Re: Compiler Generated Default Functions

"kanze" <>
26 Jul 2006 09:48:29 -0400
<> wrote:

In a recent discussion, some of us were in disagreement about
the functions the C++ compiler generates. How many functions
are generated by the compiler when you declare:

class Foo
}; ?

I thought 4. As in:

class Foo
  Foo(); // default constructor
  Foo(const Foo& f); // default copy constructor

  ~Foo(); // default destructor
  const Foo &operator(const Foo& f); // default assigment operator

I presume you mean:

    Foo const& operator=( Foo const& f ) ;

here. (The order of const and Foo doesn't make a difference to
the compiler, but if you leave out the = sign, the code is


Is this list correct? Is that all?

Yes and yes.

Someone said you must also add the new and delete operators,
so that's 6.

I'm not sure where he got this. You can add user defined
operator new and operator delete functions, but if they are not
present, the compiler uses the global ones; it does not generate
its own versions.

I have not read the standard, but I'd argue that if this is
so, then you should also count operator* () and operator& ()

The operator& is the hard one. It's defined for all objects, of
all types. The standard doesn't consider it an implicitly
defined operator; it says that the built in operator is used if
there is no user defined one. And in fact, there is no way to
define the operator to give the semantics of the built-in

Technically, when the compiler implicitly generates a function,
it behaves like a user defined function; the compiler generated
operator=, for example, has an address, participates in overload
resolution, and introduces sequence points. The global
operator& which the compiler uses if their is not a user defined
one requires an lvalue (which a user defined function cannot),
doesn't have an address, and doesn't introduce sequence points.

The operator new and operator delete are another kettle of fish,
because they are always functions. However, the way they work
with overload resolution should make it clear that there are no
default versions generated as class members:

    struct A
        void* operator new( size_t, int ) ;

    A* p = new A ; // Error, the member operator hides the
                        // global version (but would not hide a
                        // compiler generated member).


    extern void* operator new( size_t, int ) ;

    struct A {} ;

    A* p = new A ; // Not an error, although a compiler
                        // generated default would hide the
                        // global operator.

    void (*pf)( size_t ) = &A::operator new ;
                        // Error, because there is no
                        // A::operator new, not even one
                        // implicitly defined by the compiler.
                        // (initialization with &::operator new
                        // is legal, however.)

James Kanze GABI Software
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