unexpected printout for int

[AlanG]

Here is my bash session:

agauld@ubuntu:~/src/cc/tour$ cat testinc.cc
#include <iostream>
#include <stdio.h>

using namespace std;

int main(){
    int n = 0;

    cout << "n: " << n << ++n << n++ << n << "\n";

    n = 0;
    printf("n: %d%d%d%d\n",n,++n,n++,n);

}
agauld@ubuntu:~/src/cc/tour$ make testinc
make: `testinc' is up to date.
agauld@ubuntu:~/src/cc/tour$ ./testinc
n: 2202
n: 2202 

I was trying to compare the behaviour of ++n and n++
and how that worked within a cout context.

I expected, or hoped for, 0112.
What I got - 2202 - surprised me.
Can someone please explain what happened?

[Pattako]

cout << "n: " << n << ++n << n++ << n << "\n";
printf("n: %d%d%d%d\n",n,++n,n++,n);

Mixing prefix and postfix increments in the same line like that is considered undefined behavior. Other compilers may not act the same as yours.

[AlanG]

Thanks, that was actually why I did it: to see how they were treated
when on a single line. But I assumed I'd either get the result I
expected or an error, I hadn't considered "undefined behaviour" as an
outcome! :-)

Thanks again.

[JLBorges]

#include <iostream>
#include <cstdio>

int main() {

    int n = 0;

    // undefined behaviour pre-C++17
    // well defined behaviour in C++17; prints n: 0112
    // C++17: in x << y, value computation and side-effect of x
    //                   is sequenced before the value computation and side-effect of y
    std::cout << "n: " << n << ++n << n++ << n << '\n' ;

    n = 0;

    // undefined behaviour pre-C++17
    // unspecified behaviour in C++17
    // C++17: in fn(x,y), value computations and side effects of x and y
    //                    are indeterminately sequenced with respect to each other
    std::printf( "n: %d%d%d%d\n", n, ++n, n++, n );
}

[Pattako]

@JLBorges
Sorry, what is unspecified behaviour and what is the difference between undefined behaviour and unspecified behaviour?

[JLBorges]

From the horse's mouth:

undefined behavior
behavior for which this International Standard imposes no requirements
[Note: Undefined behavior may be expected when this International Standard omits any explicit definition of behavior or when a program uses an erroneous construct or erroneous data. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed. Evaluation of a constant expression never exhibits behavior explicitly specified as undefined. —end note ]

unspecified behavior
behavior, for a well-formed program construct and correct data, that depends on the implementation
[Note: The implementation is not required to document which behavior occurs. The range of possible behaviors is usually delineated by this International Standard. —end note ]

The difference between unspecified behaviour and implementation-defined behaviour is that the latter must be documented behaviour.

implementation-defined behavior
behavior, for a well-formed program construct and correct data, that depends on the implementation and that each implementation documents