I know this might be a common question but I have tried to search but still cannot find a clear answer.
I have the following code:
int* f() {
int a[] = {1,2,3};
return a;
}
int main() {
int a[] = f(); // Error here
getch();
return 0;
}
This code produces the error message: "Cannot convert from 'int *' to 'int []'"
I found this quite strange because I have read that pointer and array are similar. For example, we can use a[i] instead of *(a + i).
Can anyone give me a clear explanation, please?
There are actually two errors in this code.
Firstly, you are returning the address of a temporary (the int array within f), so its contents are undefined after the function returns. Any attempt to access the memory pointed to by the returned pointer will cause undefined behaviour.
Secondly, there is no implicit conversion from pointers to array types in C++. They are similar, but not identical. Arrays can decay to pointers, but it doesn't work the other way round as information is lost on the way - a pointer just represents a memory address, while an array represents the address of a continuous region, typically with a particular size. Also you can't assign to arrays.
For example, we can use a[i] instead of *(a + i)
This, however, has little to do with the differences between arrays and pointers, it's just a syntactic rule for pointer types. As arrays decay to pointers, it works for arrays as well.
The type int[] doesn't actually exist.
When you define and initialize an array like
int a[] = {1,2,3};
the compiler counts the elements in the initializer and creates an array of the right size; in that case, it magically becomes:
int a[3] = {1,2,3};
int[] used as a parameter to a function, instead, it's just plain int *, i.e. a pointer to the first element of the array. No other information is carried with it, in particular nothing about the size is preserved. The same holds when you return a pointer
Notice that an array is not a pointer: a pointer can be changed to point to other stuff, while an array refers always to the same memory; a pointer does not know anything about how big is the space of memory it points to, while the size of an array is always known at compile time. The confusion arises from the fact that an array decays to a pointer to its first element in many circumstances, and passing it to a function/returning it from a function are some of these circumstances.
So, why doesn't your code work? There are two big errors:
You are trying to initialize an array with a pointer. We said that an int * doesn't carry any information about the size of the array. It's just a pointer to the first element. So the compiler cannot know how big a should be made to accomodate the stuff returned by f().
In f you are returning a pointer to a variable that is local to that function. This is wrong, because a pointer does not actually store the data, it only points to where the data is stored, i.e. in your case to the a local to f. Because that array is local to the function, it ceases to exist when the function exits (i.e. at the return).
This means that the pointer you are returning points to stuff that does not exist anymore; consider the code:
int * a = f();
This initialization works, and you can try to use a later in the function, but a will be pointing to the no-longer existent array of f; in the best case your program will crash (and you'll notice immediately that you've done something wrong), in the worst it will seem to work for some time, and then start giving strange results.
int * and int [] are similar but different.
int * is a real pointer, meanwhile int[] is an array reference ( a sort of "constant pointer" to the begin of the data) wich cannot be modified. So, a int * can be threated like a int [] but not viceversa.
You can use a[b] and*(a+b) interchangeably because that is exactly how a[b] is defined when one of a or b is a pointer and the other is of integer or enumeration type.
Note: This also means that expressions like 42[a] are perfectly legal. Human readers might object strongly, but the compiler won't bat an eye at this.
Related
#include <iostream>
using namespace std;
int func(int ar[5]){
//the code is written here with returning whatever the requirement is
}
int main(){
int ar[5];
func(ar);
return 0;
}
In this kind of situation where we are passing array through a function why the address of the array is used in actual parameter whereas the array in formal parameter?
This is because C handled arrays weirdly.
In C, arrays convert to a pointer to their first element at the drop of a hat, cannot be passed as arguments to a function, cannot be returned from functions, and cannot be copied by assignment.
C++, originally based off C, carries these screwed up design decisions as a legacy issue. Fixing them would break insane amounts of code.
If you want an array that behaves more reasonable, use std::array.
void foo(int[5]);
this is actually taking an int* argument.
int arr[7];
foo(arr);
this converts the 7 element arr to a pointer and passes it to foo. Which seems to take int[5] but that 5 does nothing.
Yes this is utterly crazy.
Now,
void bar(int(&)[5])
because C does not have references, the above is an actual reference to an array if 5 elements, so
bar(arr)
won't compile.
