Why does sizeof(A), for some array A, not work inside a function? [duplicate]

Answer 1

It is because you check sizeof ( int*) which is sizeof pointer to int. This is because arr passed to function in

int arr [] = {1, 2, 3, 69203};
printArraySize(arr);

decays to a pointer to int. It is just a pointer to integer, first element of array.

C

For doing what you would like to achieve there is a macro often used (for static arrays):

#define ARRAY_SIZE(x) (sizeof((x)) / sizeof((x)[0]))

example:

int arr [] = {1, 2, 3, 69203};
printf( "%d", ARRAY_SIZE(arr));

C++

template
size_t array_size( T(&)[n]) {
    return n;
}