Let's say I have an array arr. When would the following not give the number of elements of the array: sizeof(arr) / sizeof(arr[0])?
One thing I've often seen new programmers doing this:
void f(Sample *arr)
{
int count = sizeof(arr)/sizeof(arr[0]); //what would be count? 10?
}
Sample arr[10];
f(arr);
So new programmers think the value of count will be 10. But that's wrong.
Even this is wrong:
void g(Sample arr[]) //even more deceptive form!
{
int count = sizeof(arr)/sizeof(arr[0]); //count would not be 10
}
It's all because once you pass an array to any of these functions, it becomes pointer type, and so sizeof(arr) would give the size of pointer, not array!
EDIT:
The following is an elegant way you can pass an array to a function, without letting it to decay into pointer type:
template<size_t N>
void h(Sample (&arr)[N])
{
size_t count = N; //N is 10, so would be count!
//you can even do this now:
//size_t count = sizeof(arr)/sizeof(arr[0]); it'll return 10!
}
Sample arr[10];
h(arr); //pass : same as before!