length of dynamic array in c++ [duplicate]

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<int> arr; is declaring a dynamic array. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented Aug 1, 2012 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
– Kerrek SB, Commented Aug 1, 2012 at 13:45
@KerrekSB Other than using vector as an alternative, is there a way to find length of dynamic array. — Jatin
– Jatin, Commented Aug 1, 2012 at 13:48
@Jatin No, there is no way. You have to store the length and pass it where necessary. — Daniel Fischer
– Daniel Fischer, Commented Aug 1, 2012 at 13:49

Andrew · Accepted Answer · 2012-08-01 13:39:01Z

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 1, 2012 at 13:39

Andrew

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5 Comments

Jatin Over a year ago

Is something like this valid int arr[n] where n is entered by user?

Andrew Over a year ago

@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

Jatin Over a year ago

surprisingly, It compiles on code::blocks

R. Martinho Fernandes Over a year ago

@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.

Daniel Fischer Over a year ago

@Jatin gcc has VLAs in C++ as an extension, I think Code::Blocks uses gcc.

yuri kilochek · Accepted Answer · 2012-08-01 15:30:04Z

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.

Scientist42 · Accepted Answer · 2012-08-01 13:41:59Z

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...

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