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c++gccauto-ptr

Why does this code only print 42?

Could somebody please explain to me why does this code only print "42" instead of "created\n42"?

#include <iostream>
#include <string>
#include <memory>

using namespace std;

class MyClass
{
public:
    MyClass() {cout<<"created"<<endl;};
    int solution() {return 42;}
    virtual ~MyClass() {};
};

int main(int argc, char *argv[])
{
    auto_ptr<MyClass> ptr;
    cout<<ptr->solution()<<endl;
    return 0;
}

BTW I tried this code with different values in solution and I always get the "right" value, so it doesn't seem to be a random lucky value.

Solution

  • Because it exhibits undefined behaviour - you dereference a null pointer.

    When you say:

     auto_ptr<MyClass> ptr;

    you create an autopointer which doesn't point to anything. This is equivalent to saying:

    MyClass * ptr = NULL;

    Then when you say:

    cout<<ptr->solution()<<endl;

    you dereference this null pointer. Doing that is undefined in C++ - for your implementation, it appears to work.