深度剖析“引用传递”与“指针传递”的区别

Reference Parameters

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.

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.

The reference parameter strategy: B() receives a pointer to the value of interest instead of
a copy.

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...
1. Suppose a function wants to communicate about some value of interest —
int or float or struct fraction.
2. 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.
3. 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).
4. 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
}

Don't Make Copies

Reference parameters enable communication between the callee and its caller. Another
reason to use reference parameters is to avoid making copies. For efficiency, making
copies may be undesirable if the value of interest is large, such as an array. Making the
copy requires extra space for the copy itself and extra time to do the copying. From a
design point of view, making copies may be undesirable because as soon as there are two
copies, it is unclear which one is the "correct" one if either is changed. Proverb: "A
person with one watch always knows what time it is. A person with two watches is never
sure." Avoid making copies.

Simple Reference Parameter Example — Swap()

The standard example of reference parameters is a Swap() function which exchanges the
values of two ints. It's a simple function, but it does need to change the caller's memory
which is the key feature of pass by reference.

Swap() Function

The values of interest for Swap() are two ints. Therefore, Swap() does not take ints
as its parameters. It takes a pointers to int — (int*)'s. In the body of Swap() the
parameters, a and b, are dereferenced with * to get at the actual (int) values of interest.

void Swap(int* a, int* b) {
int temp;
temp = *a;
*a = *b;
*b = temp;
}

Swap() Caller

To call Swap(), the caller must pass pointers to the values of interest...

void SwapCaller() {
int x = 1;
int y = 2;
Swap(&x, &y); // Use & to pass pointers to the int values of interest
//  (x and y).
}

The parameters to Swap() are pointers to values of interest which are back in the caller's
locals. The Swap() code can dereference the pointers to get back to the caller's memory to
exchange the values. In this case, Swap() follows the pointers to exchange the values in
the variables x and y back in SwapCaller(). Swap() will exchange any two ints given
pointers to those two ints.

Swap() With Arrays

Just to demonstrate that the value of interest does not need to be a simple variable, here's
a call to Swap() to exchange the first and last ints in an array. Swap() takes int*'s, but
the ints can be anywhere. An int inside an array is still an int.

void SwapCaller2() {
int scores[10];
scores[0] = 1;
scores[9[ = 2;
Swap(&(scores[0]), &(scores[9]));// the ints of interest do not need to be
// simple variables -- they can be any int. The caller is responsible
// for computing a pointer to the int.

The above call to Swap() can be written equivalently as Swap(scores, scores+9)
due to the array syntax in C. You can ignore this case if it is not familiar to you — it's
not an important area of the language and both forms compile to the exact same thing
anyway.

Is The & Always Necessary?

When passing by reference, the caller does not always need to use & to compute a new
pointer to the value of interest. Sometimes the caller already has a pointer to the value of
interest, and so no new pointer computation is required. The pointer to the value of
interest can be passed through unchanged.
For example, suppose B() is changed so it calls a C() function which adds 2 to the value
of interest...

// Takes the value of interest by reference and adds 2.
void C(int* worthRef) {
*worthRef = *worthRef + 2;
}
// Adds 1 to the value of interest, and calls C().
void B(int* worthRef) {
*worthRef = *worthRef + 1; // add 1 to value of interest as before
C(worthRef); // NOTE no & required. We already have
// a pointer to the value of interest, so
// it can be passed through directly.
}

What About The & Bug TAB?

All this use of & might make you nervous — are we committing the & bug from Section
2? No, it turns out the above uses of & are fine. The & bug happens when an & passes a
pointer to local storage from the callee back to its caller. When the callee exits, its localmemory is deallocated and so the pointer no longer has a pointee. In the above, correctcases, we use & to pass a pointer from the caller to the callee. The pointer remains validfor the callee to use because the caller locals continue to exist while the callee is running.
The pointees will remain valid due to the simple constraint that the caller can only exit
sometime after its callee exits. Using & to pass a pointer to local storage from the caller
to the callee is fine. The reverse case, from the callee to the caller, is the & bug.

The ** Case

What if the value of interest to be shared and changed between the caller and callee is
already a pointer, such as an int* or a struct fraction*? Does that change the
rules for setting up reference parameters? No. In that case, there is no change in the rules.
They operate just as before. The reference parameter is still a pointer to the value of
interest, even if the value of interest is itself a pointer. Suppose the value of interest isint*.

This means there is an int* value which the caller and callee want to share and
change. Then the reference parameter should be an int**. For a structfraction* value of interest, the reference parameter is struct fraction**. Asingle dereference (*) operation on the reference parameter yields the value of interest asit did in the simple cases. Double pointer (**) parameters are common in linked list orother pointer manipulating code were the value of interest to share and change is itself apointer, such as a linked list head pointer.

Reference Parameter Summary

Passing by value (copying) does not allow the callee to communicate back to its caller
and has also has the usual disadvantages of making copies. Pass by reference uses
pointers to avoid copying the value of interest, and allow the callee to communicate back
to the caller.
For pass by reference, there is only one copy of the value of interest, and pointers to that
one copy are passed. So if the value of interest is an int, its reference parameter is an int*.
If the value of interest is a struct fraction*, its reference parameters is a struct fraction**.
Functions use the dereference operator (*) on the reference parameter to see or change the
value of interest.

Extra: Reference Parameters in Java

Because Java has no */& operators, it is not possible to implement reference parameters
in Java directly. Maybe this is ok — in the OOP paradigm, you should change objects by
sending them messages which makes the reference parameter concept unnecessary. The
caller passes the callee a (shallow) reference to the value of interest (object of interest?),
and the callee can send it a message to change it. Since all objects are intrinsically
shallow, any change is communicated back to the caller automatically since the object of
interest was never copied.

ref:

http://cslibrary.stanford.edu/102/PointersAndMemory.pdf

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