CODEWITHSUNDEEP
c++move-semanticsunique-ptr
darkThoughts
darkThoughts

Reputation: 443

using unique_ptr after move

I don't understand why this code works:

class Base {
public:
    virtual void doStuff() const{}
    Base(Base & b) = delete;
    Base() {
        cout << "Base" << endl;
    }
    ~Base() {
        cout << "~Base" << endl;
    }
    friend ostream& operator<<(ostream& out, const Base& a) {
         return out << "me";
    }
};
int main(){
    unique_ptr<Base> pt = make_unique<Base>();
    auto npt = move(pt);
    auto &a = *pt;

    if (pt == nullptr)
        cout << "yes nullptr" << endl;
    cout << a << endl;
}

The output in Visual Studio 2015 is:

Base
yes nullptr
me
~Base

So it doesn't crash and pt is even usable after being moved from. In coliru online compiler, it crashes at line cout << a << endl;. I don't understand how it doesn't crash at line auto &a = *pt;, since pt is equal to nullptr at this point and the command auto &refToNull= nullptr; is a compilation error.

I will appreciate a clarification about what's going on.

Upvotes: 0

Views: 472

Answers (2)

BiagioF
BiagioF

Reputation: 9735

First of all, the statement

auto &a = *pt;

is an Undefined Behaviour, that is not defined. Dereferencing a nullptr in C++ does not make crash your program, it may happen everything.

What you probably expect from your code is a segmentation fault, but it does not occur because you actually does not ever access to the the object a.

Indeed, your operator<< take a Base object but it does not use it at all.

Instead, if you try to do something like:

friend ostream& operator<<(ostream& out, const Base& a) {
     a.do_stuff();
}

Your program will be killed by the OS because a object is referenced on a wrong memory (actually 0x0 memory).

A related question about nullptr derefereing is Here.

Upvotes: 1

duong_dajgja
duong_dajgja

Reputation: 4276

auto npt = move(pt); transfers ownership of pt to npt leaving pt a nullptr. Since pt is now a nullptr then if (pt == nullptr) cout << "yes nullptr" << endl; outputs "yes nullptr" correctly.

auto &a = *pt; means you are trying to dereference a nullptr this is simply an undefined behavior. cout << a << endl; can lead to anything can happen but not necessarily crashing program.

Upvotes: 0

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