/**

\page encoding_helper Encoding Helper

YYCC::EncodingHelper namespace include all encoding related functions:

\li The convertion between ordinary string and UTF8 string which has been introduced in chapter \ref library_encoding.
\li Windows specific convertion between \c WCHAR, UTF8 string and string encoded by other encoding.
\li The convertion among UTF8, UTF16 and UTF32.

\section encoding_helper__ordinary_utf8_conv Ordinary & UTF8 Convertion

These convertion functions have been introduced in previous page.
See \ref library_encoding for more infomation.

YYCC supports following convertions:

\li YYCC::EncodingHelper::ToUTF8: Convert ordinary string to UTF8 string.
\li YYCC::EncodingHelper::ToUTF8View: Same as ToUTF8, but return string view instead.
\li YYCC::EncodingHelper::ToOrdinary: Convert UTF8 string to ordinary string.
\li YYCC::EncodingHelper::ToOrdinaryView: Same as ToOrdinary, but return string view instead.

\section encoding_helper__win_conv Windows Specific Convertion

During Windows programming, the convertion between Microsoft specified \c wchar_t and \c char is an essential operation.
Because Windows has 2 different function system, the functions ended with A and the functions ended with W.
(Microsoft specified \c wchar_t is \c 2 bytes long. It's different with Linux defined common 4 bytes long).
Thus YYCC provides these convertion functions in Windows to help programmer have better programming experience.

These functions are Windows specific, so they will be invisible in other platforms.
Please use them carefully (make sure that you are using them only in Windows environment).

YYCC supports following convertions:

\li YYCC::EncodingHelper::WcharToChar: Convert \c wchar_t string to code page specified string.
\li YYCC::EncodingHelper::CharToWchar: The reversed convertion of WcharToChar.
\li YYCC::EncodingHelper::CharToChar: Convert string between 2 different code pages. It's a shortcut of calling CharToWchar and WcharToChar successively.
\li YYCC::EncodingHelper::WcharToUTF8: Convert \c wchar_t string to UTF8 string.
\li YYCC::EncodingHelper::UTF8ToWchar: The reversed convertion of WcharToUTF8.
\li YYCC::EncodingHelper::CharToUTF8: Convert code page specified string to UTF8 string.
\li YYCC::EncodingHelper::UTF8ToChar: The reversed convertion of CharToUTF8.

Code Page is a Windows concept.
If you don't understand it, please view corresponding Microsoft documentation.

\section encoding_helper__utf_conv UTF8 UTF16 UTF32 Convertion

The convertion between UTF8, UTF16 and UTF32 is not common but essential.
These convertions can be achieved by standard library functions and classes.
(they are actually done by standard library functions in our implementation)
But we provided functions are easy to use and have clear interface.

These functions are different with the functions introduced above.
They can be used in any platform, not confined in Windows platforms.

YYCC supports following convertions:

\li YYCC::EncodingHelper::UTF8ToUTF16: Convert UTF8 string to UTF16 string.
\li YYCC::EncodingHelper::UTF16ToUTF8: The reversed convertion of UTF8ToUTF16.
\li YYCC::EncodingHelper::UTF8ToUTF32: Convert UTF8 string to UTF32 string.
\li YYCC::EncodingHelper::UTF32ToUTF8: The reversed convertion of UTF8ToUTF32.

\section encoding_helper__overloads Function Overloads

Every encoding convertion functions (except the convertion between UTF8 and ordinary string) have 4 different overloads for different scenarios.
Take YYCC::EncodingHelper::WcharToChar for example.
There are following 4 overloads:

\code
bool WcharToChar(const std::wstring_view& src, std::string& dst, UINT code_page);
bool WcharToChar(const wchar_t* src, std::string& dst, UINT code_page);
std::string WcharToChar(const std::wstring_view& src, UINT code_page);
std::string WcharToChar(const wchar_t* src, UINT code_page);
\endcode

\subsection encoding_helper__overloads_destination Destination String

According to the return value, these 4 overload can be divided into 2 types.
The first type returns bool. The second type returns \c std::string instance.

For the first type, it always return bool to indicate whether the convertion is success.
Due to this, the function must require an argument for holding the result string.
So you can see the functions belonging to this type always require a reference to \c std::string in argument.

Oppositely, the second directly returns result by return value.
It doesn't care the success of convertion and will return empty string if convertion failed.
Programmer can more naturally use it because the retuen value itself is the result.
There is no need to declare a variable before calling convertion function for holding result.

All in all, the first type overload should be used in strict scope.
The success of convertion will massively affect the behavior of your following code.
For example, the convertion code is delivered to some system function and it should not be empty and etc.
The second type overload usually is used in lossen scenarios.
For exmaple, this overload usually is used in console output because it usually doesn't matter.
There is no risk even if the convertion failed (just output a blank string).

For the first type, please note that there is \b NO guarantee that the argument holding return value is not changed.
Even the convertion is failed, the argument holding return value may still be changed by function itself.

In this case, the type of result is \c std::string because this is function required.
In other functions, such as YYCC::EncodingHelper::WcharToUTF8, the type of result can be \c yycc_u8string or etc.
So please note the type of result is decided by convertion function itself, not only \c std::string.

\subsection encoding_helper__overloads__source Source String

According to the way providing source string,
these 4 overload also can be divided into 2 types.
The first type take a reference to constant \c std::wstring_view.
The second type take a pointer to constant wchar_t.

For first type, it will take the whole string for convertion, including \b embedded NUL terminal.
Please note we use string view as argument.
It is compatible with corresponding raw string pointer and string container.
So it is safe to directly pass \c std::wstring for this function.

For second type, it will assume that you passed argument is a NUL terminated string and send it for convertion.

The result is clear.
If you want to process string with \b embedded NUL terminal, please choose first type overload.
Otherwise the second type overload is enough.

Same as destination string, the type of source is also decided by the convertion function itself.
For exmaple, the type of source in YYCC::EncodingHelper::UTF8ToWchar is \c yycc_u8string_view and \c yycc_char8_t, 
not \c std::wstring and \c wchar_t.

\subsection encoding_helper__overloads__extra Extra Argument

There is an extra argument called \c code_page for YYCC::EncodingHelper::WcharToChar.
It indicates the code page of destination string,
because this function will convert \c wchar_t string to the string with specified code page encoding.

Some convertion functions have extra argument like this,
because they need more infomations to decide what they need to do.
Some convertion functions don't have extra argument.
For exmaple, the convertion between \c wchar_t string and UTF8 string.
Because both source string and destination string are concrete.
There is no need to provide any more infomations.

\subsection encoding_helper__overloads__conclusion Conclusion

Mixing 2 types of source string and 2 types of destination string,
we have 4 different overload as we illustrated before.
Programmer can use them freely according to your requirements.
And don't forget to provide extra argument if function required.

*/