SizeOf question C++

Question

I have a question about this code

https://code.sololearn.com/c8fswm6Kkx2g/#cpp.

I am wondering what is happening when I use the sizeof operator on the array "arr" it output 24 which is not even in the sequence. As well as when I use the sizeof operator on the "arr[0]" it outputs 4 even though the first number in the sequence is 10. This is very confusing to me if you could help explain this to me that would be wonderful, thank you.

Answer 1

It is pretty much obvious that you wanted to initialize a vector of integers using an array of integers by incorporating pointer arithmetic which is a little bit messy in terms of readability but good for compatibility with C++98/03.

For C++11 compliant implementations, you'd simply use iterators for initializing a vector by another container or in simplest form as the way you initialize an ordinary array directly. Consider the following examples.

// initialize vector using assignment operator (C++11)
std::vector<int> vec = {10, 20, 30, 40, 50};

// initialize vector using iterator and another container (C++11)
int arr[] = {10, 20, 30, 40, 50};
std::vector<int> vec(std::begin(arr), std::end(arr));

// initialize vector using pointer arithmetic and another container (Both C++11 and C++98/03)
int arr[] = {10, 20, 30, 40, 50};
std::vector<int> vec(arr, arr + sizeof(arr) / sizeof(arr[0]));

All three of them yield the same result with different methods which is usually dictated by implementation and project restraints.

Answer 2

Sizeof returns the size of an element in the memory in bytes. The compiler uses 32 bit integers, which are 4 bytes large (32/8=4). The array contains 6 integers: 6*4=24

Answer 3

Aaron Eberhardt isn‘t the name of the array basically just a pointer? if so why doesn‘t sizeof(a) return the size of pointers on the system but the lenght of the array

Answer 4

Thank you very much :)

Answer 5

Max I guess because C++ knows the array size and also that this must be a pointer to an array. Using an usual pointer instead just returns the pointers size, so these pointers must be treated differently