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 [AT] imag.fr>
COPYRIGHT
Copyright (C) 2000-2018 Pierre Saramito <Pierre.Saramito [AT] 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.