<< question

[Ralf Goertz]

(second repost, there seem to be problems)

Hi,

this might be g++-related but it raises a general question.

#include <iostream>

using namespace std;


bool getValue(int& val){
val=int(42);
return true;
}

int main(){
int i(0);
cout<< i<<" "<<getValue(i)< <" value "<<i<<endl; // (*)
}


Compiled with optimization (-O3) this yields the output

42 1 value 42

Without optimization it gives

42 1 value 0

I had expected

0 1 value 42

Do I have to read the line (*) from right to left? But that seems odd
since "endl" is at the end of the output.

Ralf

[Juha Nieminen]

Re: &lt;&lt; question

Ralf Goertz wrote:

bool getValue(int& val){
val=int(42);
return true;
}
>
int main(){
int i(0);
cout<< i<<" "<<getValue(i)< <" value "<<i<<endl; // (*)

Because your getValue() function has side effects, the results of that
last expression are undefined because there's more than one reference to
the value being modified in the same expression. This is because the
compiler is allowed to optimize that kind of expression in any way it
wants. The result is only guaranteed after the ';'.

If you want it to work "correctly" , divide each reference to 'i' into
its own expression:

cout << i << " ";
cout << getValue(i);
cout << " value " << i << endl;