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In C++, the STL includes a set of standard library classes, functions, and algorithms that simplify common tasks, including string manipulation. To convert UTF-8 strings to and from wide characters (wchar_t) in C++, we can use the following STL classes and functions:

1. std::wstringconvert - This class template provides a conversion between wide character strings and byte strings in a specified encoding. We can use std::wstringconvert to convert between UTF-8 and wide characters.

2. std::wstring - This class represents a sequence of wide characters. We can use std::wstring to store and manipulate wide character strings.

Here's an example of how to convert UTF-8 to wide characters using STL:

#include <codecvt>
#include <locale>
#include <string>

std::wstring utf8toWide(const std::string& str)
{
    std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
    return converter.from_bytes(str);
}

In this example, we use the std::codecvtutf8<wchar_t> facet to specify that we want to convert from UTF-8. We then create a std::wstringconvert object using this facet and call its from_bytes() method to convert the input UTF-8 string to wide characters.

Here's an example of how to convert wide characters to UTF-8 using STL:

std::string wideToUtf8(const std::wstring& str)
{
    std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
    return converter.to_bytes(str);
}

In this example, we use the same std::codecvtutf8<wchar_t> facet to specify that we want to convert to UTF-8. We then create a std::wstringconvert object using this facet and call its to_bytes() method to convert the input wide character string to UTF-8.