What does sizeof do?

Question

What is the main function of sizeof (I am new to C++). For instance

int k=7;
char t='Z';

What do sizeof (k) or sizeof (int) and sizeof (char) mean?

Ful

You are missing a ' in your char. – Bonehead

@Tyler McHenry Timing is little off the mark for me at 0.08 secs. – Blood

@davit: if you're going to learn a programming language you should really try reading a decent introductory book on the language to get the basics, not just leave it to trial, error, guesswork and endless questions on SO. – Oakum

It always amuses me when I Google for a question, and the first link is to SO where someone's being berated for asking rather than googling! :) – Yarn

The answer on What does sizeof do? should explain what is happening. – Naoise

I'm voting to close this question as off-topic because this question shows no effort at all. – Rutland

Answer 1

sizeof(x) returns the amount of memory (in bytes) that the variable or type x occupies. It has nothing to do with the value of the variable.

For example, if you have an array of some arbitrary type T then the distance between elements of that array is exactly sizeof(T).

int a[10];
assert(&(a[0]) + sizeof(int) == &(a[1]));

When used on a variable, it is equivalent to using it on the type of that variable:

T x;
assert(sizeof(T) == sizeof(x));

As a rule-of-thumb, it is best to use the variable name where possible, just in case the type changes:

int x;
std::cout << "x uses " << sizeof(x) << " bytes." << std::endl
// If x is changed to a char, then the statement doesn't need to be changed.
// If we used sizeof(int) instead, we would need to change 2 lines of code
// instead of one.

When used on user-defined types, sizeof still returns the amount of memory used by instances of that type, but it's worth pointing out that this does not necessary equal the sum of its members.

struct Foo { int a; char b; };

While sizeof(int) + sizeof(char) is typically 5, on many machines, sizeof(Foo) may be 8 because the compiler needs to pad out the structure so that it lies on 4 byte boundaries. This is not always the case, and it's quite possible that on your machine sizeof(Foo) will be 5, but you can't depend on it.

Creolacreole

int means that if variable t is given t=100 than sizeof(t)= 1? – Ful

sizeof has nothing to do with the value of the variable. It has to do with the type. It is telling you that on your system, all variables of type int require 4 bytes of storage. – Hufford

It shows 4 because an int typically uses up 4 bytes. sizeof has nothing to do with the value stored in the variable. – Creolacreole

nit: technically, sizeof does not report the size in bytes, but whatever unit necessary so that sizeof(char) == 1. The is nearly universally bytes, but in theory at least, not necessarily. – Hillhouse

From the standard: "The sizeof operator yields the number of bytes in the object representation of its operand." However, you are right that byte in the standard isn't explicitly defined as 8-bits as it usually is. – Creolacreole

Your first assert is wrong. ` &(a[0])` is an int pointer so pointer arithmetic will make it equal to a[4]. – Bellerophon

How efficient is the sizeof() operation? If I call sizeof() in hundreds of places, is that slow, and should I store it in a variable or something? Something like int LONG_SIZE = sizeof(long);? – Contestation

Answer 2

To add to Peter Alexander's answer: sizeof yields the size of a value or type in multiples of the size of a char---char being defined as the smallest unit of memory addressable (by C or C++) for a given architecture (and, in C++ at least, at least 8 bits in size according to the standard). This is what's generally meant by "bytes" (smallest addressable unit for a given architecture) but it never hurts to clarify, and there are occasionally questions about the variability of sizeof (char), which is of course always 1.

Roughen

A byte isn't necessarily the "smallest addressable unit for a given architecture". On some CDC Cyber computers, the smallest addressable unit is a 16-bit word (Address 0 is 16-bits, address 1 is a distinct 16-bits), and they refer to the two halves as bytes. – Hillhouse

@James: Right: my point was that "byte" is an inherently ambiguous term, and no matter how you define it, somebody somewhere will use it differently. Interesting about the CDCs. – Roughen

Most people will agree that a byte is 8 bits, and while there used to be some confusion, it's standardized nowadays: IEC 80000-13. In contrast, the smallest addressable unit in a machine is often called a word, and it is machine-dependent. There are 16-, 32-, 64-bit architectures, but even in those contexts, a byte will still be 8 bit. – Stonwin

Answer 3

sizeof() returns the size of the argument passed to it. sizeof() cpp reference

Pauly

How efficient is the sizeof() operation? If I call sizeof() in hundreds of places, is that slow, and should I store it in a variable or something? Something like int LONG_SIZE = sizeof(long);? – Contestation

sizeof() is compile time, so its the best performance you can get, since at runtime thats just a constant number in your code. – Pauly

Answer 4

sizeof is a compile time unary operator that returns size of data type. For example:

sizeof(int)

will return the size of int in byte.

Also remember that type sizes are platform dependent.

Check this page for more details: sizeof in C/C++

Elaelaborate