Re: [Dev-C++] Problem with 'std::cout' (?)

[Franco B. <fbr...@gm...>]

OK.
I 'supposed' it was an intrinsic compiler feature.
But nothing better than a detailed explanation given by an expert!

I guess, the better strategy is to use 'separate-cout' statements in similar
situations.

Thanks,
Franco


2007/10/5, matheus ribeiro <mfr...@gm...>:
>
> Elaborating a bit more on the problem. The C/C++ compiler is free to
> evaluate function parameters in any order. In the case of gcc/g++,
> function parameter evaluation happens from right to left. Also, all
> parameters have to be evaluated before function call.
>
> The function:
>
> --- ostream &operator <<(ostream &, int)
>
> acts just like:
>
> --- ostream& write(ostream &s, int i){ s.write(i); return s; }
>
> Thus, this statement:
>
> --- cout  << f1() << f2() << f3();
>
> is equivalent to:
>
> --- write3( write2( write1(cout, f1() ), f2() ), f3() );
>
> because the << operator is evaluated from left to right. I renamed
> functions to make it easier to explain (write1, write2, write3 are the
> same, like f1, f2 and f3).
>
> When compiler sees this, it:
> 1- evaluates write3 right parameter, f3r = f3().
> 2- evaluates write3 left parameter, write2( write1(cout, f1() ), f2() ).
>     2.1 - evaluates write2 right parameter, f2r = f2()
>     2.2- evaluates write2 left parameter write1(cout, f1() )
>         2.2.1- evaluates write1 right parameter, f1r = f1()
>         2.2.2- call w1r = write1(cout, f1r)
>     2.3- call w2r = write2(w1r, f2r)
> 3- call write3(w2r, f3r)
>
> So, the f functions will be called in the order: f3(), f2(), f1()
> before any of the write functions due to parameter order evaluation.
> The write functions will happen in the order write1(), write2(),
> write3().
>
> As a result, the f functions will be called in reverse order, but the
> stream will be written in correct the order. The side effect of the f
> functions are responsible for the weird result you see. If a function
> has a side effect on itself, you can not rely on the order of
> parameters if its called multiple times as parameters, since each
> compiler is free to evaluate parameters in any order.
>
> Sorry for bringing this up again, just wanted to clear it up.
>