When I want to pass an array by reference to a function I don't know what to choose.
void myFunction(int* data);
Is there a difference or a best coding way between those two cases:
myFunction(&data[0]);
Or
myFunction(data);
There is no difference. Arrays ("proper" arrays) automatically decay to pointers to their first element.
For example, lets say you have
int my_array[10];
then using plain my_array will automatically decay to a pointer to its first element, which is &my_array[0].
It is this array-to-pointer decay that allows you to use both pointer arithmetic and array indexing for both arrays and pointers. For the array above my_array[i] is exactly equal to *(my_array + i). This equivalence also exists for pointers:
int *my_pointer = my_array; // Make my_pointer point to the first element of my_array
Then my_pointer[i] is also exactly equal to *(my_pointer + i).
For curiosity (and something you should never do in real programs), thanks to the commutative property of addition an expression such as *(my_array + i) will also be equal to *(i + my_array) which is then equal to i[my_array].
An array, when passed as a parameter to a function, automatically decays to a pointer to its first element. So passing either data or &data[0] to the function are exactly equivalent.
From a readability standpoint I would opt for the former. It makes it clear to the reader that the function is potentially operating on the entire array and not just on one element.
Apart from the obvious (data being shorter than &data[0]; therefore easier to write and to read), there's no difference.
Think about what &data[0] means:
It's a pointer to data[0].
And data[0] just means *(data+0), i.e. *data.
A pointer to *data is simply data.
data is a pointer to the beginning of the array.
&data[0] is an address of the first element of an array.
When reading a code, the first option is, for the most people, more readable and i suppose is a way most programmers will and should choose
There isn't any difference as both point to the same starting location of the array i.e. a[0].
I would just use my_function(data) because why make it more confusing than it has to be?
If for some reason you needed to find the memory address of a single element somewhere in the middle of data, then my_function(&data[17]) might possibly be warranted, but there are probably better ways to handle that case too.
In general if you have to manually and specifically pick out single pieces of data like that by hand, you are probably not doing it in a very good way.
There are rare cases where it can makes sense ( like if you are parsing data from some other source and you ALWAYS 100% of the time only care about the 17th byte )... but that's not usually the case.
Consider the following:
As your code evolves and you make changes, you will probably also slightly change data structures. Data[17] might no longer be the magical byte that you need anymore. Now it might be data[18]. If you manually hard coded data[17] in 100 or 1000 different places in your code, you will now have to go manually change them all and hope that it doesn't cause any new bugs. Also... portability issues.
Instead design functions that can find and return whatever data you need from your data structures without needing any hard coded addresses. They will still work ( if designed properly ) as your code evolves and will be 1000 times more portable.
No difference. When coerced into a pointer, an array (data) decays into a pointer to its first element (&data[0]).
Remember that data[0] simply means *(data+0), so &data[0] is equivalent to &*(data+0), which simplifies to data (because &* cancels out).
Demo:
#include <stdio.h>
int main(void) {
int data[2];
printf("%p\n", (void*)data);
printf("%p\n", (void*)&*(data+0));
printf("%p\n", (void*)&data[0]);
return 0;
}
Output:
$ gcc -Wall -Wextra -pedantic a.c -o a && a
0x3c2180f4fa0
0x3c2180f4fa0
0x3c2180f4fa0
I always advice to use a general approach.
Just consider the function
void myFunction( char* data);
where the parameter has the type char * instead of int *.
And now let's assume that you want to pass to the function a string literal.
It can be done either like
myFunction( "Hello" );
or like
myFunction( &"Hello"[0] );
It is evident that the first approach is more clear and readable.
So I prefer to use the first approach.:)
In fact such an expression
&data[i];
is syntactically redundant.. In fact it looks like
&( *( data + i ) )
that is equivalent to just
data + i
When i is equal to 0 then you have
data + 0
that in expressions is equivalent (I do not take into account for example the sizeof operaor) to
data
So use data instead of &data[0].
Related
This question goes out to the C gurus out there:
In C, it is possible to declare a pointer as follows:
char (* p)[10];
.. which basically states that this pointer points to an array of 10 chars. The neat thing about declaring a pointer like this is that you will get a compile time error if you try to assign a pointer of an array of different size to p. It will also give you a compile time error if you try to assign the value of a simple char pointer to p. I tried this with gcc and it seems to work with ANSI, C89 and C99.
