Questions regarding arrays and their memory consumption, depicted using sizeof()

**Densetsu** · 11-12-2014

My Questions:
1) Why does the function sizeof() return a higher byte value when evaluating nElementIndex rather than nElementIndex [ARRAY_LENGTH] ?

2) If you remove the keyword const when declaring the integer variable, the program is unable to compile. Why is this?
(I understand it would be completely redundant doing so, but still.)

Code:

#include <iostream>
using namespace std;

int main()
{
    const int ARRAY_LENGTH = 10;
    int nElementIndex [ARRAY_LENGTH] = {0};
    
    cout << sizeof(nElementIndex) << endl;
    cout << sizeof(nElementIndex [ARRAY_LENGTH]) << endl;
    
    return 0;
}

The only logical sense I can extract is that int consumes 4 bytes of memory, of which, since the number of elements is initialized to 10, the microprocessor stores into 10 addresses. Thus, validating the result of "40" from the program. But that's where things become confusing. I would of thought "40" would of been the result of sizeof(nElementIndex [ARRAY_LENGTH]). ???

**grumpy** · 11-12-2014

sizeof(nElementIndex) gives the size of an array. sizeof(nElement[any_integral_value]) gives the size of an element of the array. Since arrays contain multiple elements, sizeof for an array can never be less than the size of an element.

Array sizes in C++ can only be constants that are known at compile time. Removing the const makes ARRAY_LENGTH a variable, which - by definition - cannot have a value that is known at compile time.

The size of an int is implementation defined. For your compiler (and chosen compiler settings) it might be consistently a value of 4 - because that's what is meant by "implementation defined". However, there is no requirement that size of an int be 4 - and there are real-world compilers that support different sizes.

**hk_mp5kpdw** · 11-13-2014

Nit:

Code:

cout << sizeof(nElementIndex [ARRAY_LENGTH]) << endl;

Since ARRAY_LENGTH is 10, the range of valid index values to use in accessing the nElementIndex array go from 0 through 9 (ARRAY_LENGTH - 1). Using ARRAY_LENGTH at this point here constitutes an attempt to access beyond the array bounds and is undefined behavior.

**grumpy** · 11-13-2014

Originally Posted by hk_mp5kpdw

Nit:

Code:

cout << sizeof(nElementIndex [ARRAY_LENGTH]) << endl;

Since ARRAY_LENGTH is 10, the range of valid index values to use in accessing the nElementIndex array go from 0 through 9 (ARRAY_LENGTH - 1). Using ARRAY_LENGTH at this point here constitutes an attempt to access beyond the array bounds and is undefined behavior.

That's not true. sizeof does not evaluate its operand - it only works out the type of the operand, and then evaluates the size of the type. So sizeof(nElementIndex [ARRAY_LENGTH]) does not attempt to access one past the end of array nElementIndex.

**hk_mp5kpdw** · 11-13-2014

Oops... see, I'm still learning new things even after all these years. Now, if I can only RETAIN said knowledge in my feeble mind.

**grumpy** · 11-13-2014

Originally Posted by hk_mp5kpdw

Now, if I can only RETAIN said knowledge in my feeble mind.

Very few people can retain everything.

The trick to learning is working out what core knowledge you need to retain. From there, it is often a case of knowing where to look (e.g. in a textbook) or how to work out (e.g. derive from the core things you know) other things.

The catch is that what works for one person doesn't necessarily work for another. And that, sometimes, most people still get things wrong. It's called being human.

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