What is a call to `char()` as a function in C++?

Question

I've never seen this call to char() as a function before. Where is this described and what does it mean? This usage is part of the example on this cppreference.com community wiki page: https://en.cppreference.com/w/cpp/string/basic_string/resize:

short_string.resize( desired_length + 3 );
std::cout << "6. After:  \"";
for (char c : short_string) {
    std::cout << (c == char() ? '@' : c);  // <=== HERE ===
}

This wording in the description also doesn't make any sense to me and I don't understand what it's saying:

Initializes appended characters to CharT().

Highlighted in context:

Adjacently related

1. What motivated me to study the std::string::resize() method was trying to learn how to pre-allocate a std::string for use in C function calls as a char* buffer. This is possible by first pre-allocating the std::string by calling the my_string.resize() function on it. Then, you can safely write into &my_string[0] as a standard char* up to index my_string.size() - 1. See also:
   1. Directly write into char* buffer of std::string
   2. Is there a way to get std:string's buffer
   3. How to convert a std::string to const char* or char*
      1. See my detailed answer to this question here.

Answer 1

It returns 0 with the specified type, same as char(0). It's called value initialization.

The syntax mimics that of calling a default constructor for a class.

Answer 2

It's the constructor^† for char; with no arguments it constructs '\0'. Rarely used since primitives offer other ways to initialize them, but you initialize them with () just like you would a user-defined class, which ensures they get initialized to something; char foo; has undefined value, while char foo = char(); or char foo{}; is definitely '\0'.

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^†_{As HolyBlackCat notes, it's not technically a constructor, because it's not a class, but it behaves like one for most purposes.}