Provided by: librheolef-dev_7.2-2_amd64 
NAME
smart_pointer - with true copy semantic (rheolef-7.2)
DESCRIPTION
Here is a convenient way to implement a true copy semantic, by using shallow copies and
reference counting, in order to minimise memory copies. This concept is generally related
to the smart pointer method for managing memory.
The true semantic copy is defined as follows: if an object A is assigned to B, such as A =
B, every further modification on A or B does not modify the other.
Notice that this class differs from the std::shared_ptr class that implements safe
pointers without the true copy semantic.
CLONE VARIANT
The smart_pointer_clone variant uses a T* T::clone() const member function instead of the
usual T::T() copy constructor for obtaining a true copy of the data. This variant is
motivated as follows: when using hierarchies of derived classes (also known as polymorphic
classes), the usual copy is not possible because c++ copy constructors cannot be virtual,
so you cannot make a copy this way. This is a well-known problem with C++'s implementation
of polymorphism.
We uses a solution to the non-virtual copy constructor problem which is suggested by Ellis
and Stroustrup in 'The Annotated LRM'. The solution is to require the 'T' class to provide
a virtual clone method for every class which makes a copy using new and the correct copy
constructor, returning the result as a pointer to the superclass 'T'. Each subclass of 'T'
overloads this function with its own variant which copies its own type. Thus the copy
operation is now virtual and furthermore is localized to the individual subclass.
NOCOPY VARIANT
The smart_pointer_nocopy variant is designed for use on objects that cannot (or must not)
be copied. An example would be when managing an object that contains, say, a file handle.
It is essential that this not be copied because then you get the problem of deciding which
copy is responsible for closing the file. To avoid the problem, wrap the file handle in a
class and then manage a unique instance of it using a smart_pointer_nocopy. This ensures
that the file handle cannot be copied and is closed when the last alias is destroyed.
The interface to the nocopy variant is the same as smart_pointer but with all operations
that perform copying forbidden. In fact, because all three variants are instances of a
common superclass, the forbidden methods do exist but will cause an error and exit if they
are called.
The following modifiers cannot be used because they use copying of the pointed-to object
and will therefore cause an error:
T* operator-> ();
T& operator* ();
T* pointer ();
T& data ();
REFERENCE
[1] A. Geron and F. Tawbi, Pour mieux developer avec C++ : design pattern, STL, RTTI et
smart pointers, InterEditions, 1999. Page 118. [2] STLplus: clone and nocopy variants,
http://stlplus.sourceforge.net/stlplus3/docs/smart_ptr.html
EXAMPLE
// data representation (could be file "container_data.h")
typedef int T;
class container_data {
private:
T *values;
int n;
public:
container_data (const container_data& x)
: values(new T[x.n]), n(x.n)
{ for (int i=0; i<n;i++) values[i]=x.values[i];}
container_data& operator= (const container_data& x) {
n = x.n;
values = new T[n];
for (int i=0; i<n;i++) values[i]=x.values[i];
return *this;
}
// a customized constructor
explicit container_data(int n1)
: values(new T[n1]), n(n1) {}
~container_data() { delete [] values; }
// read and write accessors are separated
const T& operator[](int i) const
{ return values[i]; }
T& operator[](int i)
{ return values[i]; }
};
// an interface to data via the Objet class
// that count occurrence (could be "container.h")
//
class container : private smart_pointer<container_data> {
public:
// the customized cstor
explicit container(int n = 0);
// read/write accessors
const T& operator[](int i) const;
T& operator[](int i);
};
// here is the implementation of the interface
// (could be "container.c")
//
container::container (int n)
: smart_pointer<container_data> (new container_data(n))
{}
const T&
container::operator[] (int i) const {
// use read access data()
return data().operator[] (i);
}
T&
container::operator[] (int i) {
// use write access data() that check occurrence count
return data().operator [] (i);
}
// test program
int main() {
container A(10);
A[1] = 1;
container B = A;
B[1] = 2;
if (A[1] == B[1]) {
std::cerr << "fatal: It is not a true copy semantic." << std::endl;
exit(1);
}
std::cerr << "It seems to be a true copy semantic." << std::endl;
}
IMPLEMENTATION
This documentation has been generated from file util/lib/smart_pointer.h
template <typename T>
class smart_pointer : public smart_pointer_base<T, details::constructor_copy<T> > {
typedef details::constructor_copy<T> C;
typedef smart_pointer_base<T,C> base;
public:
typedef T handled_type;
typedef typename base::internal internal;
smart_pointer (T* p = 0) : base (p) {}
smart_pointer (void* count, internal i) : base(count,i) {}
smart_pointer (const smart_pointer<T>& x) : base(x) {}
smart_pointer<T>& operator= (const smart_pointer<T>& x) {
base::operator= (x); return *this; }
~smart_pointer() {}
};
template <typename T>
class smart_pointer_clone : public smart_pointer_base<T, details::clone_copy<T> > {
typedef details::clone_copy<T> C;
typedef smart_pointer_base<T,C> base;
public:
typedef T handled_type;
typedef typename base::internal internal;
smart_pointer_clone (T* p = 0) : base (p) {}
smart_pointer_clone (void* count, internal i) : base(count,i) {}
smart_pointer_clone (const smart_pointer_clone<T>& x) : base(x) {}
smart_pointer_clone<T>& operator= (const smart_pointer_clone<T>& x) {
base::operator= (x); return *this; }
~smart_pointer_clone() {}
};
template <typename T>
class smart_pointer_nocopy : public smart_pointer_base<T, details::no_copy<T> > {
typedef details::no_copy<T> C;
typedef smart_pointer_base<T,C> base;
public:
typedef T handled_type;
typedef typename base::internal internal;
smart_pointer_nocopy (T* p = 0) : base (p) {}
smart_pointer_nocopy (void* count, internal i) : base(count,i) {}
smart_pointer_nocopy (const smart_pointer_nocopy<T>& x) : base(x) {}
smart_pointer_nocopy<T>& operator= (const smart_pointer_nocopy<T>& x) {
base::operator= (x); return *this; }
~smart_pointer_nocopy() {}
};
AUTHOR
Pierre Saramito <Pierre.Saramito@imag.fr>
COPYRIGHT
Copyright (C) 2000-2018 Pierre Saramito <Pierre.Saramito@imag.fr> GPLv3+: GNU GPL
version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you
are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by
law.