It looks to me like declaring a pointer like this would be very useful - particularly, when passing a pointer to a function. Usually, people would write the prototype of such a function like this:
void foo(char * p, int plen);
If you were expecting a buffer of an specific size, you would simply test the value of plen. However, you cannot be guaranteed that the person who passes p to you will really give you plen valid memory locations in that buffer. You have to trust that the person who called this function is doing the right thing. On the other hand:
void foo(char (*p)[10]);
..would force the caller to give you a buffer of the specified size.
This seems very useful but I have never seen a pointer declared like this in any code I have ever ran across.
My question is: Is there any reason why people do not declare pointers like this? Am I not seeing some obvious pitfall?
What you are saying in your post is absolutely correct. I'd say that every C developer comes to exactly the same discovery and to exactly the same conclusion when (if) they reach certain level of proficiency with C language.
When the specifics of your application area call for an array of specific fixed size (array size is a compile-time constant), the only proper way to pass such an array to a function is by using a pointer-to-array parameter
void foo(char (*p)[10]);
(in C++ language this is also done with references
void foo(char (&p)[10]);
).
This will enable language-level type checking, which will make sure that the array of exactly correct size is supplied as an argument. In fact, in many cases people use this technique implicitly, without even realizing it, hiding the array type behind a typedef name
typedef int Vector3d[3];
void transform(Vector3d *vector);
/* equivalent to `void transform(int (*vector)[3])` */
...
Vector3d vec;
...
transform(&vec);
Note additionally that the above code is invariant with relation to Vector3d type being an array or a struct. You can switch the definition of Vector3d at any time from an array to a struct and back, and you won't have to change the function declaration. In either case the functions will receive an aggregate object "by reference" (there are exceptions to this, but within the context of this discussion this is true).
However, you won't see this method of array passing used explicitly too often, simply because too many people get confused by a rather convoluted syntax and are simply not comfortable enough with such features of C language to use them properly. For this reason, in average real life, passing an array as a pointer to its first element is a more popular approach. It just looks "simpler".
But in reality, using the pointer to the first element for array passing is a very niche technique, a trick, which serves a very specific purpose: its one and only purpose is to facilitate passing arrays of different size (i.e. run-time size). If you really need to be able to process arrays of run-time size, then the proper way to pass such an array is by a pointer to its first element with the concrete size supplied by an additional parameter
void foo(char p[], unsigned plen);
Actually, in many cases it is very useful to be able to process arrays of run-time size, which also contributes to the popularity of the method. Many C developers simply never encounter (or never recognize) the need to process a fixed-size array, thus remaining oblivious to the proper fixed-size technique.
Nevertheless, if the array size is fixed, passing it as a pointer to an element
void foo(char p[])
is a major technique-level error, which unfortunately is rather widespread these days. A pointer-to-array technique is a much better approach in such cases.
Another reason that might hinder the adoption of the fixed-size array passing technique is the dominance of naive approach to typing of dynamically allocated arrays. For example, if the program calls for fixed arrays of type char[10] (as in your example), an average developer will malloc such arrays as
char *p = malloc(10 * sizeof *p);
This array cannot be passed to a function declared as
void foo(char (*p)[10]);
which confuses the average developer and makes them abandon the fixed-size parameter declaration without giving it a further thought. In reality though, the root of the problem lies in the naive malloc approach. The malloc format shown above should be reserved for arrays of run-time size. If the array type has compile-time size, a better way to malloc it would look as follows
char (*p)[10] = malloc(sizeof *p);
This, of course, can be easily passed to the above declared foo
foo(p);
and the compiler will perform the proper type checking. But again, this is overly confusing to an unprepared C developer, which is why you won't see it in too often in the "typical" average everyday code.
I would like to add to AndreyT's answer (in case anyone stumbles upon this page looking for more info on this topic):
As I begin to play more with these declarations, I realize that there is major handicap associated with them in C (apparently not in C++). It is fairly common to have a situation where you would like to give a caller a const pointer to a buffer you have written into. Unfortunately, this is not possible when declaring a pointer like this in C. In other words, the C standard (6.7.3 - Paragraph 8) is at odds with something like this:
int array[9];
const int (* p2)[9] = &array; /* Not legal unless array is const as well */
This constraint does not seem to be present in C++, making these type of declarations far more useful. But in the case of C, it is necessary to fall back to a regular pointer declaration whenever you want a const pointer to the fixed size buffer (unless the buffer itself was declared const to begin with). You can find more info in this mail thread: link text
This is a severe constraint in my opinion and it could be one of the main reasons why people do not usually declare pointers like this in C. The other being the fact that most people do not even know that you can declare a pointer like this as AndreyT has pointed out.
