Question

This is the C++ code:

#include<iostream>
using namespace std;


int a=8;

int fun(int &a)
{
    a=a*a;
    return a;
}

int main()
{

    cout << a << endl \
        << fun(a) << endl \
        << a << endl;
        return 0;
}

why does it output:

64 64 8

the << operator’s associativity is left to right, so why not output 8 64 64?

Does it have the relation to the sequence point and the effect side?

Answer 1

Associativity and evaluation order are not the same thing. The expression a << b << c is equivalent to (a << b) << c due to left-to-right associativity, but when it comes to evaluation order, the compiler is free to evaluate c first then a << b and, likewise, it can evaluate b before it evaluates a. In fact, it can even evaluate the terms in the order b → c → a if it wants, and it just might if it concludes that such an order will maximise performance by minimising pipeline stalls, cache misses, etc.