Element count of an array in C++

Question

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])?

I can thing of only one case: the array contains elements that are of different derived types of the type of the array.

Am I right and are there (I am almost positive there must be) other such cases?

Sorry for the trivial question, I am a Java dev and I am rather new to C++.

Thanks!

Answer 1

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!

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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!