[diff.cpp17.input.output]

Affected subclause: [istream.extractors]

Change: Character array extraction only takes array types.

Rationale: Increase safety via preventing buffer overflow at compile time.

Effect on original feature: Valid C++ 2017 code may fail to compile in this revision of C++.

[Example 1: auto p = new char[100]; char q[100]; std::cin >> std::setw(20) >> p; / ill-formed; previously well-formed std::cin >> std::setw(20) >> q; / OK — end example]

Affected subclause: [ostream.inserters.character]

Change: Overload resolution for ostream inserters used with UTF-8 literals.

Rationale: Required for new features.

Effect on original feature: Valid C++ 2017 code that passes UTF-8 literals to basic_ostream<char, ..>::operator< or basic_ostream<wchar_t, ..>::operator< is now ill-formed.

[Example 2: std::cout << u8"text"; / previously called operator<<(const char*) and printed a string; / now ill-formed std::cout << u8'X'; / previously called operator<<(char) and printed a character; / now ill-formed — end example]

Affected subclause: [ostream.inserters.character]

Change: Overload resolution for ostream inserters used with wchar_t, char16_t, or char32_t types.

Rationale: Removal of surprising behavior.

Effect on original feature: Valid C++ 2017 code that passes wchar_t, char16_t, or char32_t characters or strings to basic_ostream<char, ..>::operator< or that passes char16_t or char32_t characters or strings to basic_ostream<wchar_t, ..>::operator< is now ill-formed.

[Example 3: std::cout << u"text"; / previously formatted the string as a pointer value; / now ill-formed std::cout << u'X'; / previously formatted the character as an integer value; / now ill-formed — end example]

Affected subclause: [fs.class.path]

Change: Return type of filesystem path format observer member functions.

Rationale: Required for new features.

Effect on original feature: Valid C++ 2017 code that depends on the u8string() and generic_u8string() member functions of std::filesystem::path returning std::string is not valid in this revision of C++.

[Example 4: std::filesystem::path p; std::string s1 = p.u8string(); / ill-formed; previously well-formed std::string s2 = p.generic_u8string(); / ill-formed; previously well-formed — end example]