DGtal::ConstAlias< T > Class Template Reference

Aim: This class encapsulates its parameter class so that to indicate to the user that the object/pointer will be only const aliased (and hence left unchanged). Therefore the user is reminded that the argument parameter is given to the function without any additional cost and may not be modified, while he is aware that the lifetime of the argument parameter must be at least as long as the object itself. Note that an instance of ConstAlias<T> is itself a light object (it holds only an enum and a pointer). More...

#include <DGtal/base/ConstAlias.h>

Public Member Functions
	ConstAlias (const ConstAlias &other)=default
	~ConstAlias ()
	ConstAlias (const T &t)
	ConstAlias (const T *ptrT)
	ConstAlias (const CowPtr< T > &)=delete
	ConstAlias (const CountedPtr< T > &shT)
	ConstAlias (const CountedPtrOrPtr< T > &shT)
	ConstAlias (const CountedConstPtrOrConstPtr< T > &shT)
	ConstAlias (T &&)=delete
	operator const T & () const
const T *	operator& () const
	operator CountedConstPtrOrConstPtr< T > () const
const T *	operator-> () const

Protected Types
enum	Parameter {   CONST_LEFT_VALUE_REF , LEFT_VALUE_REF , PTR , CONST_PTR ,   COW_PTR , COUNTED_PTR , RIGHT_VALUE_REF , COUNTED_PTR_OR_PTR ,   COUNTED_CONST_PTR_OR_CONST_PTR }
	Internal class that allows to distinguish the different types of parameters. More...

Private Member Functions
ConstAlias &	operator= (const ConstAlias &other)

Private Attributes
const Parameter	myParam
	Characterizes the type of the input parameter at clone instanciation. More...
const void *const	myPtr
	Stores the address of the input parameter for further use. More...

Detailed Description

template<typename T>
class DGtal::ConstAlias< T >

Aim: This class encapsulates its parameter class so that to indicate to the user that the object/pointer will be only const aliased (and hence left unchanged). Therefore the user is reminded that the argument parameter is given to the function without any additional cost and may not be modified, while he is aware that the lifetime of the argument parameter must be at least as long as the object itself. Note that an instance of ConstAlias<T> is itself a light object (it holds only an enum and a pointer).

Description of template class 'ConstAlias'

(For a complete description, see Parameter passing, cloning and referencing).

It is used in methods or functions to encapsulate the parameter types. The following conversion from input parameter to data member or variable are possible:

Argument	const T&	const T*	CountedPtr<T>	CountedPtrOrPtr<T>	CountedConstPtrOrConstPtr<T>
To: const T&	Shared. O(1)	Shared. O(1)
To: const T*	Shared. O(1)	Shared. O(1)
To: CountedConstPtrOrConstPtr<T>	Shared. O(1)	Shared. O(1)	Shared. O(1), secure	Shared. O(1), secure	Shared. O(1), secure

Argument conversion to member is automatic except when converting to a pointer const T*: the address operator (operator&) must be used in this case.

For the last row (case where the programmer choose a CountedConstPtrOrConstPtr to hold the const alias), the user can thus enforce a secure const aliasing by handling a variant of CountedPtr as argument. In this case, even if the aliased object is destroyed in the caller context, it still exists in the callee context.

Note

The usage of ConstAlias<T> instead of const T & or const T * in parameters is recommended when the lifetime of the parameter must exceed the lifetime of the called method/function/constructor (often the case in constructor or init methods).

The usage of const T & or const T * instead of ConstAlias<T> is recommended when the lifetime of the parameter is not required to exceed the lifetime of the called method/function/constructor (often the case in standard methods, where the parameter is only used at some point, but not referenced after in some data member).

If the developer (not the user) wishes to enforce a secure const aliasing in all cases, he should probably clone the argument, hence use class Clone.

Template Parameters

T	is any type.

See also

Alias

Clone

It can be used as follows. Consider this simple example where class A is a big object.

const int N = 10000;
struct A { ...
  int table[N];
};
 
// When looking at the signature of B1's constructor, there is an
// ambiguity on the role of parameter a and its life-time. Here
// a's lifetime should be longer than the construction. Generally
// the ambiguity is removed by adding comments or, for the
// experienced developper, by looking at other parts of the code.
 
