Shared pointer to an immutable type has value semantics

Question

Sean Parent gave a talk at Going Native 2013 titled Inheritance Is The Base Class of Evil. At 20 minutes, 50 seconds in, he makes the statement that a shared pointer to an i

Answer 1

I am giving 3 examples. In all three cases I create a variable a with the content "original value". Then I create another variable b by saying auto b = a; and after this statement I assign a the content "new value".

If a and b have value semantics, I expect b's content to be the "original content". And in fact exactly this happens with string and shared_ptr. The conceptual meaning of auto b = a; is the same with these types. Not so much with shared_ptr, b will have the content "new value".

Code (online demo):

#include 
#include 
#include 

using namespace std;

void string_example() {

    auto a = string("original value");

    auto b = a; // true copy by copying the value

    a = string("new value");

    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    cout << boolalpha << "&a == &b ? " << (&a==&b) << endl;
}

void shared_ptr_example() {

    auto a = make_shared("original value");

    auto b = a; // not a copy, just and alias

    *a = string("new value"); // and this gonna hurt b

    cout << "a = " << *a << endl;
    cout << "b = " << *b << endl;
    cout << boolalpha << "&a == &b ? " << (&a==&b) << endl;
}

void shared_ptr_to_const_example() {

    auto a = make_shared("original value");

    auto b = a;

    //*a = string("new value"); // <-- now won't compile
    a = make_shared("new value");

    cout << "a = " << *a << endl;
    cout << "b = " << *b << endl;
    cout << boolalpha << "&a == &b ? " << (&a==&b) << endl;
}

int main() {

    cout << "--------------" << endl;
    cout << "string example" << endl;
    string_example();

    cout << "------------------" << endl;
    cout << "shared_ptr example" << endl;
    shared_ptr_example();

    cout << "---------------------------" << endl;
    cout << "shared_ptr to const example" << endl;
    shared_ptr_to_const_example();
}

Output:

--------------
string example
a = new value
b = original value
&a == &b ? false
------------------
shared_ptr example
a = new value
b = new value
&a == &b ? false
---------------------------
shared_ptr to const example
a = new value
b = original value
&a == &b ? false

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Having said that, I wish he had had a little more time: There are a few things I am still wondering about after that presentation. I am pretty convinced it was only the lack of time, he seems like a excellent presenter.