Confusion with unique_ptr

Question

I would expect the following code to crash at runtime with a null pointer error:

#include <memory>
#include <iostream>
#include <cassert>

struct Foo {
    void echo() {std::cout << "Echo" << std::endl;}
};

int main()
{
    std::unique_ptr<Foo> up(new Foo());
    up.reset(nullptr);

    assert(up.get() == nullptr);

    up.get()->echo();
}

however both gcc (4.7/4.8/4.9), msvc (2013 and upcoming 2015) and clang (3.5) happily outputs:

Echo

and assert is not firing so up.get() is nullptr.

UB is UB, you should not expect anything. – Deduplicator Nov 23 '14 at 15:57 — Deduplicator, Nov 23 '14 at 15:57
How can I call a method on a nullptr? – ismail Nov 23 '14 at 15:59 — ismail, Nov 23 '14 at 15:59
Finally found the duplicate again. – Deduplicator Nov 23 '14 at 16:01 — Deduplicator, Nov 23 '14 at 16:01
BTW: The `unique_ptr` is a red hering in your question. – Deduplicator Nov 23 '14 at 16:13 — Deduplicator, Nov 23 '14 at 16:13
@Deduplicator well I get that now. – ismail Nov 23 '14 at 16:13 — ismail, Nov 23 '14 at 16:13

score 2 · Accepted Answer · answered Nov 23 '14 at 16:00

2

You are essentially calling a member function with an invalid object argument - which is generally considered to trigger undefined behavior. However, you are not using this inside the member funtion, so no Segfaults should occur practically.

Try adding a member and accessing that member inside echo. That should crash your program. I.e.

struct Foo
{
    int i;
    void echo() {std::cout << i << std::endl;}
};

answered Nov 23 '14 at 16:00

Columbo

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The error is earlier, see the duplicate. – Deduplicator Nov 23 '14 at 16:02
@Deduplicator What do you mean? Dereferencing a null pointer? dyp says otherwise. I've used the same argumentation myself some time ago. – Columbo Nov 23 '14 at 16:06
@Columbo: Dyp says dereferencing a null-pointer *in C++* is fine? Where, and what are his arguments? I hope it's not "it seems to work". Because I would be *very* interested in anything allowing dereferencing null-pointers, especially as it breaks the guarantee that references always represent an object. – Deduplicator Nov 23 '14 at 16:10
Dereferencing a null-pointer yields an lvalue. No reference is created per se. I'll find the comment where we had the discussion. – Columbo Nov 23 '14 at 16:13
@Deduplicator I mixed it up a little bit. [**Here**](http://stackoverflow.com/questions/26168164/lambda-expressions-as-class-template-parameters-in-c14#comment41046731_26170459) are the comments. Also consider hvd's last comment. – Columbo Nov 23 '14 at 16:19

score 0 · Answer 2 · answered Nov 23 '14 at 16:00

0

Calling member function through NULL pointer is surely an undefined behavior.

Your call to that member function is successful as you are not making use of this pointer.

answered Nov 23 '14 at 16:00

ravi

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Confusion with unique_ptr

2 Answers2