// Only aliasing, but for a long lifetime.
struct B1 {
  B1( const A & a ) // ambiguous, cost is O(1) here and lifetime of a should exceed constructor.
  : myA( a ) {}
...
  const A & myA;
};

Sometimes it is very important that the developper that uses the library is conscious that an object, say b, may require that an instance a given as parameter should have a lifetime longer than b itself (case for an instance of B1 above). Classes Clone, Alias, ConstAlias exist for these reasons. The class above may be rewritten as follows.

// ConstAliasing for a long lifetime is visible.
struct B1_v2_1 {
  B1_v2_1( ConstAlias<A> a ) // not ambiguous, cost is O(1) here and lifetime of a should be long enough
  : myA( a ) {}
...
  const A & myA;
};
 
// ConstAliasing for a long lifetime is visible.
struct B1_v2_2 {
  B1_v2_2( ConstAlias<A> a ) // not ambiguous, cost is O(1) here and lifetime of a should be long enough
  : myA( &a ) {} // Note the use of the address operator because of the pointer member
...
  const A* myA;
};
 
// ConstAliasing for a long lifetime is visible.
struct B1_v2_3 {
  B1_v2_3( ConstAlias<A> a ) // not ambiguous, cost is O(1) here and lifetime of a should be long enough
  : myA( a ) {}
...
  CountedConstPtrOrConstPtr<A> myA;
};
 
...
A a1;
CountedPtr<A> counted_a1( new A( a1 ) );
B1 ( a1 ); // not duplicated
B1_v2_1 ( a1 ); // not duplicated
B1_v2_2 ( a1 ); // not duplicated
B1_v2_3 ( counted_a1 );  // not duplicated, even better the user choose a secure variant of const alias.

Note

The user should not use ConstAlias<T> instead of const T & for data members. It works in most cases, but there are some subtle differences between the two behaviors.

ConstAlias have a default copy constructor, so as to let the user forward an ConstAlias<T> parameter.

Examples

dec/exampleDECSurface.cpp.

Definition at line 186 of file ConstAlias.h.

Member Enumeration Documentation

Parameter

template<typename T >

enum DGtal::ConstAlias::Parameter

protected

Internal class that allows to distinguish the different types of parameters.

Enumerator
CONST_LEFT_VALUE_REF
LEFT_VALUE_REF
PTR
CONST_PTR
COW_PTR
COUNTED_PTR
RIGHT_VALUE_REF
COUNTED_PTR_OR_PTR
COUNTED_CONST_PTR_OR_CONST_PTR

Definition at line 193 of file ConstAlias.h.

                   { CONST_LEFT_VALUE_REF, LEFT_VALUE_REF, PTR, CONST_PTR,
        COW_PTR, COUNTED_PTR, RIGHT_VALUE_REF, COUNTED_PTR_OR_PTR,
        COUNTED_CONST_PTR_OR_CONST_PTR };

Constructor & Destructor Documentation

ConstAlias() [1/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const ConstAlias< T > &  other )

default

Default copy constructor.

Parameters

other

object to copy.

~ConstAlias()

template<typename T >

DGtal::ConstAlias< T >::~ConstAlias ( )

inline

Destructor. Does nothing.

Definition at line 209 of file ConstAlias.h.

{}

ConstAlias() [2/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const T &  t )

inline

Constructor from const reference to an instance of T. The object is pointed in 'this'.

Parameters

t	any const reference to an object of type T.

Definition at line 216 of file ConstAlias.h.

      : myParam( CONST_LEFT_VALUE_REF ), myPtr( static_cast<const void*>( &t ) ) {}

ConstAlias() [3/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const T *  ptrT )

inline

Constructor from const pointer to an instance of T. The object is pointed in 'this'.

Parameters

ptrT	any const pointer to an object of type T.

Definition at line 224 of file ConstAlias.h.

      : myParam( CONST_PTR ), myPtr( static_cast<const void*>( ptrT ) ) {}

ConstAlias() [4/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const CowPtr< T > &  )

delete

Constructor from a const reference to a copy-on-write pointer on T. Deleted.

Const-aliasing a copy-on-write pointer has no meaning. Consider Clone instead.