The obvious reason is that this code doesn't compile:
extern void foo(char (*p)[10]);
void bar() {
char p[10];
foo(p);
}
The default promotion of an array is to an unqualified pointer.
Also see this question, using foo(&p) should work.
I also want to use this syntax to enable more type checking.
But I also agree that the syntax and mental model of using pointers is simpler, and easier to remember.
Here are some more obstacles I have come across.
Accessing the array requires using (*p)[]:
void foo(char (*p)[10])
{
char c = (*p)[3];
(*p)[0] = 1;
}
It is tempting to use a local pointer-to-char instead:
void foo(char (*p)[10])
{
char *cp = (char *)p;
char c = cp[3];
cp[0] = 1;
}
But this would partially defeat the purpose of using the correct type.
One has to remember to use the address-of operator when assigning an array's address to a pointer-to-array:
char a[10];
char (*p)[10] = &a;
The address-of operator gets the address of the whole array in &a, with the correct type to assign it to p. Without the operator, a is automatically converted to the address of the first element of the array, same as in &a[0], which has a different type.
Since this automatic conversion is already taking place, I am always puzzled that the & is necessary. It is consistent with the use of & on variables of other types, but I have to remember that an array is special and that I need the & to get the correct type of address, even though the address value is the same.
One reason for my problem may be that I learned K&R C back in the 80s, which did not allow using the & operator on whole arrays yet (although some compilers ignored that or tolerated the syntax). Which, by the way, may be another reason why pointers-to-arrays have a hard time to get adopted: they only work properly since ANSI C, and the & operator limitation may have been another reason to deem them too awkward.
When typedef is not used to create a type for the pointer-to-array (in a common header file), then a global pointer-to-array needs a more complicated extern declaration to share it across files:
fileA:
char (*p)[10];
fileB:
extern char (*p)[10];
Well, simply put, C doesn't do things that way. An array of type T is passed around as a pointer to the first T in the array, and that's all you get.
This allows for some cool and elegant algorithms, such as looping through the array with expressions like
*dst++ = *src++
The downside is that management of the size is up to you. Unfortunately, failure to do this conscientiously has also led to millions of bugs in C coding, and/or opportunities for malevolent exploitation.
What comes close to what you ask in C is to pass around a struct (by value) or a pointer to one (by reference). As long as the same struct type is used on both sides of this operation, both the code that hand out the reference and the code that uses it are in agreement about the size of the data being handled.
Your struct can contain whatever data you want; it could contain your array of a well-defined size.
Still, nothing prevents you or an incompetent or malevolent coder from using casts to fool the compiler into treating your struct as one of a different size. The almost unshackled ability to do this kind of thing is a part of C's design.
You can declare an array of characters a number of ways:
char p[10];
char* p = (char*)malloc(10 * sizeof(char));
The prototype to a function that takes an array by value is:
void foo(char* p); //cannot modify p
or by reference:
void foo(char** p); //can modify p, derefernce by *p[0] = 'f';
or by array syntax:
void foo(char p[]); //same as char*
I would not recommend this solution
typedef int Vector3d[3];
since it obscures the fact that Vector3D has a type that you
must know about. Programmers usually dont expect variables of the
same type to have different sizes. Consider :
void foo(Vector3d a) {
Vector3d b;
}
where sizeof a != sizeof b
Maybe I'm missing something, but... since arrays are constant pointers, basically that means that there's no point in passing around pointers to them.
Couldn't you just use void foo(char p[10], int plen); ?
type (*)[];
// points to an array e.g
int (*ptr)[5];
// points to an 5 integer array
// gets the address of the array
type *[];
// points to an array of pointers e.g
int* ptr[5]
// point to an array of five integer pointers
// point to 5 adresses.
On my compiler (vs2008) it treats char (*p)[10] as an array of character pointers, as if there was no parentheses, even if I compile as a C file. Is compiler support for this "variable"? If so that is a major reason not to use it.
Some blogs and sites were talking about pointers are beneficial one of the causes was because the "execution speed" will be better in a program with pointers than without pointers. The thing I can work out is that:
Dereferencing a single location requires two (or more) memory accesses (depending on number of indirection). Which will increase the execution time, compared to if it was used directly.