There is nothing weird, nor screwed up about how arrays are passed in C. C is expressed pretty simply, really. C /does/ have references; they are explicit, so, easy to see when they're in use. We call them pointers.
One simply needs to understand that there is not dedicated storage for array types, that arrays are not first-class types, so, won't behave like first-class types. Can't be assigned into (directly), nor passed into and out of functions directly.
In C/C++ we use pointers to refer to a chunk of memory where the contents of the array are stored.
So, all that's left is that we understand that when we declare an array, the name of the array is really a pointer to the beginning of the array.
int a[12] ;
/* a's type is this: */
int *a ;
So, when we "pass an array" to a function, we are copying the pointer into the function. The semantics are straightforward, and entirely consistent (unlike Java).
The array, itself, is never passed directly.
Finally, realise that there is no difference between a pointer to a single integer, and a pointer to a contiguous chunk of integers in memory.
Remember, a[i] is equivalent to *(a+i). So, in this example:
int i = 12 ;
int *p = &i ;
, *p is the same as *(p+0); the p can be thought of as an array of length 1.
With a slight change in perspective you should be able to see the simplicity, the elegance, that is C.
I'm trying to understand the nature of type-decay. For example, we all know arrays decay into pointers in a certain context. My attempt is to understand how int[] equates to int* but how two-dimensional arrays don't correspond to the expected pointer type. Here is a test case:
std::is_same<int*, std::decay<int[]>::type>::value; // true
This returns true as expected, but this doesn't:
std::is_same<int**, std::decay<int[][1]>::type>::value; // false
Why is this not true? I finally found a way to make it return true, and that was by making the first dimension a pointer:
std::is_same<int**, std::decay<int*[]>::type>::value; // true
And the assertion holds true for any type with pointers but with the last being the array. For example (int***[] == int****; // true).
Can I have an explanation as to why this is happening? Why doesn't the array types correspond to the pointer types as would be expected?
Why does int*[] decay into int** but not int[][]?
Because it would be impossible to do pointer arithmetic with it.
For example, int p[5][4] means an array of (length-4 array of int). There are no pointers involved, it's simply a contiguous block of memory of size 5*4*sizeof(int). When you ask for a particular element, e.g. int a = p[i][j], the compiler is really doing this:
char *tmp = (char *)p // Work in units of bytes (char)
+ i * sizeof(int[4]) // Offset for outer dimension (int[4] is a type)
+ j * sizeof(int); // Offset for inner dimension
int a = *(int *)tmp; // Back to the contained type, and dereference
Obviously, it can only do this because it knows the size of the "inner" dimension(s). Casting to an int (*)[4] retains this information; it's a pointer to (length-4 array of int). However, an int ** doesn't; it's merely a pointer to (pointer to int).
For another take on this, see the following sections of the C FAQ:
6.18: My compiler complained when I passed a two-dimensional array to a function expecting a pointer to a pointer.
6.19: How do I write functions which accept two-dimensional arrays when the width is not known at compile time?
6.20: How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions?
(This is all for C, but this behaviour is essentially unchanged in C++.)
C was not really "designed" as a language; instead, features were added as needs arose, with an effort not to break earlier code. Such an evolutionary approach was a good thing in the days when C was being developed, since it meant that for the most part developers could reap the benefits of the earlier improvements in the language before everything the language might need to do was worked out. Unfortunately, the way in which array- and pointer handling have evolved has led to a variety of rules which are, in retrospect, unfortunate.
In the C language of today, there is a fairly substantial type system, and variables have clearly defined types, but things were not always thus. A declaration char arr[8]; would allocate 8 bytes in the present scope, and make arr point to the first of them. The compiler wouldn't know that arr represented an array--it would represent a char pointer just like any other char*. From what I understand, if one had declared char arr1[8], arr2[8];, the statement arr1 = arr2; would have been perfectly legal, being somewhat equivalent conceptually to char *st1 = "foo, *st2 = "bar"; st1 = st2;, but would have almost always represented a bug.
