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CMapStringToOb Class

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The latest version of this topic can be found at CMapStringToOb Class.

A dictionary collection class that maps unique CString objects to CObject pointers.

Syntax

class CMapStringToOb : public CObject

Members

Public Constructors

Name Description
CMapStringToOb::CMapStringToOb Constructor.

Public Methods

Name Description
CMapStringToOb::GetCount Returns the number of elements in this map.
CMapStringToOb::GetHashTableSize Determines the current number of elements in the hash table.
CMapStringToOb::GetNextAssoc Gets the next element for iterating.
CMapStringToOb::GetSize Returns the number of elements in this map.
CMapStringToOb::GetStartPosition Returns the position of the first element.
CMapStringToOb::HashKey Calculates the hash value of a specified key.
CMapStringToOb::InitHashTable Initializes the hash table.
CMapStringToOb::IsEmpty Tests for the empty-map condition (no elements).
CMapStringToOb::Lookup Looks up a void pointer based on the void pointer key. The pointer value, not the entity it points to, is used for the key comparison.
CMapStringToOb::LookupKey Returns a reference to the key associated with the specified key value.
CMapStringToOb::RemoveAll Removes all the elements from this map.
CMapStringToOb::RemoveKey Removes an element specified by a key.
CMapStringToOb::SetAt Inserts an element into the map; replaces an existing element if a matching key is found.

Public Operators

Name Description
CMapStringToOb::operator [ ] Inserts an element into the map — operator substitution for SetAt.

Remarks

Once you have inserted a CString- CObject* pair (element) into the map, you can efficiently retrieve or delete the pair using a string or a CString value as a key. You can also iterate over all the elements in the map.

A variable of type POSITION is used for alternate entry access in all map variations. You can use a POSITION to "remember" an entry and to iterate through the map. You might think that this iteration is sequential by key value; it is not. The sequence of retrieved elements is indeterminate.

CMapStringToOb incorporates the IMPLEMENT_SERIAL macro to support serialization and dumping of its elements. Each element is serialized in turn if a map is stored to an archive, either with the overloaded insertion ( <<) operator or with the Serialize member function.

If you need a diagnostic dump of the individual elements in the map (the CString value and the CObject contents), you must set the depth of the dump context to 1 or greater.

When a CMapStringToOb object is deleted, or when its elements are removed, the CString objects and the CObject pointers are removed. The objects referenced by the CObject pointers are not destroyed.

Map class derivation is similar to list derivation. See the article Collections for an illustration of the derivation of a special-purpose list class.

Inheritance Hierarchy

CObject

CMapStringToOb

Requirements

Header: afxcoll.h

CMapStringToOb::CMapStringToOb

Constructs an empty CString-to- CObject* map.

CMapStringToOb(INT_PTR nBlockSize = 10);

Parameters

nBlockSize
Specifies the memory-allocation granularity for extending the map.

Remarks

As the map grows, memory is allocated in units of nBlockSize entries.

The following table shows other member functions that are similar to CMapStringToOb:: CMapStringToOb.

Class Member Function
CMapPtrToPtr CMapPtrToPtr( INT_PTR nBlockSize = 10 );
CMapPtrToWord CMapPtrToWord( INT_PTR nBlockSize = 10 );
CMapStringToPtr CMapStringToPtr( INT_PTR nBlockSize = 10 );
CMapStringToString CMapStringToString( INT_PTR nBlockSize = 10 );
CMapWordToOb CMapWordToOb( INT_PTR nBlockSize = 10 );
CMapWordToPtr MapWordToPtr( INT_PTR nBlockSize = 10 );

Example

         CMapStringToOb map(20);  // Map on the stack with blocksize of 20

         CMapStringToOb* pm = new CMapStringToOb;  // Map on the heap
                                                   // with default blocksize

See CObList::CObList for a listing of the CAge class used in all collection examples.

CMapStringToOb::GetCount

Determines how many elements are in the map.

INT_PTR GetCount() const;

Return Value

The number of elements in this map.

Remarks

The following table shows other member functions that are similar to CMapStringToOb::GetCount.

