length of dynamic array in c++

Question

Possible Duplicate:
How to find the sizeof(a pointer pointing to an array)

I declared a dynamic array like this:

int *arr = new int[n];   //n is entered by user

Then used this to find length of array:

int len = sizeof(arr)/sizeof(int);

It gives len as 1 instead of n . Why is it so?

`int *arr = new int[n];` is declaring a pointer. `std::vector arr;` is declaring a dynamic array. — R. Martinho Fernandes, Aug 01 '12 at 13:42
You don't have a dynamic array -- you only have a pointer to the first element of a dynamic array. — Kerrek SB, Aug 01 '12 at 13:45
@KerrekSB Other than using vector as an alternative, is there a way to find length of dynamic array. — Jatin, Aug 01 '12 at 13:48
@Jatin No, there is no way. You have to store the length and pass it where necessary. — Daniel Fischer, Aug 01 '12 at 13:49
@Jatin: No. *You*'re the one who specified the length in the first place, so you should be the one remembering it. — Kerrek SB, Aug 01 '12 at 13:50
@Jatin, What does it give you, if you take: int lenArr = sizeof(arr); — Vikram, Aug 01 '12 at 15:10

score 12 · Answer 1 · answered Aug 01 '12 at 13:39

12

Because sizeof does not work for dynamic arrays. It gives you the size of pointer, since int *arr is a pointer

You should store the size of allocated array or better use std::vector

answered Aug 01 '12 at 13:39

Andrew

Is something like this valid `int arr[n]` where n is entered by user? – Jatin Aug 01 '12 at 13:42
@Jatin: `n` must be `const` or `const-expression`. Otherwise it will not compile since the compiler does not know how much memory to allocate at compile time – Andrew Aug 01 '12 at 13:43
surprisingly, It compiles on code::blocks – Jatin Aug 01 '12 at 13:45
@Jatin Set your compiler to standards-compliant mode and turn all the warnings on (`-std=c++98` or `-std=c++0x` or `-std=c++11` and all of `-Wall -Wextra -pedantic`). It probably compiles because of a GNU extension. – R. Martinho Fernandes Aug 01 '12 at 13:47
@Jatin gcc has VLAs in C++ as an extension, I think Code::Blocks uses gcc. – Daniel Fischer Aug 01 '12 at 13:47

yuri kilochek · Accepted Answer · 2012-08-01T15:30:04.357

11

Because arr is not an array, but a pointer, and you are running on an architecture where size of pointer is equal to the size of int.

edited Aug 01 '12 at 15:30

answered Aug 01 '12 at 13:40

yuri kilochek

score 0 · Answer 3 · answered Aug 01 '12 at 13:41

0

Andrew is right. You have to save n somewhere (depends on where do you use it). Or if you are using .NET you could use Array or List...

answered Aug 01 '12 at 13:41

Scientist42

3 Answers3