Question

During work over a simple project I have found situation that I don’t fully understand. Consider following code:

#include <iostream>

using namespace std;

bool test(int k)
{
    cout << "start " << k << endl;

    bool result; // it is important that result's value is opposite to initial value of recheck in main()
    result = false;

    return result;
}

int main()
{
    bool recheck;
    recheck = true;
    for (int i = 2; i > -1; i--)
    {
      recheck = (recheck || test(i));   // (1)
      cout << i << " ???" <<endl;
    }
    cout << "----------------------------" << endl;
    cout << endl;

    recheck = true;
    for (int i = 2; i > -1; i--)
    {
        recheck = (test(i) || recheck);  //different order that in (1)
        cout << i << "???" <<endl;
    }

    return 0;
}

It returns completely different results from for loops:

2 ???
1 ???
0 ???
----------------------------

start 2
2???
start 1
1???
start 0
0???

It seems that it first one test(int k) is not even invoked. I suspect it has something to do with || operator. Could anybody explain such a behavior?

Answer 1

The built-in || short-circuits: if the left operand is true, the right operand is not evaluated (it doesn’t matter what the value of the right operand is, because the value of the || expression is guaranteed to be true in this case).

^{For completeness, but not particularly relevant to the question: In c++, the || operator is overloadable, just as many other operators are. If an overload is used, short circuiting does not take place.}