Why is it different from putting a return value of a function directly in a c++ for statement conditional expression?

#include <iostream>
using namespace std;

#define MAXN (100000)
int N;
int sol[MAXN + 10];
int wp, rp = 0;
int que[MAXN + 10];

void push(int d) {
    if (wp < MAXN) {
        que[wp] = d;
        ++wp;
    }
}
int front() {
    if (rp != wp) {
        int t = que[rp];
        ++rp;
        return t;
    }
}
bool empty() {
    if (rp == wp)
        return true;
    else
        return false;
}
int end() {
    if (!empty()) 
        return que[wp-1];
}


int main() {
    cin >> N;
    for (int i = 0; i < N; i++) 
        push(i+1);
    for (int j = 0; j < end() /2; j++) {
        push(front());
        cout<<"j"  << j <<endl;
    }
    cout << front() << "\n";
    return 0;
}