structure and functions in C++

Question

Can someone explain why this code gives the output 10? When I try to analyse it my logic gives as result 11.

#include <iostream>
using namespace std; 
class A { 
public:     
    A() { a.a = a.b = 1; }   
    struct { int a,b; } a;  
    int b(void); 
};

int A::b(void) { 
    int x=a.a;
    a.a=a.b;
    a.b=x; 
    return x; 
};

int main(void) {     
    A a;
    a.a.a = 0;
    a.b();
    cout << a.b() << a.a.b << endl;
    return 0;
}

Answer 1

In the cout line, ab() could either be called before or after aab is evaluated. Beginners sometimes assume left-to-right evaluation in this sort of code but actually that is not a rule of C++.

Those two different possibilities would explain your 10 and 11 . To avoid ambiguity you could write:

cout << a.b();
cout << a.a.b << endl;   

(assuming that order was your intent).

Note: C++17 may change this and define left-right evaluation order for this code.

Answer 2

Other than using a debugger, you can also use cout statements to help keep track of when things are called.

To kind of help myself out tracing your program I fixed a bit of the indentation and added comments as to when things are happening:

#include <iostream>
using namespace std; 
class A {
public:
    A() {
        a.a = a.b = 1;
    }
    struct {
        int a,b;
    } a;
    int b(void); 
};

int A::b(void) {
    cout << "Within A::b()" << endl;
    // swap a.a, a.b
    int x=a.a;
    a.a=a.b;
    a.b=x;

    cout << "a.a.a = " << a.a << " a.a.b: " << a.b << endl;

    return x;
};

int main(void) {
    // sets a.a.a = 1, a.a.b = 1
    A a;
    // sets a.a.a = 0, a.a.b = 1
    a.a.a = 0;
    // a.a.a = 1, a.a.b = 0
    a.b();
    // check output here
    cout << a.b() << a.a.b << endl;
    return 0; 
}

The above program results with the following output on http://cpp.sh/ :

Within A::b()
a.a.a = 1 a.a.b: 0
Within A::b()
a.a.a = 0 a.a.b: 1
10

In all, it depends on whether or not ab() or aab is resolved first when you call cout . In this case, operator precedence is undefined due to how cout works. This stackoverflow post has some good info on this.