How does sizeof() behaves in C++11?

Question

The sizeof() function in c++ is behaving very weird. I am unable to make any sense out of it. I was writing code to implement the binary search algorithm to find an element in an array. To have a lesser number of arguments in the function, I decided to get the length of the array using sizeof() function. Here is the code:

#include <bits/stdc++.h>

using namespace std;

int binarySearch(int arr[], int target) {
    int low = 0;
    int high = sizeof(arr) / sizeof(arr[0]) - 1;

    cout << "high is: " << high << endl;

    while (low <= high) {
        int mid = low + (high - low) / 2;

        if (arr[mid] == target) return mid;

        else if (arr[mid] > target) high = mid - 1;

        else low = mid + 1;
    }
    return -1;
}

int main() {
    int array[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    cout << binarySearch(array, 10) << endl;
    cout << "size of behaviour in main: " << sizeof(array);
    return 0;
}

The output is:

high is: 0
-1
size of behaviour in main: 40

On debugging, I realised that inside binarySearch function, the sizeof(arr) is giving me the size of int type i.e. 4. so sizeof(arr)/sizeof(arr[0])-1 = 4/4-1 = 0.

However, in int main, the sizeof(array) is giving me the size of int times length of the array i.e. 4*10 = 40.

What explains this change in behaviour of the sizeof() function with respect to its usage in int main and in function definition?

Answer 1

That doesn't work as you're getting the sizeof of a pointer. Refactoring the code to use a template is one solution:

#include <iostream>

using namespace std;

template<std::size_t N>
int binarySearch(int (&arr)[N], int target) {
    int low = 0;
    int high = sizeof(arr) / sizeof(arr[0]) - 1;

    cout << "high is: " << high << endl;

    while (low <= high) {
        int mid = low + (high - low) / 2;

        if (arr[mid] == target) return mid;

        else if (arr[mid] > target) high = mid - 1;

        else low = mid + 1;
    }
    return -1;
}

int main() {
    int array[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    cout << binarySearch(array, 10) << endl;
    cout << "size of behaviour in main: " << sizeof(array);
    return 0;
}

Using std::array is probably a better option. Both templates and std::array have sizes known at compile time. If you want a runtime approach, take a look at std::vector.