c++ logical expression optimization

Question

Is it okay for me to presume in C or C++ or JavaScript or any other modern language that if I do…

bool funt1(void) {…}
bool funt2(void) {…}
if (funt1() && funt2()) {Some code}

… Am I guaranteed that both functions get called or if funt1 returns false can the compiler bail on me and never call funt2?

Answer 1

In C, C++, and Javascript, the logical operators && and || short circuit, i.e. in A && B, A is evaluated first and B evaluated if and only if A returned true. Similarly, in A || B, B is evaluated if and only if A returned false. These are guaranteed by the language standards and apply even if B has side effects (in fact, this can be used to control those side effects).

In C++, these rules only apply to the logical operators applied to built in types, but not to user-defined logical operators of the same name. However, in your code snippet, two bool are compared, so this cannot be a user-defined &&.

Answer 2

There is a pitfall in C++: If the first and the second function return objects and both are needed for the logical operation, there is no short circuit.

#include <iostream>

struct A {};
struct B {};

bool operator || (const A&, const B&) { return true; }

A a() { std::cout << "A\n"; return A(); }
B b() { std::cout << "B\n"; return B(); }

bool f() { std::cout << "f\n"; return true; }
bool g() { std::cout << "g\n"; return true; }

int main(int argc, char* argv[])
{
    if(a() || b());
    if(f() || g());
}