Passing a pointer to a large datatype to a function, like a structure can be beneficial, as only the address of the structure/union is getting copied and it's not getting passed by value. Therefore it should be faster in this case.
For example just by force introducing pointers without any need as:
int a, b, *p, *q, c, *d;
p = &a;
q = &b;
d = &c
// get values in a, b
*d = *p + *q; // why the heck this would be faster
c = a + b; // than this code?
I checked the assembler output using gcc -S -masm=intel file.c The pointer version has a lot of loading memory and storing for the dereferences than the direct method.
Am I missing something?
Note: The question is not related to just the code. The code is just an example. Not considering compiler optimizations.
I think your conclusions are basically right. The author did not mean that using more pointers will always speed up all code. That's obviously nonsense.
But there are times when it is faster to pass a pointer to data instead of copying that data.
As you pointed out: Passing a pointer to a large datatype to a function; here the structure is an int, so it's hardly large. BTW: I guess gcc will optimize away the pointer accesses when you use -O2.
Apart from that your understanding is correct.
You are right in your example - that code would run slower. One place where it can be faster is when making a function call:
void foo( Object Obj );
void bar( const Object * pObj );
void main()
{
Object theObject;
foo( theObject ); // Creates a copy of theObject which is then used in the function.
bar( &theObject ); // Creates a copy of the memory address only, then the function references the original object within.
}
bar is faster as we don't need to copy the entire object (assuming the object is more than just a base data type). Most people would use a reference rather than a pointer in this instance, however.
void foobar( const Object & Obj );
Mark Byers is absolutely right. You cannot judge the power of pointers in such simple program.They are used to optimize the memory management and faster execution of programs where there are excessive use of data structures and references are done through addresses.
Consider when you start a program it takes some time in loading but with efficient use of pointers and skills if the program loads even 1 second earlier that's a large accomplishment.
This question goes out to the C gurus out there:
In C, it is possible to declare a pointer as follows:
char (* p)[10];
.. which basically states that this pointer points to an array of 10 chars. The neat thing about declaring a pointer like this is that you will get a compile time error if you try to assign a pointer of an array of different size to p. It will also give you a compile time error if you try to assign the value of a simple char pointer to p. I tried this with gcc and it seems to work with ANSI, C89 and C99.
It looks to me like declaring a pointer like this would be very useful - particularly, when passing a pointer to a function. Usually, people would write the prototype of such a function like this:
void foo(char * p, int plen);
If you were expecting a buffer of an specific size, you would simply test the value of plen. However, you cannot be guaranteed that the person who passes p to you will really give you plen valid memory locations in that buffer. You have to trust that the person who called this function is doing the right thing. On the other hand:
void foo(char (*p)[10]);
..would force the caller to give you a buffer of the specified size.
This seems very useful but I have never seen a pointer declared like this in any code I have ever ran across.
My question is: Is there any reason why people do not declare pointers like this? Am I not seeing some obvious pitfall?
What you are saying in your post is absolutely correct. I'd say that every C developer comes to exactly the same discovery and to exactly the same conclusion when (if) they reach certain level of proficiency with C language.
When the specifics of your application area call for an array of specific fixed size (array size is a compile-time constant), the only proper way to pass such an array to a function is by using a pointer-to-array parameter
void foo(char (*p)[10]);
(in C++ language this is also done with references
void foo(char (&p)[10]);
).
This will enable language-level type checking, which will make sure that the array of exactly correct size is supplied as an argument. In fact, in many cases people use this technique implicitly, without even realizing it, hiding the array type behind a typedef name
typedef int Vector3d[3];
void transform(Vector3d *vector);
/* equivalent to `void transform(int (*vector)[3])` */
...
Vector3d vec;
...
transform(&vec);
Note additionally that the above code is invariant with relation to Vector3d type being an array or a struct. You can switch the definition of Vector3d at any time from an array to a struct and back, and you won't have to change the function declaration. In either case the functions will receive an aggregate object "by reference" (there are exceptions to this, but within the context of this discussion this is true).
However, you won't see this method of array passing used explicitly too often, simply because too many people get confused by a rather convoluted syntax and are simply not comfortable enough with such features of C language to use them properly. For this reason, in average real life, passing an array as a pointer to its first element is a more popular approach. It just looks "simpler".
