Ignored parameters in std::cout after NULL

Question

If I write this code:

const char * b = NULL;
std::cout << "Hello!" << b;

I get this output:

Hello!

However, if I change the order to:

const char * b = NULL;
std::cout << b << "Hello!";

I get no output whatsoever, so I'm curious about it.

I can guess that probably the << operator for cout reads until a NULL, so everything after it is ignored, but wonder if someone could give me some more insight about it.

Possible duplicate of: [Is printing a null-pointer Undefined Behavior?](http://stackoverflow.com/questions/23283772/is-printing-a-null-pointer-undefined-behavior) — David G, Dec 05 '14 at 15:32

score 4 · Accepted Answer · edited Dec 05 '14 at 15:38

4

The operator has a precondition:

Requires: s shall not be a null pointer.

By breaking this precondition, you cause undefined behaviour. Anything could happen.

(As mentioned in the comments, one popular implementation sets failbit and badbit in this case, which would explain the behaviour you see. But that's not something you can rely on.)

edited Dec 05 '14 at 15:38

David G

94,763
41
167
253

answered Dec 05 '14 at 15:31

Mike Seymour

249,747
28
448
644

2

[And, in practice, GCC (rather, libstdc++) sets badbit](http://stackoverflow.com/a/7019483/560648). – Lightness Races in Orbit Dec 05 '14 at 15:32
@LightnessRacesinOrbit: That's interesting, and explains the observed behaviour. But it definitely can't be relied on. – Mike Seymour Dec 05 '14 at 15:33
[It also sets `badbit` on GCC](http://ideone.com/B06vm0) – David G Dec 05 '14 at 15:35
@0x499602D2: You're absolutely right; I typo'd badbit for failbit in the above comment. :P Anyway, my answer that I linked to gives all the details. – Lightness Races in Orbit Dec 05 '14 at 15:37
@MikeSeymour: Nope of course not :) – Lightness Races in Orbit Dec 05 '14 at 15:37

Ignored parameters in std::cout after NULL

1 Answers1