Thread: Vectors and memory

I just wanted to know "do vectors store there values in memory consecutively like a normal array from start to end? Or can they be broken up.
By this i just mean in one instance of time, Im not talking about reallocation after the string has been modifyed or anything like that

A normal implementation of a vector will store its elements in a consecutive block of memory, just like a dynamically allocated array. I'm not sure if the standard guarantees this or not, but if you use gcc at least that will be the representation.

The idea of a vector is that it support random access fairly efficiently -- if that's what you need, you can use a vector without really worrying about how it is implemented . . . .

Originally Posted by Matty_Alan

I just wanted to know "do vectors store there values in memory consecutively like a normal array from start to end? Or can they be broken up.
By this i just mean in one instance of time, Im not talking about reallocation after the string has been modifyed or anything like that

The standard does not guarantee it, but this is considered an oversight in the standard. All realistic implementations of std::vector store their contents in a contiguous array of memory. If it were otherwise, the vector would not be able to satisfy the complexity requirements dictated by the standard.

So you can assume that the contents of the vector occupy a contiguous region of memory from &vec.front() to &vec.back().

Thanks guys, Im mainly just playing with vectors iv'e never really used them before but the general idea that iv'e picked up is that there just an array thats with a little more flexiablity.

I have seen some earlier versions that (depending on certain system constraints and compile switches) would say start with a contiguous block of 64 elements, then if more were needed it could either allocate a larger block and move the old contents to the new location OR it would just allocate a new smaller or equal sized block and just maintain a link between the two depending on the indices requested, saving (temporarily) the time needed to move all of the old data. I am not saying this was right or even still in play but I did see it...

Originally Posted by brewbuck

The standard does not guarantee it, but this is considered an oversight in the standard. All realistic implementations of std::vector store their contents in a contiguous array of memory. If it were otherwise, the vector would not be able to satisfy the complexity requirements dictated by the standard.

So you can assume that the contents of the vector occupy a contiguous region of memory from &vec.front() to &vec.back().

According to my searches, that was true of the early standard of C++, but this oversight was corrected in C++03
Is std::vector contiguous?