But in reality, using the pointer to the first element for array passing is a very niche technique, a trick, which serves a very specific purpose: its one and only purpose is to facilitate passing arrays of different size (i.e. run-time size). If you really need to be able to process arrays of run-time size, then the proper way to pass such an array is by a pointer to its first element with the concrete size supplied by an additional parameter
void foo(char p[], unsigned plen);
Actually, in many cases it is very useful to be able to process arrays of run-time size, which also contributes to the popularity of the method. Many C developers simply never encounter (or never recognize) the need to process a fixed-size array, thus remaining oblivious to the proper fixed-size technique.
Nevertheless, if the array size is fixed, passing it as a pointer to an element
void foo(char p[])
is a major technique-level error, which unfortunately is rather widespread these days. A pointer-to-array technique is a much better approach in such cases.
Another reason that might hinder the adoption of the fixed-size array passing technique is the dominance of naive approach to typing of dynamically allocated arrays. For example, if the program calls for fixed arrays of type char[10] (as in your example), an average developer will malloc such arrays as
char *p = malloc(10 * sizeof *p);
This array cannot be passed to a function declared as
void foo(char (*p)[10]);
which confuses the average developer and makes them abandon the fixed-size parameter declaration without giving it a further thought. In reality though, the root of the problem lies in the naive malloc approach. The malloc format shown above should be reserved for arrays of run-time size. If the array type has compile-time size, a better way to malloc it would look as follows
char (*p)[10] = malloc(sizeof *p);
This, of course, can be easily passed to the above declared foo
foo(p);
and the compiler will perform the proper type checking. But again, this is overly confusing to an unprepared C developer, which is why you won't see it in too often in the "typical" average everyday code.
I would like to add to AndreyT's answer (in case anyone stumbles upon this page looking for more info on this topic):
As I begin to play more with these declarations, I realize that there is major handicap associated with them in C (apparently not in C++). It is fairly common to have a situation where you would like to give a caller a const pointer to a buffer you have written into. Unfortunately, this is not possible when declaring a pointer like this in C. In other words, the C standard (6.7.3 - Paragraph 8) is at odds with something like this:
int array[9];
const int (* p2)[9] = &array; /* Not legal unless array is const as well */
This constraint does not seem to be present in C++, making these type of declarations far more useful. But in the case of C, it is necessary to fall back to a regular pointer declaration whenever you want a const pointer to the fixed size buffer (unless the buffer itself was declared const to begin with). You can find more info in this mail thread: link text
This is a severe constraint in my opinion and it could be one of the main reasons why people do not usually declare pointers like this in C. The other being the fact that most people do not even know that you can declare a pointer like this as AndreyT has pointed out.
The obvious reason is that this code doesn't compile:
extern void foo(char (*p)[10]);
void bar() {
char p[10];
foo(p);
}
The default promotion of an array is to an unqualified pointer.
Also see this question, using foo(&p) should work.
I also want to use this syntax to enable more type checking.
But I also agree that the syntax and mental model of using pointers is simpler, and easier to remember.
Here are some more obstacles I have come across.
Accessing the array requires using (*p)[]:
void foo(char (*p)[10])
{
char c = (*p)[3];
(*p)[0] = 1;
}
It is tempting to use a local pointer-to-char instead:
void foo(char (*p)[10])
{
char *cp = (char *)p;
char c = cp[3];
cp[0] = 1;
}
But this would partially defeat the purpose of using the correct type.
One has to remember to use the address-of operator when assigning an array's address to a pointer-to-array:
char a[10];
char (*p)[10] = &a;
The address-of operator gets the address of the whole array in &a, with the correct type to assign it to p. Without the operator, a is automatically converted to the address of the first element of the array, same as in &a[0], which has a different type.
Since this automatic conversion is already taking place, I am always puzzled that the & is necessary. It is consistent with the use of & on variables of other types, but I have to remember that an array is special and that I need the & to get the correct type of address, even though the address value is the same.
One reason for my problem may be that I learned K&R C back in the 80s, which did not allow using the & operator on whole arrays yet (although some compilers ignored that or tolerated the syntax). Which, by the way, may be another reason why pointers-to-arrays have a hard time to get adopted: they only work properly since ANSI C, and the & operator limitation may have been another reason to deem them too awkward.
When typedef is not used to create a type for the pointer-to-array (in a common header file), then a global pointer-to-array needs a more complicated extern declaration to share it across files:
fileA:
char (*p)[10];
fileB:
extern char (*p)[10];
Well, simply put, C doesn't do things that way. An array of type T is passed around as a pointer to the first T in the array, and that's all you get.