Class Member Function
CMapPtrToPtr INT_PTR GetCount( ) const;
CMapPtrToWord INT_PTR GetCount( ) const;
CMapStringToPtr INT_PTR GetCount( ) const;
CMapStringToString INT_PTR GetCount( ) const;
CMapWordToOb INT_PTR GetCount( ) const;
CMapWordToPtr INT_PTR GetCount( ) const;

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;

         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Homer"), new CAge(36));
         ASSERT(map.GetCount() == 2);

CMapStringToOb::GetHashTableSize

Determines the current number of elements in the hash table.

UINT GetHashTableSize() const;

Return Value

Returns the number of elements in the hash table.

Remarks

The following table shows other member functions that are similar to CMapStringToOb::GetHashTableSize.

Class Member Function
CMapPtrToPtr UINT GetHashTableSize( ) const;
CMapPtrToWord UINT GetHashTableSize( ) const;
CMapStringToPtr UINT GetHashTableSize( ) const;
CMapStringToString UINT GetHashTableSize( ) const;
CMapWordToOb UINT GetHashTableSize( ) const;
CMapWordToPtr UINT GetHashTableSize( ) const;

CMapStringToOb::GetNextAssoc

Retrieves the map element at rNextPosition, then updates rNextPosition to refer to the next element in the map.

void GetNextAssoc(
    POSITION& rNextPosition,  
    CString& rKey,  
    CObject*& rValue) const;

Parameters

rNextPosition
Specifies a reference to a POSITION value returned by a previous GetNextAssoc or GetStartPosition call.

rKey
Specifies the returned key of the retrieved element (a string).

rValue
Specifies the returned value of the retrieved element (a CObject pointer). See Remarks for more about this parameter.

Remarks

This function is most useful for iterating through all the elements in the map. Note that the position sequence is not necessarily the same as the key value sequence.

If the retrieved element is the last in the map, then the new value of rNextPosition is set to NULL.

For the rValue parameter, be sure to cast your object type to CObject*&, which is what the compiler requires, as shown in the following example:

         CObject* ob;
         map.GetNextAssoc(pos, key, (CObject*&)ob);

This is not true of GetNextAssoc for maps based on templates.

The following table shows other member functions that are similar to CMapStringToOb::GetNextAssoc.

Class Member Function
CMapPtrToPtr void GetNextAssoc( POSITION& rNextPosition , void*& rKey , void*& rValue ) const;
CMapPtrToWord void GetNextAssoc( POSITION& rNextPosition , void*& rKey , WORD& rValue ) const;
CMapStringToPtr void GetNextAssoc( POSITION& rNextPosition , CString& rKey , void*& rValue ) const;
CMapStringToString void GetNextAssoc( POSITION& rNextPosition , CString& rKey , CString& rValue ) const;
CMapWordToOb void GetNextAssoc( POSITION& rNextPosition , WORD& rKey , CObject*& rValue ) const;
CMapWordToPtr void GetNextAssoc( POSITION& rNextPosition , WORD& rKey , void*& rValue ) const;

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;
         POSITION pos;
         CString key;
         CAge* pa;
         
         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Lisa"), new CAge(11));
         map.SetAt(_T("Homer"), new CAge(36));
         map.SetAt(_T("Marge"), new CAge(35));
         // Iterate through the entire map, dumping both name and age.
         for (pos = map.GetStartPosition(); pos != NULL;)
         {
            map.GetNextAssoc(pos, key, (CObject*&)pa);
            #ifdef _DEBUG
                  afxDump << key << _T(" : ") << pa << _T("\n");
            #endif
         }

The results from this program are as follows:

Lisa : a CAge at $4724 11

Marge : a CAge at $47A8 35

Homer : a CAge at $4766 36

Bart : a CAge at $45D4 13

CMapStringToOb::GetSize

Returns the number of map elements.

INT_PTR GetSize() const;

Return Value

The number of items in the map.

Remarks

Call this method to retrieve the number of elements in the map.

The following table shows other member functions that are similar to CMapStringToOb::GetSize.

