c++arrayssizeof

Element count of an array in C++

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!

Solution

  • 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!