Solved

In C++, how much more do dynamic multi-dimensional arrays use?

Posted on 2014-03-06
7
515 Views
Last Modified: 2014-03-06
In looking at the available Physical Memory in Task Manager while my program is executing in the debugger, I've realized that the dynamically allocated multi-dimensional arrays in my VS2012 C++ application use A LOT MORE MEMORY than I would expect.  

Let's assume for this discussion that my computer needs 8 bytes to store a variable of data type "double".  Well, I would've expected that the amount of memory (ie, the heap) needed for a two-dimensional array of size [1000][50] (using the "new" keyword) would simply be 1000 * 50 * 8 bytes =  400,000 bytes (ie, 400 K).  Well, I must be misunderstanding the algorithm, because more than twice this much memory is actually be "taken" from the supply of available memory.

So, if someone can explain the algorithm for the use of dynamically allocated memory with regard to arrays, and in particular multi-dimensional arrays, I would greatly appreciate it.
0
Comment
Question by:cjm20
7 Comments
 
LVL 37

Expert Comment

by:TommySzalapski
ID: 39909255
Are you using "managed c++" or unmanaged in your project properties? The memory management stuff that VC++ tacks on can be rather large.

The way operating systems allocate memory is not simple at all. Your program may ask for 400k and the OS might give it a 1MB block.

In Task Manager, go to the Performance tab, click Resource Monitor...
Then click on the Memory tab. See if the numbers there for your process make more sense.
0
 
LVL 10

Expert Comment

by:aboo_s
ID: 39909276
I think it depends on the architecture of your machine OS , 32bit or 64bit depends how large is one word in your machine's memory.
0
 
LVL 10

Expert Comment

by:aboo_s
ID: 39909281
if a variable needs 8 bytes to be stored and a word in your machine is 32bytes long than 8 bytes are stores in 32.
0
Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

 
LVL 37

Accepted Solution

by:
TommySzalapski earned 500 total points
ID: 39909314
A 64bit system will generally use 64bits for its default allocation "word"
But 64 bits is 8 bytes. Also arrays are typically packed anyway.
If you declare 1 character, you'll see that it takes up 8 bytes.
If you declare an array of 5 characters... 8 bytes
11 characters... 16 bytes, etc etc. It will typically round up to the nearest 8 bytes.

But the memory that the OS allocates to your process is a different story. If you declare a large array, it can give you twice that memory in case you allocate more so it can be efficient and give you memory near what you already have. Looking at the free physical memory isn't really a good indication of how much memory a c++ object is taking up.
0
 
LVL 33

Expert Comment

by:sarabande
ID: 39909388
if you build the debug configuration the extra space is reserved for the debugger.

Sara
0
 

Author Closing Comment

by:cjm20
ID: 39909978
Thanks Tom (and to everyone else who chimed in).  The Resource Viewer in Task Manger is nice, but still didn't help pinpoint exactly why the excess memory for my dynamic arrays is, well, so excessive.  But i think you've hit on it when you mentioned the OS trying to be efficient in its anticipation of the application allocating MORE memory than it just asked for.  This makes sense.  The OS, for my big arrays, is allocating about 2.5 times the amount I would think my array would need.  I've also read elsewhere that for multi-dimensional dynamically-sized arrays, the pointers to the arrays within arrays also need to reside in memory. Said pointers for an array of the "double" data types take 4 bytes I'm told, so with a very large array, this is more a lot more overheard.
0
 
LVL 37

Expert Comment

by:TommySzalapski
ID: 39910061
Sara is right too. If you were running a debug build, then that would explain most of the extra memory.

In C++ a two dimensional array like that does not store pointers to "arrays within arrays." If you had some type of two dimensional array that had rows of different lengths that would apply, but double x[1000][50] is just going to give you one array. x[ i ][ j ] just translates to something like x[i*50+j].

On a 64 bit system, pointers take up 8 bytes.
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Many modern programming languages support the concept of a property -- a class member that combines characteristics of both a data member and a method.  These are sometimes called "smart fields" because you can add logic that is applied automaticall…
Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
This tutorial explains how to use the VisualVM tool for the Java platform application. This video goes into detail on the Threads, Sampler, and Profiler tabs.
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…

863 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

24 Experts available now in Live!

Get 1:1 Help Now