Why can I not declare a function before I declare its contents? (c++)

Question

I have this program that adds powers of numbers and for some reason it keeps throwing an error at me when I run it, the program runs fine if I declare and define the contents before the main function but I don't understand why that's necessary... Here's the code that's giving me problems:

#include <iostream>
#include <math.h>
using namespace std;

long long addPow(int n, int p);

int main() {
    cout << addPow(100, 1) * addPow(100, 1) - addPow(100, 2) << endl;
    return 0;
}

addPow(int n, int p) {
    long long sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += pow(i, p);
    }
    return sum;
}

changing it to this fixes it all, but I don't really know why...

#include <iostream>
#include <math.h>
using namespace std;

long long addPow(int n, int p) {
    long long sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += pow(i, p);
    }
    return sum;
}

int main() {
    cout << addPow(100, 1) * addPow(100, 1) - addPow(100, 2) << endl;
    return 0;
}

If someone could help me out I'd really appreciate it!

Answer 1

The function in the first code block, defined as

addPow(int n, int p) {

needs the extra information (namely the return type) you put into the prototype. It should look like this:

long long addPow(int n, int p) {