7/31/08

Auto Pointer: auto_ptr

Auto Pointer:

The C++ standard library contains a class template called auto_ptr, which is just a pointer like object that knows enough to delete what it is pointing to when the pointer goes out of scope. The auto_ptr class takes a pointer to a heap object in its constructor and deletes it in its destructor, freeing the user from responsibility of deleting the pointer.

Auto pointers should be used in a function with multiple exit points. So that programmer need not explicitly delete the pointer at multiple places.

e.g. In the example below, there are multiple exit points in the function, thus ideal case for using auto pointer.

# include

# include

# include

# include

vector* getList()

{

vector<>* pvNumbers = new vector<>;

int number;

cout<<"give the 5 numbers";

for( int i = 0; i <>

{

cin>>number;

pvNumbers->push_back( number );

}

return pvNumbers;

}

void checkValidity()

{

//this function will check the validity of the

//numbers given by the user. The number will be invalid

//if it is equal to 13.

auto_ptr<> > apvNumbers(getList());

//this pointer should be deleted in this function.

if( apvNumbers->size() == 0 )

{

return;

}

unsigned unSize = apvNumbers->size();

for( int i = 0; i <>

{

if( (* (apvNumbers.get()) )[i] == 13 )

{

cout<< " You have entered invalid number."<

return;

}

}

cout<< " All the numbers are valid."<

//As we are using auto_ptr we need not delete the pointer explicitly

//Otherwise we will have to delete the pointer at

//three places in this function.

}

int main()

{

checkValidity();

return 0;

}

Use auto_ptr in functions with multiple exit points, so that the pointer is deleted on exit from function.

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