why is 1

Question

hello, I want to get number of elements of a array, but I encounter a doubt about the following codes:
/*
correct array size
*/
#include <iostream>
using namespace std;

int main()
{
int numbers[100];
cout << sizeof(numbers) / sizeof(int);

return 0;
}

/*
uncorrect array size
*/
#include <iostream>
using namespace std;

// Define a function to get size of an array.
int getSize(int arr[])
{
return sizeof(arr)/sizeof(arr[0]);
}

int main()
{
int numbers[100];
cout << getSize(numbers); // the result is 1, why ?

return 0;
}

So, why I get the result is '1' from 2nd program, Thank you !

Answer 1

Arrays decay to pointers when passed as function parameters so sizeof is not going to help you.

Refer:
https://stackoverflow.com/questions/968001/determine-size-of-array-if-passed-to-function

Answer 2

in simple word, when you pass an array to a function you send the first element of array to that function (as a pointer), so you get the size of the first element obviously

Answer 3

You can use a template/generic.

#include <iostream>
using namespace std;

#include <iostream>
using namespace std;

template <typename T, int N>
int getSize(T (&arr) [N])
{
// You could just return N instead here though
// Or get rid of N completely using:
// template <typename T>
// int getSize(T &arr)
return sizeof(arr)/sizeof(arr[0]);
}

int main()
{
int numbers[100];
cout << getSize(numbers);

return 0;
}

Answer 4

This is one of the reasons why you should use std::array, it keeps track of its own size without having to keep track of it yourself