C++ sizeof Operator – How is sizeof Implemented?

c++sizeof

Can someone point me the to the implementation of sizeof operator in C++ and also some description about its implementation.

sizeof is one of the operator that cannot be overloaded.

So it means we cannot change its default behavior?

Best Answer

sizeof is not a real operator in C++. It is merely special syntax which inserts a constant equal to the size of the argument. sizeof doesn't need or have any runtime support.

Edit: do you want to know how to determine the size of a class/structure looking at its definition? The rules for this are part of the ABI, and compilers merely implement them. Basically the rules consist of

size and alignment definitions for primitive types;
structure, size and alignment of the various pointers;
rules for packing fields in structures;
rules about virtual table-related stuff (more esoteric).

However, ABIs are platform- and often vendor-specific, i.e. on x86 and (say) IA64 the size of A below will be different because IA64 does not permit unaligned data access.

struct A
{
    char i ;
    int  j ;
} ;

assert (sizeof (A) == 5)  ; // x86, MSVC #pragma pack(1)
assert (sizeof (A) == 8)  ; // x86, MSVC default
assert (sizeof (A) == 16) ; // IA64

Related Solutions

C sizeof – Mechanism of sizeof() in C and C++

You know, there's a reason why there are standard documents (3.8MB PDF); C99, section 6.5.3.4, §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.

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...

C++ – Understanding and Implementing sizeof Operator

sizeof is a compiler built-in operator. It is evaluated at compile-time by the compiler, and there is no runtime code behind it. You cannot write your own.

Asking this is akin to asking how you would write your own version of return.

Best Answer

Related Solutions

C sizeof – Mechanism of sizeof() in C and C++

C++ – Understanding and Implementing sizeof Operator

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