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Pointers And Memory - Contents


Reference Parameter

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    In the simplest "pass by value" or "value parameter" scheme, each function has separate, local memory and parameters are copied from the caller to the callee at the moment of the function call. But what about the other direction? How can the callee communicate back to its caller? Using a "return" at the end of the callee to copy a result back to the caller works for simple cases, but does not work well for all situations. Also, sometimes copying values back and forth is undesirable. "Pass by reference" parameters solve all of these problems.

    For the following discussion, the term "value of interest" will be a value that the caller and callee wish to communicate between each other. A reference parameter passes a pointer to the value of interest instead of a copy of the value of interest. This technique uses the sharing property of pointers so that the caller and callee can share the value of interest.

   Bill Gates Example Suppose functions A() and B() both do computations involving Bill Gates' net worth measured in billions of dollars the value of interest for this problem. A() is the main function and its stores the initial value (about 55 as of 1998). A() calls B() which tries to add 1 to the value of interest

   Bill Gates By Value Here is the code and memory drawing for a simple, but incorrect implementation where A() and B() use pass by value. Three points in time, T1, T2, and T3 are marked in the code and the state of memory is shown for each state...


void B(int worth) {
worth = worth + 1;
// T2
}
void A() {
int netWorth;
netWorth = 55; // T1
B(netWorth);
// T3 -- B() did not change netWorth
}


B() adds 1 to its local worth copy, but when B() exits, worth is deallocated, so changing it was useless. The value of interest, netWorth, rests unchanged the whole time in A()'s local storage. A function can change its local copy of the value of interest, but that change is not reflected back in the original value. This is really just the old "independence" property of local storage, but in this case it is not what is wanted

    Vector allows us to view it's insides using an Enumerator; a class to go through objects. It is very useful to first be able to look what you're looking for, and only later decide whether you'd like to remove it or not. A sample program that uses java.util.Vector for it's storage follows.

    By Reference: The reference solution to the Bill Gates problem is to use a single netWorth variable for the value of interest and never copy it. Instead, each function can receives a pointer to netWorth. Each function can see the current value of netWorth by dereferencing its pointer. More importantly, each function can change the net worth just dereference the pointer to the centralized netWorth and change it directly. Everyone agrees what the current value of netWorth because it exists in only one place everyone has a pointer to the one master copy. The following memory drawing shows A() and B() functions changed to use "reference" parameters. As before, T1, T2, and T3 correspond to points in the code (below), but you can study the memory structure without looking at the code yet

    Passing By Reference:
Here are the steps to use in the code to use the pass-by-reference strategy

  • Have a single copy of the value of interest. The single "master" copy.
  • Pass pointers to that value to any function which wants to see or change the value.
  • Functions can dereference their pointer to see or change the value of interest.
  • Functions must remember that they do not have their own local copies. If they dereference their pointer and change the value, they really are changing the master value. If a function wants a local copy to change safely, the function must explicitly allocate and initialize such a local copy.

    Syntax: The syntax for by reference parameters in the C language just uses pointer operations on the parameters

  • Suppose a function wants to communicate about some value of interest int or float or struct fraction.
  • The function takes as its parameter a pointer to the value of interest an int* or float* or struct fraction*. Some programmers will add the word "ref" to the name of a reference parameter as a reminder that it is a reference to the value of interest instead of a copy.
  • .At the time of the call, the caller computes a pointer to the value of interest and passes that pointer. The type of the pointer (pointer to the value of interest) will agree with the type in (2) above. If the value of interest is local to the caller, then this will often involve a use of the & operator (Section 1).
  • When the callee is running, if it wishes to access the value of interest, it must dereference its pointer to access the actual value of interest. Typically, this equates to use of the dereference operator (*) in the function to see the value of interest.

    Bill Gates By Reference: Here is the Bill Gates example written to use reference parameters. This code now matches the by-reference memory drawing above.


// B() now uses a reference parameter -- a pointer to
// the value of interest. B() uses a dereference (*) on the
// reference parameter to get at the value of interest.
void B(int* worthRef) { // reference parameter
*worthRef = *worthRef + 1; // use * to get at value of interest
// T2
}
void A() {
int netWorth;
netWorth = 55; // T1 -- the value of interest is local to A()
B(&netWorth); // Pass a pointer to the value of interest.
// In this case using &.
// T3 -- B() has used its pointer to change the value of interest
}

    I guess that covers the Vector class. If you need to know more about it, you're welcome to read the API specs for it. I also greatly encourage you to look at java.util.Vector source, and see for yourself what's going on inside that incredibly simple structure.


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