The rule that arrays decompose into pointers stemmed from a time when arrays and pointers really were the same thing. Since then, arrays have come to be recognized as a distinct type, but the language needed to remain essentially compatible with the days when they weren't. When the rules were being formulated, the question of how two-dimensional arrays should be handled wasn't an issue because there was no such thing. One could do something like char foo[20]; char *bar[4]; int i; for (i=0; i<4; i++) bar[i] = foo + (i*5); and then use bar[x][y] in the same way as one would now use a two-dimensional array, but a compiler wouldn't view things that way--it just saw bar as a pointer to a pointer. If one wanted to make foo[1] point somewhere completely different from foo[2], one could perfectly legally do so.
When two two-dimensional arrays were added to C, it was not necessary to maintain compatibility with earlier code that declared two-dimensional arrays, because there wasn't any. While it would have been possible to specify that char bar[4][5]; would generate code equivalent to what was shown using the foo[20], in which case a char[][] would have been usable as a char**, it was thought that just as assigning array variables would have been a mistake 99% of the time, so too would have been re-assignment of array rows, had that been legal. Thus, arrays in C are recognized as distinct types, with their own rules which are a bit odd, but which are what they are.
Because int[M][N] and int** are incompatible types.
However, int[M][N] can decay into int (*)[N] type. So the following :
std::is_same<int(*)[1], std::decay<int[1][1]>::type>::value;
should give you true.
Two dimensional arrays are not stored as pointer to pointers, but as a contiguous block of memory.
An object declared as type int[y][x] is a block of size sizeof(int) * x * y whereas, an object of type int ** is a pointer to an int*
For example, can you explain what would happen in the following code?
class Vector{
int v[3];
Vector(int *x);//parameterized constructor created
};
Vector::Vector(int *x)//definition of the parameterized constructor
{
for (int i=0;i<size;i++)
v[i]=x[i];//what happens here?? why did we take pointer as arguement?
}
From my understanding, by putting v[i]=x[i] we created a new array in which all elements of v are now in x. Why did this require a pointer argument? Couldn't it have been done with a reference &?
This goes back to older style C habits, when you can use a pointer as an array, by "indexing" it's elements.
Taken from: https://en.wikibooks.org/wiki/C_Programming/Pointers_and_arrays
A variable declared as an array of some type acts as a pointer to that type. When used by itself, it points to the first element of the array.
A pointer can be indexed like an array name.
However, a few notes:
v[i]=x[i] we created a new array
No, you did not create a new array here, the array was already created when the body of the constructor got executed. What happens here is that TO the value v[i] we will assign the value of: *(x + i) ie. the ith. element from the address x points to. Unless you know how x was created and initialized this is pretty dangerous code. Just imagine you can call this method with the address of a single int. I suppose, size is 3 or less, otherwise this code has serious security issues.
You always should check for null pointers, before trying to access the value they point to.
You can pass in the array by reference if you know the size of x at compile time:
Vector(int (&x)[3]);
If you don't know the size at compile time then what you're doing goes from being unsafe code, to blatantly wrong code.
Another option is to use std::array if you know the size at compile time, and std::vector if you don't.
Just to add a bit to previous answers, the indexing operator [] actually dereferences a pointer and shifts it by index*sizeof(type) at the same time. The same relates to declaration of an array. Say, if you declare int a[1]; this means that a is now a pointer to int, i.e. int*. So if you wanted to pass it to a function as an argument, you would need to specify its type as int*.
I'm trying to understand the nature of type-decay. For example, we all know arrays decay into pointers in a certain context. My attempt is to understand how int[] equates to int* but how two-dimensional arrays don't correspond to the expected pointer type. Here is a test case:
std::is_same<int*, std::decay<int[]>::type>::value; // true
This returns true as expected, but this doesn't:
std::is_same<int**, std::decay<int[][1]>::type>::value; // false
Why is this not true? I finally found a way to make it return true, and that was by making the first dimension a pointer:
std::is_same<int**, std::decay<int*[]>::type>::value; // true
And the assertion holds true for any type with pointers but with the last being the array. For example (int***[] == int****; // true).
Can I have an explanation as to why this is happening? Why doesn't the array types correspond to the pointer types as would be expected?
Why does int*[] decay into int** but not int[][]?
Because it would be impossible to do pointer arithmetic with it.
