How can "sizeof(array)" get the size of array with variable length?

Question

As Johannes Schaub says here, the sizeof operand evaluate the size of arrays with variable size in runtime, but... How? Where is that size stored? Why doesn't it return the size of the pointer type?

Example code:

#include <iostream>

using namespace std;

int main(int argc, char** argv)
{
    int array[argc];
    cout << sizeof array << endl;

    return 0;
}

This is more of a question for C99 than C++11--variable length arrays are still a compiler extension in C++. — jaggedSpire, Jan 19 '17 at 23:46
@GManNickG I'm assuming that's the pointer the array would decay to. — jaggedSpire, Jan 19 '17 at 23:56
In C, an array expression does *not* decay to a pointer if it is (a) the operand of unary `&`, (b) the operand of unary `sizeof`, or (c) a string literal in an initializer used to initialize an array object. — Keith Thompson, Jan 20 '17 at 00:38
In C, VLAs (variable length arrays) are nonexistent in C90, mandatory in C99, and optional in C11. They do not exist in any standard version of C++. — Keith Thompson, Jan 20 '17 at 00:39
I thought arrays are the same as pointers (You can use the deference operator with both, and brackets operator too!), but I had been wrong — Adrian, Jan 20 '17 at 11:37

eerorika · Accepted Answer · 2017-01-20T00:19:32.330

2

How? Where is that size stored?

Where ever the implementation decides to store it. Probably on the stack within the frame with the other local variables, or perhaps it only exists within a register.

Why doesn't it return the size of the pointer type?

sizeof returns the size of a pointer type only when you apply it to a pointer type or an object with a pointer type. array doesn't have a pointer type so there is no reason to return such size.

P.S. VLA do not exist in standard C++.

edited Jan 20 '17 at 00:19

answered Jan 20 '17 at 00:02

eerorika

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@4dr14n31t0rTh3G4m3r you thought wrongly. Pointers and arrays behave similarly because arrays implicitly convert to a pointer to first element in many contexts. `sizeof` is not such context. – eerorika Jan 20 '17 at 11:39

How can "sizeof(array)" get the size of array with variable length?

1 Answers1