References & Pointers

CS-1030Dr. Mark L. Hornick

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References & Pointers


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Parameter passing in Java and C++

Both Java and C++ pass primitives by value Meaning that a copy of the primitive is passed into a

method If the value of the primitive is changed in the method, only

the copy of the primitive is changed

Java always passes objects by reference

C++, however… By default, objects are passed by value Meaning that a copy of an object is passed into a method


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Pass by value: a good thing?

Passing entire objects by value in C++ is not always desireable Objects can be big, leading to inefficiency Copy constructor must be called to make the copy

of the object…

If a method is supposed to modify the object, a copy of the object must be returned… Copy constructor must be called again to make a

copy of the modified object…


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Another way: References

Passing entire objects by value in C++ is not always desireable Objects can be big, leading to inefficiency

References were created to make life easier C++ references are similar to Java

references


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References – special syntax

Review: Here’s a declaration of a method that passes an object by value: void printName(Employee e); Usage: printName( anEmp );

Here’s the modified declaration of the method to tell it to pass the object by reference instead: void printName(Employee& e); Usage: printName( anEmp ); // same as above

References are only 4 bytes long (8 in Windows Vista)


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Passing primitives by reference

Review: Here’s a declaration of a method that passes a primitive by value: void setValue( int val );

Here’s the modified declaration of the method to tell it to pass a primitive by reference instead: void setValue( int& val );

Could be useful if you want setValue() to modify the actual val variable you are passing into the method


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But there’s yet another way

C++ has another kind of datatype called a pointer Pointers are used to store the address (of an item)

An address is legitimate data by itself Every memory location has an address Java has no capability for this

Example of a pointer datatype Review: int is a datatype int* (“pointer to int”) is a separate datatypeint x = 3; // int x is assigned a value of 3

int* px = 0; // int* px is assigned to point // to memory address 0


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Pointer syntax

Declaration of a pointer uses the * prefix int *pValue; // pointer to an integer int* pValue; // another way of declaring the same

Be careful with multiple declarations on one line: int *pValue1, *pValue2; int* pValue1, pValue2; // No!!!


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How can you assign a pointer?

int x = 3; // int x is assigned a value of 3

int* px = &x; // int* px is assigned to point // to memory address where x is

New unary operator:& prefix – “address of” operator


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How do you get the value of what’s at the location where the pointer is pointing?



int y; // another int variable

y = *px; // y is assigned the value at the

// memory location px points to

Another new unary operator:* prefix – dereference operator


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More unary operators

++ prefix or postfix – increment the address stored in the pointer

int aValue = 3;int* pValue = &aValue; // addr might be 0x1234pValue++; // new addr is 0x1238; why not 0x1235????

-- prefix or postfix – decrement the address stored in the pointer


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What about objects?

for any “regular” datatype, you can also declare a corresponding datatype:

SearchResult sr1; // an object

SearchResult sr2; // another object

SearchResult* psr = &sr1; // ptr to sr1

psr = &sr2; // now points to sr2


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Ran out of time here


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How do you get the value of what’s at the location where the pointer is pointing?



int y; // another int variable

y = *px; // y is assigned the value at the

// memory location px points to

Another new unary operator:* prefix – dereference operator


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What about objects?

Dereferencing object pointers

SearchResult sr1; // an objectSearchResult* psr = &sr1; // ptr to sr1

SearchResults sr2 = *psr; // same as sr2=sr1


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Passing items by address

Review: Here’s a declaration of a method that passes a primitive by value: void setValue( int val );

Here’s the modified declaration of the method to tell it to pass a primitive by address instead: void setValue( int* val );

Usage: setValue( &x ); // must pass address of x


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Confusing syntax

int aValue = 1; // an intint anotherValue = 2;

int* pValue = &aValue; // ptr to aValue// above line is same as:// int* pValue; // ptr, but not init’d// pValue = &aValue; // now init’d

pValue = &anotherValue // points to anotherValue

// whatever pValue points to now equals aValue: *pValue = aValue;// same as anotherValue=aValue;


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More Confusing syntax

int aValue = 1; // an intint anotherValue = 2;

int *p1 = &aValue; // ptr to aValueint *p2 = &anotherValue;

*p1 = *p2; // same as aValue=anotherValue

p1 = p2; // now both point to anotherValue


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The this keyword in C++

Sometimes an object must refer to itself Java and C++ both use the this keyword

In Java, this is a reference to the current object

In C++ this– a pointer to the current object *this – a dereferenced pointer = the current

object Accessible in member functions Usage is implied in member functions


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Method access via pointers

Employee anEmp; // an object

Employee* pe = &anEmp; // ptr to object

int id;

id = anEmp.getID(); // method access

id = pe->getID(); // via pointer

Another new unary operator:-> the indirection operator (called a bigraph)


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The NULL pointer Since a pointer holds an address, the pointer can be

made to point anywhere in memory.int *pValue;pValue = 0x1234; // points to whatever is there…

Pointers should only be made to point at valid addresses The exception is the NULL pointer, whereint *pValue = NULL; // points to address 0

This is a convention that allows you to check to see if a pointer is valid

References & Pointers

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