Annex C (informative) Compatibility [diff]

C.5 C++ and ISO C++ 2017 [diff.cpp17]

C.5.12 [input.output]: input/output library [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 International Standard:
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
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 ISO C++ 2017 code that passes UTF-8 literals to basic_­ostream<char, ...>​::​operator<< or basic_­ostream<wchar_­t, ...>​::​operator<< is now ill-formed.
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
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 ISO 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.
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
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 ISO 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 International Standard.
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