Class Member Function
CMapPtrToPtr INT_PTR GetSize( ) const;
CMapPtrToWord INT_PTR GetSize( ) const;
CMapStringToPtr INT_PTR GetSize( ) const;
CMapStringToString INT_PTR GetSize( ) const;
CMapWordToOb INT_PTR GetSize( ) const;
CMapWordToPtr INT_PTR GetSize( ) const;

Example

         CMapStringToOb map;

         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Homer"), new CAge(36));
         ASSERT(map.GetSize() == 2);

CMapStringToOb::GetStartPosition

Starts a map iteration by returning a POSITION value that can be passed to a GetNextAssoc call.

POSITION GetStartPosition() const;

Return Value

A POSITION value that indicates a starting position for iterating the map; or NULL if the map is empty.

Remarks

The iteration sequence is not predictable; therefore, the "first element in the map" has no special significance.

The following table shows other member functions that are similar to CMapStringToOb::GetStartPosition.

Class Member Function
CMapPtrToPtr POSITION GetStartPosition( ) const;
CMapPtrToWord POSITION GetStartPosition( ) const;
CMapStringToPtr POSITION GetStartPosition( ) const;
CMapStringToString POSITION GetStartPosition( ) const;
CMapWordToOb POSITION GetStartPosition( ) const;
CMapWordToPtr POSITION GetStartPosition( ) const;

Example

See the example for CMapStringToOb::GetNextAssoc.

CMapStringToOb::HashKey

Calculates the hash value of a specified key.

UINT HashKey(LPCTSTR key) const;

Parameters

key
The key whose hash value is to be calculated.

Return Value

The Key's hash value

Remarks

The following table shows other member functions that are similar to CMapStringToOb::HashKey.

Class Member Function
CMapPtrToPtr UINT HashKey( void* key ) const;
CMapPtrToWord UINT HashKey( void* key ) const;
CMapStringToString UINT HashKey( LPCTSTR key ) const;
CMapStringToPtr UINT HashKey( LPCTSTR key ) const;
CMapWordToOb UINT HashKey( WORD key ) const;
CMapWordToPtr UINT HashKey( WORD key ) const;

CMapStringToOb::InitHashTable

Initializes the hash table.

void InitHashTable(
    UINT hashSize,  
    BOOL bAllocNow = TRUE);

Parameters

hashSize
Number of entries in the hash table.

bAllocNow
If TRUE, allocates the hash table upon initialization; otherwise the table is allocated when needed.

Remarks

For best performance, the hash table size should be a prime number. To minimize collisions, the size should be roughly 20 percent larger than the largest anticipated data set.

The following table shows other member functions that are similar to CMapStringToOb::InitHashTable.

Class Member Function
CMapPtrToPtr void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );
CMapPtrToWord void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );
CMapStringToString void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );
CMapStringToPtr void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );
CMapWordToOb void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );
CMapWordToPtr void InitHashTable( UINT hashSize , BOOL bAllocNow = TRUE );

CMapStringToOb::IsEmpty

Determines whether the map is empty.

BOOL IsEmpty() const;

Return Value

Nonzero if this map contains no elements; otherwise 0.

Example

See the example for RemoveAll.

Remarks

The following table shows other member functions that are similar to CMapStringToOb:: IsEmpty.

Class Member Function
CMapPtrToPtr BOOL IsEmpty( ) const;
CMapPtrToWord BOOL IsEmpty( ) const;
CMapStringToPtr BOOL IsEmpty( ) const;
CMapStringToString BOOL IsEmpty( ) const;
CMapWordToOb BOOL IsEmpty( ) const;
CMapWordToPtr BOOL IsEmpty( ) const;

CMapStringToOb::Lookup

Returns a CObject pointer based on a CString value.

BOOL Lookup(
    LPCTSTR key,  
    CObject*& rValue) const;

Parameters

key
Specifies the string key that identifies the element to be looked up.

rValue
Specifies the returned value from the looked-up element.

Return Value

Nonzero if the element was found; otherwise 0.

Remarks

Lookup uses a hashing algorithm to quickly find the map element with a key that matches exactly ( CString value).

The following table shows other member functions that are similar to CMapStringToOb::LookUp.

