undefined behavior in C++

#ifndef _STACK_H
#define _STACK_H
const int maxStack=20;
template <class T>
class Stack
{
public:
       Stack():_top(0){}
       T pop();
       void push(T);
private:
        T _array[maxStack];
        int _top;
};

#endif

 

I found the output from the expression:

cout << is.pop()<<is.pop()<<is.pop<<endl;

is not my expection. I googled the problem. Got answer from the C++ creator as below, it is not undefined behavior:

C++ : What's the value of i++ + i++?

It's undefined !! Basically, in C and C++, if you read a variable twice in an expression where you also write it, the result is undefined. Don't do that. Another example is:

 v[i] = i++;

Related example:

 f(v[i],i++);

Here, the result is undefined because the order of evaluation of function arguments are undefined.

You can even check it from Bjarne's own blog on C++ FAQs.