How sizeof works in C/C++
what's the mechanism of sizeof() in C/C++? - Stack Overflow
Understanding the sizeof operator in C - Stack Overflow
Does sizeof return the number of bytes or the number of octets of a type in C? - Stack Overflow
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You know, there's a reason why there are standard documents (3.8MB PDF); C99, section 6.5.3.4, ยง2:
The
sizeofoperator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.
In response to ibread's comment, here's an example for the C99 variable length array case:
#include <stdio.h>
size_t sizeof_int_vla(size_t count)
{
int foo[count];
return sizeof foo;
}
int main(void)
{
printf("%u", (unsigned)sizeof_int_vla(3));
}
The size of foo is no longer known at compile-time and has to be determined at run-time. The generated assembly looks quite weird, so don't ask me about implementation details...
sizeof is an operator, not a function.
It's usually evaluated as compile time - the exception being when it's used on C99-style variable length arrays.
Your example is evaluating sizeof(int), which is of course known at compile time, so the code is replaced with a constant and therefore the ++ doesn't exist at run-time to be executed.
int i=0;
cout << sizeof(++i) << endl;
cout << i << endl;
It's also worth noting that since it's an operator, it can be used without the brackets on values:
int myVal;
cout << sizeof myVal << endl;
cout << sizeof(myVal) << endl;
Are equivalent.
Answer: sizeof returns the size of the type in bytes.
Example: sizeof(char) is 100% guaranteed to be 1, but this does not mean, that it's one octet (8 bits).
Proved by the standard:
in 6.5.3.4, point 2:
The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.
...
When applied to an operand that has type char, unsigned char, or signed char, (or a quali๏ฌed version thereof) the result is 1. When applied to an operand that has array type, the result is the total number of bytes in the array) When applied to an operand that has structure or union type, the result is the total number of bytes in such an object, including internal and trailing padding.
Also, in Section 3.6, point 3:
A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined
sizeof always returns size as the number of bytes. But according to wikipedia:
In the programming languages C and C++, the unary operator sizeof is used to calculate the size of any datatype, measured in the number of bytes required to represent the type. A byte in this context is the same as an unsigned char, and may be larger than 8 bits, although that is uncommon.
I perfer sizeof(variable) over sizeof(type). Consider:
int a1;
float a2;
memset(&a1,0,sizeof(a1));
memset(&a2,0,sizeof(a2));
vs.
int a1;
float a2;
memset(&a1,0,sizeof(int));
memset(&a2,0,sizeof(float));
In the first case, it's easy to verify that the right sizes are being passed to memset. In the second case, you need to constantly review top and bottom sections to make sure you are consistent.
The preference (as always) is to reflect your intention as directly as possible.
Is the intention to operate against an existing variable's memory? If so, then use sizeof(variable), as this shows as closely as possible that it's the variable itself's memory that you care about.
Is the intention to perform some calculation on the type, for example to determine how much memory should be allocated for a new instance? If so, then use sizeof(type).
That is, I prefer
struct foo *bar;
bar = (struct foo *) malloc(sizeof(struct foo));
over
bar = (struct foo *) malloc(sizeof(*bar));
as the latter case looks like you're trying to access a variable that doesn't exist, yet.
On the other hand, I prefer
char Buffer[256];
memset(Buffer, 0, sizeof(Buffer));
over
char Buffer[256];
memset(Buffer, 0, 256 * sizeof(char));
as the intention is clearly to zero-fill the contents of the variable, so it's the variable we should operate against. Trying to use the type metadata just confuses things, here.