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?
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221answers:
3
+16
A:
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.
Peter Alexander
2010-07-08 11:48:46
int means that if variable t is given t=100 than sizeof(t)= 1?
2010-07-08 11:53:41
but it show me 4 why
2010-07-08 11:54:27
`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.
Tyler McHenry
2010-07-08 11:56:33
It shows 4 because an `int` typically uses up 4 bytes. `sizeof` has nothing to do with the value stored in the variable.
Peter Alexander
2010-07-08 11:57:13
Upvoting your answer since you were faster than my and you edited it with more information and the link to cpp reference :) nice answer!
PeterK
2010-07-08 12:12:49
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.
James Curran
2010-07-08 14:44:03
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.
Peter Alexander
2010-07-09 06:58:40
+5
A:
sizeof() returns the size of the argument passed to it. sizeof() cpp reference
PeterK
2010-07-08 11:49:57
+3
A:
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.
Derrick Turk
2010-07-08 12:27:18
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.
James Curran
2010-07-08 14:50:11
@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.
Derrick Turk
2010-07-08 16:02:27