shared_ptr 的未知行为

Question

I have a sample code as below

#include <iostream>
#include <memory>

using namespace std;

class A {
    public:
    int n = 0;
};

class B {
    public:
    shared_ptr<A> ba;
};

class C {
    public:
    shared_ptr<A> ca;
    shared_ptr<B> cb;
};

int main () {
    cout << "Hello\n";

    C c;
    shared_ptr<A> a(new A);

    c.ca = a;
    c.cb->ba = a; // MAYBE PROBLEM HERE

    return 0;
}

This code was compiled successfully, but when I run, it even doesn't print "Hello". I'm newbie to C++ as well as smart pointer. Please help me explain the problem here. Thanks in advance!

Answer 1

Possible fix for your code:

#include <iostream>
#include <memory>

using namespace std;

class A {
    public:
    int n = 0;
};

class B {
    public:
    shared_ptr<A> ba;
};

class C {
    public:
        C() {
            cb = make_shared<B>();
        }
    shared_ptr<A> ca;
    shared_ptr<B> cb;
};

int main () {
    cout << "Hello\n";

    C c;
    shared_ptr<A> a = make_shared<A>();

    c.ca = a;
    c.cb->ba = a; // PROBLEM FIXED in constructor

    return 0;
}

Answer 2

This is undefined behaviour. You never create an instance of B and on the line c.cb->ba = a; // MAYBE PROBLEM HERE c.cb->ba = a; // MAYBE PROBLEM HERE you are de-referencing a null pointer, which is UB.

Note that shared pointers are default constructed to nullptr, not the default constructor of the owned type.