Pre-increment and post-increment in C++ not working correctly when used as arguments

Asked Feb 09 '18 at 20:06

Active Feb 09 '18 at 20:13

Viewed 45 times

I came across this problem in c++ randomly. By my understanding this code:

#include <iostream>

using namespace std;

void foo(int a, int b){
    cout<<"a = "<<a;
    cout<<"\n";
    cout<<"b = "<<b; }

int main() {
    int i = 1;
    foo(++i,i++);

    return 0; }

should print out:

a = 2 
b = 2

But instead it prints out

a = 3
b = 1

How is that possible?

asked Feb 09 '18 at 20:06

Angeal

2

Who said function arguments are evaluated left to right? – StoryTeller - Unslander Monica Feb 09 '18 at 20:07
1

Sigh, not another one of these questions... In this case, the arguments are evaluated from right to left. i = 1, then it is incremented but since you are using post increment, the original 1 is passed for b, but i is 2. Next, you use pre-increment, and pass that incremented value, which is now 3, to a. – Arnav Borborah Feb 09 '18 at 20:07
[Order of evaluation of function arguments](https://stackoverflow.com/search?q=%5Bc%2B%2B%5D+or+%5Bc%2B%2B11%5D+order+of+evaluation+of+function+arguments) – R Sahu Feb 09 '18 at 20:12
@StoryTeller standard did. – SergeyA Feb 09 '18 at 20:13
@SergeyA - Cite it then – StoryTeller - Unslander Monica Feb 09 '18 at 20:14
Reopened, since duplicate is out of date. Will answer with latest state of the affairs. – SergeyA Feb 09 '18 at 20:14
Closed again, since there are many updated answers in the duplicate. For c++11, 14 and 17. – StoryTeller - Unslander Monica Feb 09 '18 at 20:16
1

@SergeyA You should put that answer in the canonical and leave it closed to that. – NathanOliver Feb 09 '18 at 20:17
@NathanOliver I do not see how people will get past legacy wall of text in the canonical. C++ 17 (the most important one!) down below, and I bet no more than 1 out of 10 readers gets to it. – SergeyA Feb 09 '18 at 20:21
2

@SergeyA That still doesn't change the fact that all the consensus on meta says that is the way it should be done. – NathanOliver Feb 09 '18 at 20:22

Pre-increment and post-increment in C++ not working correctly when used as arguments

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