This allows for some cool and elegant algorithms, such as looping through the array with expressions like
*dst++ = *src++
The downside is that management of the size is up to you. Unfortunately, failure to do this conscientiously has also led to millions of bugs in C coding, and/or opportunities for malevolent exploitation.
What comes close to what you ask in C is to pass around a struct (by value) or a pointer to one (by reference). As long as the same struct type is used on both sides of this operation, both the code that hand out the reference and the code that uses it are in agreement about the size of the data being handled.
Your struct can contain whatever data you want; it could contain your array of a well-defined size.
Still, nothing prevents you or an incompetent or malevolent coder from using casts to fool the compiler into treating your struct as one of a different size. The almost unshackled ability to do this kind of thing is a part of C's design.
You can declare an array of characters a number of ways:
char p[10];
char* p = (char*)malloc(10 * sizeof(char));
The prototype to a function that takes an array by value is:
void foo(char* p); //cannot modify p
or by reference:
void foo(char** p); //can modify p, derefernce by *p[0] = 'f';
or by array syntax:
void foo(char p[]); //same as char*
I would not recommend this solution
typedef int Vector3d[3];
since it obscures the fact that Vector3D has a type that you
must know about. Programmers usually dont expect variables of the
same type to have different sizes. Consider :
void foo(Vector3d a) {
Vector3d b;
}
where sizeof a != sizeof b
Maybe I'm missing something, but... since arrays are constant pointers, basically that means that there's no point in passing around pointers to them.
Couldn't you just use void foo(char p[10], int plen); ?
type (*)[];
// points to an array e.g
int (*ptr)[5];
// points to an 5 integer array
// gets the address of the array
type *[];
// points to an array of pointers e.g
int* ptr[5]
// point to an array of five integer pointers
// point to 5 adresses.
On my compiler (vs2008) it treats char (*p)[10] as an array of character pointers, as if there was no parentheses, even if I compile as a C file. Is compiler support for this "variable"? If so that is a major reason not to use it.
I've got some code I'm mantaining with the following variable declaration:
char tmpry[40];
It's being used with this function:
char *SomeFunction(char *tmpryP) {
// Do stuff.
}
The function call is:
SomeFunction(&tmpry[0]);
I'm pretty damn sure that's just the same as:
SomeFunction(tmpry);
I've even checked that the char* pointer in SomeFunction ends up pointing to the same memory location as the array in both cases.
My question is a sanity check as to whether the two function calls are identical (and therefore the original programmer was just being nasty)?
they are exactly the same.
someArray[i]
means exactly
*(someArray + i)
where someArray in the second case is a pointer to the memory location. Similarly,
&someArray[i]
means exactly
(someArray + i)
In both these cases the terms are pointers to memory locations.
The two are equivalent and I think SomeFunction(tmpry); is more readable.
It may be significant, if SomeFunction is a macro and takes "sizeof" of one of its arguments, because sizeof(tmpry) may not necessarily be equal to sizeof(&tmpry[0]).
Otherwise, as others pointed out, they are exactly the same.
The C Programming FAQ section on arrays and pointers tackles this (and many other common C questions and confusions.
As everyone else said, the two notations are equivalent.
I would normally use the simpler one, unless there are multiple calls like this:
SomeFunction(&tmparry[0]);
SomeFunction(&tmparry[10]);
SomeFunction(&tmparry[NNN]);
Where ideally all the constants (magic numbers) would be enum (or #define) constants.
Both are one and the same although second one looks nice and clarifies the intention of passing the array to the function. But how does the SomeFunction know the size of the array being passed, is it always assumed as 40 ? I feel it is better to pass the size also as the parameter to SomeFunction.
These two variants are equivalent and passing like this
SomeFunction(tmpry);
looks cleaner.
The declaration
int a[10];
int *pa;
There is one difference between an array and a pointer that must be kept in mind.
A pointer is a variable, so pa=a and pa++ are legal.
But an array name is not a variable; construction like a=pa and a++ are illegal
As format parameters in a function definition,
char s[]
and
char *s
are equivalent;
From: The C Programming Language 2th, Page 99-100
I'm preparing some slides for an introductory C class, and I'm trying to present good examples (and motivation) for using pointer arithmetic over array subscripting.
A lot of the examples I see in books are fairly equivalent. For example, many books show how to reverse the case of all values in a string, but with the exception of replacing an a[i] with a *p the code is identical.
