Annex C (informative) Compatibility [diff]

Change: pp-number can contain one or more single quotes.

Rationale: Necessary to enable single quotes as digit separators.

Effect on original feature: Valid C++ 2011 code may fail to compile or may change meaning in this International Standard.

For example, the following code is valid both in C++ 2011 and in this International Standard, but the macro invocation produces different outcomes because the single quotes delimit a character-literal in C++ 2011, whereas they are digit separators in this International Standard:

#define M(x, ...) __VA_ARGS__
int x[2] = { M(1'2,3'4, 5) };
// int x[2] = { 5 };      --- C++ 2011
// int x[2] = { 3'4, 5 }; --- this International Standard

Change: New usual (non-placement) deallocator.

Rationale: Required for sized deallocation.

Effect on original feature: Valid C++ 2011 code could declare a global placement allocation function and deallocation function as follows:

void* operator new(std::size_t, std::size_t);
void operator delete(void*, std::size_t) noexcept;

In this International Standard, however, the declaration of operator delete might match a predefined usual (non-placement) operator delete ([basic.stc.dynamic]).

If so, the program is ill-formed, as it was for class member allocation functions and deallocation functions ([expr.new]).

Change: A conditional expression with a throw expression as its second or third operand keeps the type and value category of the other operand.

Rationale: Formerly mandated conversions (lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions), especially the creation of the temporary due to lvalue-to-rvalue conversion, were considered gratuitous and surprising.

Effect on original feature: Valid C++ 2011 code that relies on the conversions may behave differently in this International Standard:

struct S {
  int x = 1;
  void mf() { x = 2; }
};
int f(bool cond) {
  S s;
  (cond ? s : throw 0).mf();
  return s.x;
}

In C++ 2011, f(true) returns 1.

In this International Standard, it returns 2.

sizeof(true ? "" : throw 0)

In C++ 2011, the expression yields sizeof(const char*).

In this International Standard, it yields sizeof(const char[1]).

Change: constexpr non-static member functions are not implicitly const member functions.

Rationale: Necessary to allow constexpr member functions to mutate the object.

Effect on original feature: Valid C++ 2011 code may fail to compile in this International Standard.

For example, the following code is valid in C++ 2011 but invalid in this International Standard because it declares the same member function twice with different return types:

struct S {
  constexpr const int &f();
  int &f();
};

Change: Classes with default member initializers can be aggregates.

Rationale: Necessary to allow default member initializers to be used by aggregate initialization.

Effect on original feature: Valid C++ 2011 code may fail to compile or may change meaning in this International Standard.

For example:

struct S {          // Aggregate in C++ 2014 onwards.
  int m = 1;
};
struct X {
  operator int();
  operator S();
};
X a{};
S b{a};             // uses copy constructor in C++ 2011,
                    // performs aggregate initialization in this International Standard

Change: New header.

Rationale: New functionality.

Effect on original feature: The C++ header <shared_­mutex> ([shared.mutex.syn]) is new.

Valid C++ 2011 code that #includes a header with that name may be invalid in this International Standard.

Change: gets is not defined.

Rationale: Use of gets is considered dangerous.

Effect on original feature: Valid C++ 2011 code that uses the gets function may fail to compile in this International Standard.