Length of array using pointer
Author
Hi! Everyone,
int *array;
Suppose, I've initialized it in somewhere.
Now, how do I know the length of the array using this pointer?
int getLength() {
return ..... what?
}
Thanks in advance.
You can't.
You can either inject a terminator character in that array, and check for it or store the length in a variable.
Maybe you can create a struct with an int to indicate the length:
You can either inject a terminator character in that array, and check for it or store the length in a variable.
Maybe you can create a struct with an int to indicate the length:
struct intArray{int* dataint length;};
To expand on the above suggestion to keep the length around yourself, your main alternatives are, in C++, to use a container (std::vector offers all the storage benefits of raw memory buffers, without the disadvantages), and in C to keep the size of the object around (this generally forces you to implement the array bookkeeping for every type).
A generic but type-unsafe system would be:
A generic type-safe template-like system could look like:
WIth the obvious "macros are evil" restriction that a should always be an array variable and not an expression.
A generic but type-unsafe system would be:
typedef struct { size_t size; void * data;};A generic type-safe template-like system could look like:
#define ARRAY(T) struct { T *data; size_t size; }#define ALLOC_ARRAY(a,size) do { \ a.data = malloc( size * sizeof(*a.data) ); \ a.size = size; } while(0)#define RESIZE_ARRAY(a,size) do { \ a.data = realloc( a.data, size * sizeof(*a.data) ); \ a.size = size; } while(0)#define FREE_ARRAY(a,size) do { \ free(a.data); a.data = 0; } while(0)#define VALID_ARRAY(a) (a.data != 0)#define ARRAY_GET(a,i) (a.data)#define ARRAY_SET(a,i,x) do { \ a.data = (x); } while(0)WIth the obvious "macros are evil" restriction that a should always be an array variable and not an expression.
As Fase said, you can't so you have to keep the length around. If this is C++, it would be more convenient to use std::vector which is a fancy array and does remember it's length.
There are compiler dependent methods for determining the size of a dynamically allocated array. For instance on MSVC, you can use _msize() on a pointer to determine how many bytes were allocated on that pointer.
Previous thread on this topic.
Previous thread on this topic.
Quote:Original post by moo_gamer
Hi! Everyone,
int *array;
Suppose, I've initialized it in somewhere.
Now, how do I know the length of the array using this pointer?
int getLength() {
return ..... what?
}
Thanks in advance.
you can't, and it's not good practise. You must keep the length somewhere.
I suggest you use some sort of container (Like a Linked List or a Vector or some sort of ArrayList, etc..)
Just to clarify:
Quote:Original post by SiCrane
There are compiler dependent methods, for some compilers/platforms, for determining the size of a dynamically allocated array.
Quote:Original post by memento_mori
you can't portably, and it's not good practise. ...the length must be kept somewhere.
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