I am looking for a good (and short) example with single-dimensional arrays where pointer arithmetic can produce significantly more elegant code. Any ideas?
Getting a pointer again instead of a value:
One usually uses pointer arithmetic when they want to get a pointer again. To get a pointer while using an array index: you are 1) calculating the pointer offset, then 2) getting the value at that memory location, then 3) you have to use & to get the address again. That's more typing and less clean syntax.
Example 1: Let's say you need a pointer to the 512th byte in a buffer
char buffer[1024]
char *p = buffer + 512;
Is cleaner than:
char buffer[1024];
char *p = &buffer[512];
Example 2: More efficient strcat
char buffer[1024];
strcpy(buffer, "hello ");
strcpy(buffer + 6, "world!");
This is cleaner than:
char buffer[1024];
strcpy(buffer, "hello ");
strcpy(&buffer[6], "world!");
Using pointer arithmetic ++ as an iterator:
Incrementing pointers with ++, and decrementing with -- is useful when iterating over each element in an array of elements. It is cleaner than using a separate variable used to keep track of the offset.
Pointer subtraction:
You can use pointer subtraction with pointer arithmetic. This can be useful in some cases to get the element before the one you are pointing to. It can be done with array subscripts too, but it looks really bad and confusing. Especially to a python programmer where a negative subscript is given to index something from the end of the list.
char *my_strcpy(const char *s, char *t) {
char *u = t;
while (*t++ = *s++);
return u;
}
Why would you want to spoil such a beauty with an index? (See K&R, and how they build on up to this style.)There is a reason I used the above signature the way it is. Stop editing without asking for a clarification first. For those who think they know, look up the present signature -- you missed a few restrict qualifications.
Structure alignment testing and the offsetof macro implementation.
Pointer arithmetic may look fancy and "hackerish", but I have never encountered a case it was FASTER than the standard indexing. Just the opposite, I often encountered cases when it slowed the code down by a large factor.
For example, typical sequential looping through an array with a pointer may be less efficient than looping with a classic index on a modern processors, that support SSE extensions. Pointer arithmetic in a loop sufficiently blocks compilers from performing loop vectorization, which can yield typical 2x-4x performance boost. Additionally, using pointers instead of simple integer variables may result in needless memory store operations due to pointer aliasing.
So, generally pointer arithmetic instead of standard indexed access should NEVER be recommended.
iterating through a 2-dimensional array where the position of a datum does not really matter
if you dont use pointers, you would have to keep track of two subscripts
with pointers, you could point to the top of your array, and with a single loop, zip through the whole thing
If you were using an old compiler, or some kind of specialist embedded systems compiler, there might be slight performance differences, but most modern compilers would probably optimize these (tiny) differences out.
The following article might be something you could draw on - depends on the level of your students:
http://geeks.netindonesia.net/blogs/risman/archive/2007/06/25/Pointer-Arithmetic-and-Array-Indexing.aspx
You're asking about C specifically, but C++ builds upon this as well:
Most pointer arithmetic naturally generalizes to the Forward Iterator concept. Walking through memory with *p++ can be used for any sequenced container (linked list, skip list, vector, binary tree, B tree, etc), thanks to operator overloading.
Something fun I hope you never have to deal with: pointers can alias, whereas arrays cannot. Aliasing can cause all sorts of non-ideal code generation, the most common of which is using a pointer as an out parameter to another function. Basically, the compiler cannot assume that the pointer used by the function doesn't alias itself or anything else in that stack frame, so it has to reload the value from the pointer every time it's used. Or rather, to be safe it does.
Often the choice is just one of style - one looks or feels more natural than the other for a particular case.
There is also the argument that using indexes can cause the compiler to have to repeatedly recalculate offsets inside a loop - I'm not sure how often this is the case (other than in non-optimized builds), but I imagine it happens, but it's probably rarely a problem.
One area that I think is important in the long run (which might not apply to an introductory C class - but learn 'em early, I say) is that using pointer arithmetic applies to the idioms used in the C++ STL. If you get them to understand pointer arithmetic and use it, then when they move on to the STL, they'll have a leg up on how to properly use iterators.
#include ctype.h
void skip_spaces( const char **ppsz )
{
const char *psz = *ppsz;
while( isspace(*psz) )
psz++;
*ppsz = psz;
}
void fn(void)
{
char a[]=" Hello World!";
const char *psz = a;
skip_spaces( &psz );
printf("\n%s", psz);
}