For example, int p[5][4] means an array of (length-4 array of int). There are no pointers involved, it's simply a contiguous block of memory of size 5*4*sizeof(int). When you ask for a particular element, e.g. int a = p[i][j], the compiler is really doing this:
char *tmp = (char *)p // Work in units of bytes (char)
+ i * sizeof(int[4]) // Offset for outer dimension (int[4] is a type)
+ j * sizeof(int); // Offset for inner dimension
int a = *(int *)tmp; // Back to the contained type, and dereference
Obviously, it can only do this because it knows the size of the "inner" dimension(s). Casting to an int (*)[4] retains this information; it's a pointer to (length-4 array of int). However, an int ** doesn't; it's merely a pointer to (pointer to int).
For another take on this, see the following sections of the C FAQ:
6.18: My compiler complained when I passed a two-dimensional array to a function expecting a pointer to a pointer.
6.19: How do I write functions which accept two-dimensional arrays when the width is not known at compile time?
6.20: How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions?
(This is all for C, but this behaviour is essentially unchanged in C++.)
C was not really "designed" as a language; instead, features were added as needs arose, with an effort not to break earlier code. Such an evolutionary approach was a good thing in the days when C was being developed, since it meant that for the most part developers could reap the benefits of the earlier improvements in the language before everything the language might need to do was worked out. Unfortunately, the way in which array- and pointer handling have evolved has led to a variety of rules which are, in retrospect, unfortunate.
In the C language of today, there is a fairly substantial type system, and variables have clearly defined types, but things were not always thus. A declaration char arr[8]; would allocate 8 bytes in the present scope, and make arr point to the first of them. The compiler wouldn't know that arr represented an array--it would represent a char pointer just like any other char*. From what I understand, if one had declared char arr1[8], arr2[8];, the statement arr1 = arr2; would have been perfectly legal, being somewhat equivalent conceptually to char *st1 = "foo, *st2 = "bar"; st1 = st2;, but would have almost always represented a bug.
The rule that arrays decompose into pointers stemmed from a time when arrays and pointers really were the same thing. Since then, arrays have come to be recognized as a distinct type, but the language needed to remain essentially compatible with the days when they weren't. When the rules were being formulated, the question of how two-dimensional arrays should be handled wasn't an issue because there was no such thing. One could do something like char foo[20]; char *bar[4]; int i; for (i=0; i<4; i++) bar[i] = foo + (i*5); and then use bar[x][y] in the same way as one would now use a two-dimensional array, but a compiler wouldn't view things that way--it just saw bar as a pointer to a pointer. If one wanted to make foo[1] point somewhere completely different from foo[2], one could perfectly legally do so.
When two two-dimensional arrays were added to C, it was not necessary to maintain compatibility with earlier code that declared two-dimensional arrays, because there wasn't any. While it would have been possible to specify that char bar[4][5]; would generate code equivalent to what was shown using the foo[20], in which case a char[][] would have been usable as a char**, it was thought that just as assigning array variables would have been a mistake 99% of the time, so too would have been re-assignment of array rows, had that been legal. Thus, arrays in C are recognized as distinct types, with their own rules which are a bit odd, but which are what they are.
Because int[M][N] and int** are incompatible types.
However, int[M][N] can decay into int (*)[N] type. So the following :
std::is_same<int(*)[1], std::decay<int[1][1]>::type>::value;
should give you true.
Two dimensional arrays are not stored as pointer to pointers, but as a contiguous block of memory.
An object declared as type int[y][x] is a block of size sizeof(int) * x * y whereas, an object of type int ** is a pointer to an int*
So, I always knew that the array "objects" that are passed around in C/C++ just contained the address of the first object in the array.
How can the pointer to the array "object" and it's contained value be the same?
Could someone point me towards more information maybe about how all that works in assembly, maybe.
Short answer: A pointer to an array is defined to have the same value as a pointer to the first element of the array. That's how arrays in C and C++ work.
Pedantic answer:
C and C++ have rvalue and lvalue expressions. An lvalue is something to which the & operator may be applied. They also have implicit conversions. An object may be converted to another type before being used. (For example, if you call sqrt( 9 ) then 9 is converted to double because sqrt( int ) is not defined.)
