Order of execution in operator <<

The compiler can evaluate the function printcbase() as this:

void printcbase(cbase* b ){
    int a = b->FooA();    // line 1
    int b = b->FooB();    // line 2
    std::cout << a;       // line 3
    std::cout << b;       // line 4
    stc::cout << std::endl;
}

or some of many permutatins of lines marked as 1 - 4. You are only guaranteed that that the line 1 is done before the line 3, and line 2 before the line 4 (and of course line 3 before line 4). Standard does not say more and indeed you can expect different results with different C++ compilers.

The order of execution of << is well defined but the order of evaluation of sub-expressions is not defined in C++. This article and the C code example illustrates the problem you mentioned.

BA12 and AB12 are both correct. In the following code:

std::cout<< b->fooA() << b->fooB()

1 will appear before 2 but A could appear before or after B since the compiler does not promise whether it will evaluate fooA or fooB first.