Annex C (informative) Compatibility [diff]

C.1 C++ and ISO C++ 2017 [diff.cpp17]

C.1.13 [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 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 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 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