An lvalue of array type implicitly converts to a pointer. The implicit conversion changes array to &array[0]. This may also be written out explicitly as static_cast< int * >( array ), in C++.
Doing that is OK. Casting to void* is another story. void* is a bit ugly. And casting with the parentheses as (void*)array is also ugly. So please, avoid (void*) a in actual code.
You are mixing two unrelated (and, actually, mutually exclusive) things, which creates more confusion.
Firstly, you are correctly stating that "array objects that are passed around in C/C++ just contained the address of the first object in the array". The key words here are "passed around". In reality arrays cannot be passed around as array objects. Arrays are not copyable. Whenever you are using an array-style declaration in function parameter list it is actually interpreted as pointer declaration, i.e. it is a pointer that you are "passing around", not the array. However, in such situations your equality does not hold
void foo(int a[]) {
assert((void *) &a == (void *) a); // FAIL!!!
}
The above assertion is guaranteed to fail - the equality does not hold. So, within the context of this question you have to forget about arrays that you "pass around" (at least for the syntax used in the above example). Your equality does not hold for arrays that have been replaced by pointer objects.
Secondly, actual array objects are not pointers. And there's no need to take the term object into quotation markes. Arrays are full-fledged objects, albeit with some peculiar properties. The equality in question does indeed hold for the actual arrays that have not lost their "arrayness", i.e. array object that have not been replaced by pointer objects. For example
int a[10];
assert((void *) &a == (void *) a); // Never fails
What it means is that numerically the address of the entire array is the same as the address of its first element. Nothing unusual here. In fact, the very same (in nature) equality can be observed with struct types in C/C++
struct S { int x; } a;
assert((void *) &a == (void *) &a.x); // Never fails
I.e. the address of the entire struct object is the same as the address of its first field.
How can the pointer to the array "object" and it's contained value be the same?
An array is a contiguous block of memory which stores several elements.
Obviously, the first element in the array is located at some address.
There's no data "in between" the first element and the beginning of the actual array.
Therefore, the first element has the same address as the array.
Please read the following thread
http://www.cplusplus.com/forum/beginner/29595/
It basically explains that (&a != a) due to the type difference (since &a returns the pointer to the array and a to the first element) even though they both point to the same address.
Since you are casting them both to (void*) only the address value is compared and found to be equal, meaning that ((void*) a == (void*)&a) as you've stated. This makes sense since the array's address has to be the same as the first elements.
Let's look at these two declarations:
int a[4];
int * b;
Both a and b have a type compatible with int * and can, for example, be passed as an argument to a function expecting int *:
void f(int * p);
f(a); // OK
f(b); // OK
In case of a, the compiler allocates space for 4 int values. When you use the name a, such as when calling f(a), the compiler just substitutes the address of where it allocated the first of those int values, since it knows.
In case of b, the compiler allocates space for one pointer. When you use the name b, such as when calling f(b), the compiler generates code for retrieveing the pointer value from the allocated storage.
When it comes to &, that's when the difference between a and b becomes apparent. & always means the address of the storage the compiler has allocated for your variable: &a is the address of those four int values (therefore coinciding with just a), while &b is the address of the pointer value. They have different types, too.
&a is not exactly the same as a, though, even though they compare as equal. They have a different type: &a is a pointer and a is an array. You can notice the difference, for example, if you apply the sizeof operator to these expressions: sizeof(a) will evaluate to the size of four int values, while sizeof(&a) is the size of a pointer.
Ok, So what I thought happened is that when you created an array, you allocated space for the array somewhere and you created a pointer to its first object somewhere else, and what you passed around in your code was the pointer.
This is actually the behavior of what happens when you create an array with new in C++ or with malloc in C/C++. As such,
int * a = new a[SIZE];
assert((void*)&a==(void*)a); // Always fails
What I learned is that for arrays declared in the style of int a[SIZE];, a pointer to the first element is created when you try to pass the array to a function (this is called array-pointer decay). It's interesting to note that, indeed, as AndreyT writes,
void foo(int a[]) {
assert((void *) &a == (void *) a); // Always fails
}
This shows that it's only when you try to pass arrays around that a pointer is created for arrays in the style of int a[SIZE];.