Class Member Function
CMapPtrToPtr BOOL Lookup( void* key , void*& rValue ) const;
CMapPtrToWord BOOL Lookup( void* key , WORD& rValue ) const;
CMapStringToPtr BOOL Lookup( LPCTSTR key , void*& rValue ) const;
CMapStringToString BOOL Lookup( LPCTSTR key , CString& rValue ) const;
CMapWordToOb BOOL Lookup( WORD key , CObject*& rValue ) const;
CMapWordToPtr BOOL Lookup( WORD key , void*& rValue ) const;

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;
         CAge* pa;

         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Lisa"), new CAge(11));
         map.SetAt(_T("Homer"), new CAge(36));
         map.SetAt(_T("Marge"), new CAge(35));
         ASSERT(map.Lookup(_T("Lisa"), (CObject*&)pa)); // Is "Lisa" in the map?
         ASSERT(*pa ==  CAge(11)); // Is she 11?

CMapStringToOb::LookupKey

Returns a reference to the key associated with the specified key value.

BOOL LookupKey(
    LPCTSTR key,  
    LPCTSTR& rKey) const;

Parameters

key
Specifies the string key that identifies the element to be looked up.

rKey
The reference to the associated key.

Return Value

Nonzero if the key was found; otherwise 0.

Remarks

Using a reference to a key is unsafe if used after the associated element was removed from the map or after the map was destroyed.

The following table shows other member functions that are similar to CMapStringToOb:: LookupKey.

Class Member Function
CMapStringToPtr BOOL LookupKey( LPCTSTR key , LPCTSTR& rKey ) const;
CMapStringToString BOOL LookupKey( LPCTSTR key , LPCTSTR& rKey ) const;

CMapStringToOb::operator [ ]

A convenient substitute for the SetAt member function.

CObject*& operator[](LPCTSTR key);

Return Value

A reference to a pointer to a CObject object; or NULL if the map is empty or key is out of range.

Remarks

Thus it can be used only on the left side of an assignment statement (an l-value). If there is no map element with the specified key, then a new element is created.

There is no "right side" (r-value) equivalent to this operator because there is a possibility that a key may not be found in the map. Use the Lookup member function for element retrieval.

The following table shows other member functions that are similar to CMapStringToOb::operator [].

Class Member Function
CMapPtrToPtr void*& operator[]( void* key );
CMapPtrToWord WORD& operator[]( void* key );
CMapStringToPtr void*& operator[]( LPCTSTR key );
CMapStringToString CString& operator[]( LPCTSTR key );
CMapWordToOb CObject*& operator[]( WORD key );
CMapWordToPtr void*& operator[]( WORD key );

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;
         
         map[_T("Bart")] = new CAge(13);
         map[_T("Lisa")] = new CAge(11);
         #ifdef _DEBUG
            afxDump.SetDepth(1);
            afxDump << _T("Operator [] example: ") << &map << _T("\n");
         #endif

The results from this program are as follows:

Operator [] example: A CMapStringToOb with 2 elements

[Lisa] = a CAge at $4A02 11

[Bart] = a CAge at $497E 13

CMapStringToOb::RemoveAll

Removes all the elements from this map and destroys the CString key objects.

void RemoveAll();

Remarks

The CObject objects referenced by each key are not destroyed. The RemoveAll function can cause memory leaks if you do not ensure that the referenced CObject objects are destroyed.

The function works correctly if the map is already empty.

The following table shows other member functions that are similar to CMapStringToOb::RemoveAll.

Class Member Function
CMapPtrToPtr void RemoveAll( );
CMapPtrToWord void RemoveAll( );
CMapStringToPtr void RemoveAll( );
CMapStringToString void RemoveAll( );
CMapWordToOb void RemoveAll( );
CMapWordToPtr void RemoveAll( );

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         {
            CMapStringToOb map;

            CAge age1(13); // Two objects on the stack
            CAge age2(36);
            map.SetAt(_T("Bart"), &age1);
            map.SetAt(_T("Homer"), &age2);
            ASSERT(map.GetCount() == 2);
            map.RemoveAll(); // CObject pointers removed; objects not removed.
            ASSERT(map.GetCount() == 0);
            ASSERT(map.IsEmpty());
         } // The two CAge objects are deleted when they go out of scope.

