that converts a string from a specified code page to a wide-character (Unicode UTF-16) string. In the provided code, it is used to determine the required size for the output std::wstring and then to perform the conversion of a null-terminated const char* string (using the ANSI code page, CP_ACP) into a std::wstring.

Documentation

• Function Signature:

  int MultiByteToWideChar(
      UINT CodePage,
      DWORD dwFlags,
      LPCCH lpMultiByteStr,
      int cbMultiByte,
      LPWSTR lpWideCharStr,
      int cchWideChar
  );

• Parameters:
  • CodePage: The code page to use for the conversion (e.g., CP_ACP for the system's ANSI code page, CP_UTF8 for UTF-8).
  • dwFlags: Flags controlling the conversion (e.g., 0 for default behavior, or flags like MB_PRECOMPOSED).
  • lpMultiByteStr: Pointer to the input multi-byte string to convert.
  • cbMultiByte: Size (in bytes) of the input string. If -1, the function assumes a null-terminated string and calculates the length.
  • lpWideCharStr: Pointer to the output buffer for the wide-character string. If nullptr, the function returns the required buffer size.
  • cchWideChar: Size (in wide characters) of the output buffer. If 0, the function returns the required size without performing the conversion.
• Return Value:
  • If cchWideChar is 0, returns the required size (in wide characters) for the output buffer, including the null terminator.
  • Otherwise, returns the number of wide characters written to lpWideCharStr (excluding the null terminator).
  • Returns 0 on error; use GetLastError() to retrieve the error code.
• Behavior:
  • Converts a multi-byte string to a wide-character (Unicode UTF-16) string based on the specified code page.
  • When called with lpWideCharStr = nullptr and cchWideChar = 0, it computes the required size for the output buffer.
  • When called with a valid lpWideCharStr and cchWideChar, it performs the conversion and writes the result to the output buffer.
• Exceptions: Does not throw exceptions but returns 0 on failure, with error details available via GetLastError().
• Reference: Microsoft Learn - MultiByteToWideChar
• Example Usage in Code:

  int size = MultiByteToWideChar(CP_ACP, 0, str, -1, nullptr, 0); // Get required size
  MultiByteToWideChar(CP_ACP, 0, str, -1, &result[0], size); // Perform conversion