Why is C preincrement addition uniquely different from javascript, C#, etc.? (Sequence points?)-CodePudding

I have been trying to digest such references as Undefined behavior and sequence points and am interested as to why the outcome of the C variation of the following code is different from the outcome of a C# or Javascript variation of this code (see code samples, below).

Can you elaborate on this, what I think is an anomaly in this C-family variation? I appreciate it.

EDIT: This question is about a code example that would never exist in a real-world code base. The construct is undefined in C , and why would you ever do such a thing? As @Sebastian says,

You have two pre-increments and one addition. C choses (in this case with your compiler, no guarantee) to combine the two pre-increments to 2 and then sum up 7 7. That is more performant. If you want to have a defined order, use more variables or put the operations into functions. You could write i = (i 1) (i 2). That ist much easier to understand for humans, too!

EDIT: This appears then to be just a mind f**k for interviewees during a job interview. Best answer: "This is undefined."

C Code Example ( Link: https://g.nw7us.us/3nVy02j )

// CPP program to demonstrate special
// case of post increment operator
#include <iostream>
using namespace std;
 
int main()
{
    int i = 5;
    cout << "Value of i before pre-incrementing";
    cout << "\ni = " << i;
    i =   i     i;
    cout << "\nValue of i after pre-incrementing";
    cout << "\ni = " << i;
    cout << "\n                     \n";
    i = 5;
    cout << "Value of i before pre-incrementing";
    cout << "\ni = " << i;
    i =   i;
    cout << "\ni = " << i;
    i = i     i;
    cout << "\ni = " << i;
    return 0;
}

Output:

Value of i before pre-incrementing
i = 5
Value of i after pre-incrementing
i = 14
                     
Value of i before pre-incrementing
i = 5
i = 6
i = 14

Now, here is the C# version ( link: https://g.nw7us.us/3nScCLz )

using System;
                    
public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
        
        int i = 5;
        Console.WriteLine("i = ["   i   "]");
        i =   i     i;
        Console.WriteLine("i = ["   i   "]");
    }
}

Output:

Hello World
i = [5]
i = [13]

Finally, a Javascript example ( link: https://onecompiler.com/javascript/3xravf59k )

console.log("Preprocessing example...");
let i = 5;
console.log("i = ["   i   "]")
i =   i     i;
console.log("i = ["   i   "]")

Output:

Preprocessing example...
i = [5]
i = [13]

I appreciate clarification on sequence points in C , as it applies to this example.

CodePudding user response：

Because (1) you never need to write cumbersome expressions like i = i i and (2) C and C are all about not giving away performance, the behaviour is undefined.

(Although C 17 does define some more of these class of expressions).

On (2), different architectures may be able to optimise increments differently. That would be given away - with no real advantage gained - if the behaviour of expressions such as yours were defined.