CMapStringToOb::RemoveKey

Looks up the map entry corresponding to the supplied key; then, if the key is found, removes the entry.

BOOL RemoveKey(LPCTSTR key);

Parameters

key
Specifies the string used for map lookup.

Return Value

Nonzero if the entry was found and successfully removed; otherwise 0.

Remarks

This can cause memory leaks if the CObject object is not deleted elsewhere.

The following table shows other member functions that are similar to CMapStringToOb::RemoveKey.

Class Member Function
CMapPtrToPtr BOOL RemoveKey( void* key );
CMapPtrToWord BOOL RemoveKey( void* key );
CMapStringToPtr BOOL RemoveKey( LPCTSTR key );
CMapStringToString BOOL RemoveKey( LPCTSTR key );
CMapWordToOb BOOL RemoveKey( WORD key );
CMapWordToPtr BOOL RemoveKey( WORD key );

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;
         
         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Lisa"), new CAge(11));
         map.SetAt(_T("Homer"), new CAge(36));
         map.SetAt(_T("Marge"), new CAge(35));
         map.RemoveKey(_T("Lisa")); // Memory leak: CAge object not
                                    // deleted.
         #ifdef _DEBUG
            afxDump.SetDepth(1);
            afxDump << _T("RemoveKey example: ") << &map << _T("\n");
         #endif

The results from this program are as follows:

RemoveKey example: A CMapStringToOb with 3 elements

[Marge] = a CAge at $49A0 35

[Homer] = a CAge at $495E 36

[Bart] = a CAge at $4634 13

CMapStringToOb::SetAt

The primary means to insert an element in a map.

void SetAt(
    LPCTSTR key,  
    CObject* newValue);

Parameters

key
Specifies the string that is the key of the new element.

newValue
Specifies the CObject pointer that is the value of the new element.

Remarks

First, the key is looked up. If the key is found, then the corresponding value is changed; otherwise a new key-value element is created.

The following table shows other member functions that are similar to CMapStringToOb::SetAt.

Class Member Function
CMapPtrToPtr void SetAt( void* key , void* newValue );
CMapPtrToWord void SetAt( void* key , WORD newValue );
CMapStringToPtr void SetAt( LPCTSTR key , void* newValue );
CMapStringToString void SetAt( LPCTSTR key , LPCTSTR newValue );
CMapWordToOb void SetAt( WORD key , CObject* newValue );
CMapWordToPtr void SetAt( WORD key , void* newValue );

Example

See CObList::CObList for a listing of the CAge class used in all collection examples.

         CMapStringToOb map;
         CAge* pa;

         map.SetAt(_T("Bart"), new CAge(13));
         map.SetAt(_T("Lisa"), new CAge(11)); // Map contains 2 
                                                 // elements.
         #ifdef _DEBUG
            afxDump.SetDepth(1);
            afxDump << _T("before Lisa's birthday: ") << &map << _T("\n");
         #endif
         if (map.Lookup(_T("Lisa"), (CObject *&)pa))
         { // CAge 12 pointer replaces CAge 11 pointer.
            map.SetAt(_T("Lisa"), new CAge(12));
            delete pa;  // Must delete CAge 11 to avoid memory leak.
         }
         #ifdef _DEBUG
            afxDump << _T("after Lisa's birthday: ") << &map << _T("\n");
         #endif

The results from this program are as follows:

before Lisa's birthday: A CMapStringToOb with 2 elements

[Lisa] = a CAge at $493C 11

[Bart] = a CAge at $4654 13

after Lisa's birthday: A CMapStringToOb with 2 elements

[Lisa] = a CAge at $49C0 12

[Bart] = a CAge at $4654 13

See Also

CObject Class
Hierarchy Chart
CMapPtrToPtr Class
CMapPtrToWord Class
CMapStringToPtr Class
CMapStringToString Class
CMapWordToOb Class
CMapWordToPtr Class