Different results when using sizeof to count number of elements in array

Question

I'm writing a simple C++ code to count the number of elements in an array. I use template so that I can deal with arrays of different type.

template <typename T>
void count(T a[]){

    cout << "size of array: " << sizeof(a) << endl;
    cout << "size of element: " << sizeof(*a) << endl;
    cout << "count number: " << sizeof(a)/sizeof(*a) << endl;
}

int main()
{
    int x[6] ={1,2,3,4};

    count(x);
    cout << sizeof(x) << endl;
    cout << sizeof(*x) << endl;
    cout << sizeof(x)/sizeof(*x) << endl;

    return 0;
}

But when I run this code, I got different results by using function count and by just copying the same code in main. I can't figure out why. The results are like this:

size of array: 8
size of element: 4
count number: 2
24
4
6

What's more, when I use g++ compiler to run the code, there is a warning message:

warning:sizeof on array function parameter 'a' will return size of int* [-Wsizeof-array-argument]
  cout << "size of array: " << sizeof(a) << endl;

This message did't show up when I use Code::Blocks running the code, but I think it might show my problem.

Where do these two different results come from?

Answer 1

It's because your template:

template <typename T>
void count(T a[])

decays to:

template <typename T>
void count(T a*)

so you're printing the sizeof a pointer, not of the array as your main is.

The error message says this:

will return size of int*