ConstAlias() [5/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const CountedPtr< T > &  shT )

inline

Constructor from a const reference to a shared pointer on T. The object is pointed in 'this'.

Parameters

shT	any const reference to a shared pointer to an object of type T.

Definition at line 241 of file ConstAlias.h.

      : myParam( COUNTED_PTR ), myPtr( static_cast<const void*>( &shT ) ) {}

ConstAlias() [6/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const CountedPtrOrPtr< T > &  shT )

inline

Constructor from a const reference to a shared or simple const pointer on T. The object is pointed in 'this'.

Parameters

shT	any const reference to a shared or simple const pointer to an object of type T.

Definition at line 249 of file ConstAlias.h.

      : myParam( COUNTED_PTR_OR_PTR ), myPtr( static_cast<const void*>( &shT ) ) {}

ConstAlias() [7/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( const CountedConstPtrOrConstPtr< T > &  shT )

inline

Constructor from a const reference to a shared or simple const pointer on T. The object is pointed in 'this'.

Parameters

shT	any const reference to a shared or simple const pointer to an object of type T.

Definition at line 257 of file ConstAlias.h.

      : myParam( COUNTED_CONST_PTR_OR_CONST_PTR ), myPtr( static_cast<const void*>( &shT ) ) {}

ConstAlias() [8/8]

template<typename T >

DGtal::ConstAlias< T >::ConstAlias ( T &&  )

delete

Constructor from right-reference value. Delete.

Const-aliasing a rvalue ref has no meaning. Consider Clone instead.

Member Function Documentation

operator const T &()

template<typename T >

DGtal::ConstAlias< T >::operator const T & ( ) const

inline

Cast operator to a T const-reference. The object is never duplicated. Allowed input parameters are:

• const A & -> const A & // no duplication
• const A* -> const A & // no duplication, exception if null

Definition at line 273 of file ConstAlias.h.

      {
        switch( myParam ) {
          case CONST_LEFT_VALUE_REF:
          case CONST_PTR:
            return *( static_cast< const T* >( myPtr ) );
          default: ASSERT( false && "[ConstAlias::operator const T&() const] Invalid cast for given type. Consider passing a const left-value reference or a const pointer as a parameter." );
                   return *( static_cast< const T* >( myPtr ) );
        }
      }

References DGtal::ConstAlias< T >::CONST_LEFT_VALUE_REF, DGtal::ConstAlias< T >::CONST_PTR, DGtal::ConstAlias< T >::myParam, and DGtal::ConstAlias< T >::myPtr.

operator CountedConstPtrOrConstPtr< T >()

template<typename T >

DGtal::ConstAlias< T >::operator CountedConstPtrOrConstPtr< T > ( ) const

inline

Cast operator to a shared or simple const pointer. The object is never duplicated. Allowed input parameters are:

• const A& -> CountedConstPtrOrConstPtr // no duplication
• const A* -> CountedConstPtrOrConstPtr // no duplication
• CountedPtr -> CountedConstPtrOrConstPtr // no duplication
• CountedPtrOrPtr -> CountedConstPtrOrConstPtr // no duplication
• CountedConstPtrOrConstPtr -> CountedConstPtrOrConstPtr // no duplication

Definition at line 311 of file ConstAlias.h.

      {
        switch( myParam ) {
          case CONST_LEFT_VALUE_REF:
          case CONST_PTR:
            return CountedConstPtrOrConstPtr<T>( static_cast< const T* >( myPtr ), false );
          case COUNTED_PTR:
            return CountedConstPtrOrConstPtr<T>( *( static_cast< const CountedPtr<T>* >( myPtr ) ) );
          case COUNTED_PTR_OR_PTR:
            return CountedConstPtrOrConstPtr<T>( *( static_cast< const CountedPtrOrPtr<T>* >( myPtr ) ) );
          case COUNTED_CONST_PTR_OR_CONST_PTR:
            return CountedConstPtrOrConstPtr<T>( *( static_cast< const CountedPtrOrPtr<T>* >( myPtr ) ) );
          default: ASSERT( false && "[ConstAlias::operator CowPtr<T>() const] Invalid cast for given type. Consider passing a CountedPtr or a CowPtr as a parameter." );
                   return CountedConstPtrOrConstPtr<T>( 0, false );
        }
    }

