Question

I have a custom stack class. Most of the code can be seen here:
Member functions of a templated class, that take a template type as argument

I fill the stack like so:

stack <int> Astack;
Astack.Push(1); Astack.Push(2); Astack.Push(3); Astack.Push(4);

Then I do this:

cout << Astack.Pop() << Astack.Pop() << Astack.Pop() << Astack.Pop() <<endl;

and get this: 1234
However, if I do this:

cout << Astack.Pop(); cout << Astack.Pop(); cout << Astack.Pop(); cout << Astack.Pop();

I get this: 4321, which is obviously what I want.

So, what gives?

Answer 1

The order of evaluation of the function calls is unspecified. Your first expression basically boils down to this:

cout << a << b << c << d;

Each of a, b, c, and d are calls to Astack.Pop(). The compiler can generate code that evaluates those calls in any order it chooses.

You should avoid writing expressions that rely on a particular order of evaluation of parts of the expression. In general, it’s not safe (and even when it is safe, it is usually quite confusing).