Problem in incrementing

Question

I have a variable declared like this:

int j=0;

When I write:

cout<<j++<<" "<<j++<<" "<<j++<<"\n";

I receive this output:

2 1 0

I expect to receive this output:

0 1 2

Could you explain the result?

Answer 1

this is confusing but normal because order of evaluation for this operator is right to left.

Answer 2

The spec says that if you modify the same variable more than once within the same sequence point, the result is "undefined".

Sequence points are between ; (and also , is a sequence point, not sure if there are others).

What you're trying is the same as the better known trivia question, "what's the value of x after the second assignment?"

int x;
x = 0;
x = x++;

The answer is "undefined".

Answer 3

This is because your compiler is likely evaluating the equation from right to left.

check out this question for more info.

Edit: Tested on g++ 4.4.0

#include <iostream>

int main (int argc, char **argv) {
int j = 0;
std::cout << j++ << " " << j++ << " " << j++;
return 0;
}

[john@awesome]g++ rtl.cpp -o rtl
[john@awesome]./rtl
0 1 2
[john@awesome]

Answer 4

This code is equivalent to

... operator<<( operator<<( operator<<( operator<<(cout,j++), " " ), j++ ), "\n" ); ...

Though order of function call is given, order of parameters evaluation is not. So there is no guarantees which j++ is evaluated first. There are modifications of j without sequence points between them, so you see result of Undefined Behaviour.

[Edit1] There is inaccuracy in previous. operator<<(int) is a member function of basic_ostream. Denote operator<<(int) as f, operator<<(ostream&,const char*) as g. Then we have

...g(f(j++)," ").f(j++)...

Order of evaluation still can be: eval(j++) -> eval(j++) -> sequence point -> call(f) -> sequence point -> call(g) -> sequence point -> call(f). This is because of following quote from standard [expr.4]:

Except where noted, the order of evaluation of operands of individual operators and subexpressions of individual expressions, and the order in which side effects take place, is unspecified