References DGtal::ConstAlias< T >::CONST_LEFT_VALUE_REF, DGtal::ConstAlias< T >::CONST_PTR, DGtal::ConstAlias< T >::COUNTED_CONST_PTR_OR_CONST_PTR, DGtal::ConstAlias< T >::COUNTED_PTR, DGtal::ConstAlias< T >::COUNTED_PTR_OR_PTR, DGtal::ConstAlias< T >::myParam, and DGtal::ConstAlias< T >::myPtr.

operator&()

template<typename T >

const T* DGtal::ConstAlias< T >::operator& ( ) const

inline

Cast operator to a T const-pointer. The object is never duplicated. Allowed input parameters are:

• const A & -> const A* // no duplication
• const A* -> const A* // no duplication

Definition at line 290 of file ConstAlias.h.

      {
        switch( myParam ) {
          case CONST_LEFT_VALUE_REF:
          case CONST_PTR:
            return static_cast< const T* >( myPtr );
          default: ASSERT( false && "[const T* ConstAlias::operator&() const] Invalid address operator for given type. Consider passing a const left-value reference or a const pointer as a parameter." );
                   return static_cast< const T* >( myPtr );
        }
    }

References DGtal::ConstAlias< T >::CONST_LEFT_VALUE_REF, DGtal::ConstAlias< T >::CONST_PTR, DGtal::ConstAlias< T >::myParam, and DGtal::ConstAlias< T >::myPtr.

operator->()

template<typename T >

const T* DGtal::ConstAlias< T >::operator-> ( ) const

inline

Definition at line 328 of file ConstAlias.h.

      {
        switch( myParam ) {
          case CONST_LEFT_VALUE_REF:
          case CONST_PTR:
            return static_cast< const T* >( myPtr );
          case COUNTED_PTR:
            return ( static_cast< const CountedPtr<T>* >( myPtr ) )->operator->();
          case COUNTED_PTR_OR_PTR:
            return ( static_cast< const CountedPtrOrPtr<T>* >( myPtr ) )->operator->();
          case COUNTED_CONST_PTR_OR_CONST_PTR:
            return ( static_cast< const CountedConstPtrOrConstPtr<T>* >( myPtr ) )->operator->();
          default: ASSERT( false && "[ConstAlias::operator->() const] Invalid cast for given type. Consider passing a CountedPtr or a CowPtr as a parameter." );
                   return 0;
      }
    }

References DGtal::ConstAlias< T >::CONST_LEFT_VALUE_REF, DGtal::ConstAlias< T >::CONST_PTR, DGtal::ConstAlias< T >::COUNTED_CONST_PTR_OR_CONST_PTR, DGtal::ConstAlias< T >::COUNTED_PTR, DGtal::ConstAlias< T >::COUNTED_PTR_OR_PTR, DGtal::ConstAlias< T >::myParam, and DGtal::ConstAlias< T >::myPtr.

operator=()

template<typename T >

ConstAlias& DGtal::ConstAlias< T >::operator= ( const ConstAlias< T > &  other )

private

Assignment.

Parameters

other

the object to copy.

Returns

a reference on 'this'. Forbidden (otherwise the user might be tempted to use it as a member).

Field Documentation

myParam

template<typename T >

const Parameter DGtal::ConstAlias< T >::myParam

private

Characterizes the type of the input parameter at clone instanciation.

Definition at line 348 of file ConstAlias.h.

Referenced by DGtal::ConstAlias< T >::operator const T &(), DGtal::ConstAlias< T >::operator CountedConstPtrOrConstPtr< T >(), DGtal::ConstAlias< T >::operator&(), and DGtal::ConstAlias< T >::operator->().

myPtr

template<typename T >

const void* const DGtal::ConstAlias< T >::myPtr

private

Stores the address of the input parameter for further use.

Definition at line 350 of file ConstAlias.h.

Referenced by DGtal::ConstAlias< T >::operator const T &(), DGtal::ConstAlias< T >::operator CountedConstPtrOrConstPtr< T >(), DGtal::ConstAlias< T >::operator&(), and DGtal::ConstAlias< T >::operator->().

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The documentation for this class was generated from the following file:

• ConstAlias.h