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hi(pointer: to member: defining)
Pointers to members are defined by prefixing the normal pointer notation
with the appropriate class plus scope resolution operator. Therefore, in the
previous section, we used tt(char const * (String::*d_sp)() const) to indicate
that tt(d_sp)
itemization(
it() is a pointer (tt(*d_sp));
it() points to something in the class tt(String) (tt(String::*d_sp));
it() is a pointer to a tt(const) function, returning a tt(char const *)
(tt(char const * (String::*d_sp)() const)).
)
The prototype of a matching function is therefore:
verb( char const *String::somefun() const;)
which is any tt(const) parameterless function in the class
tt(String), returning a tt(char const *).
When defining pointers to members the standard procedure for constructing
pointers to functions can still be applied:
itemization(
it() put parentheses around the fully qualified function name (i.e., the
function's header, including the function's class name):
verb(char const * ( String::somefun ) () const)
it() Put a pointer (a star (tt(*))) character immediately before the
function name itself:
verb(char const * ( String:: * somefun ) () const)
it() Replace the function name with the name of the pointer variable:
verb(char const * (String::*d_sp)() const)
)
hi(pointer: to data member)
Here is another example, defining a pointer
to a data member. Assume the class tt(String) contains a tt(string d_text)
member. How to construct a pointer to this member? Again we follow standard
procedure:
itemization(
it() put parentheses around the fully qualified variable name:
verb(std::string (String::d_text))
it() Put a pointer (a star (tt(*))) character immediately before the
variable-name itself:
verb(std::string (String::*d_text))
it() Replace the variable name with the name of the pointer variable:
verb(std::string (String::*tp))
In this case, the parentheses are superfluous and may be omitted:
verb(string String::*tp)
)
Alternatively, a very simple i(rule of thumb) is
itemization(
it() Define a normal (i.e., global) pointer variable,
it() Prefix the class name to the pointer character, once you point to
something inside a class
)
For example, the following pointer to a global function
verb( char const * (*sp)() const;)
becomes a pointer to a member function after prefixing the class-scope:
verb( char const * (String::*sp)() const;)
Nothing forces us to define pointers to members in their target
(tt(String)) classes. Pointers to members em(may) be defined in their target
classes (so they become data members), or in another class, or as a local
variable or as a global variable. In all these cases the pointer to member
variable can be given the address of the kind of member it points to. The
important part is that a pointer to member can be initialized or assigned
without requiring the existence of an object of the pointer's target class.
Initializing or assigning an address to such a pointer merely indicates to
which member the pointer points. This can be considered some kind of
emi(relative address); relative to the object for which the function is
called. No object is required when pointers to members are initialized or
assigned. While it is allowed to initialize or assign a pointer to member, it
is (of course) not possible to em(call) those members without specifying an
object of the correct type.
hi(pointer: to member: assignment)
hi(assignment: pointer to member)
In the following example initialization of and assignment to
pointers to members is illustrated (for illustration purposes all members of
the class tt(PointerDemo) are defined tt(public)). In the example itself the
tt(&)-operator is used to determine the addresses of the members. These
operators as well as the class-scopes are required. Even when used inside
member implementations:
verbinclude(-a examples/initializing.cc)
This involves nothing special. The difference with pointers at
global scope is that we're now restricting ourselves to the scope of the
tt(PointerDemo) class. Because of this restriction, all em(pointer)
definitions and all variables whose addresses are used must be given the
tt(PointerDemo) class scope.
hi(pointer: to virtual member)
Pointers to members can also be used with tt(virtual)
member functions. No special syntax is required when pointing to virtual
members. Pointer construction, initialization and assignment is done
identically to the way it is done with non-virtual members.
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