Passing shared pointers

shared_ptr.cpp

#include <iostream>
#include <memory>
 
struct Base
{
    Base() { std::cout << "  Base::Base()\n"; }
    ~Base() { std::cout << "  Base::~Base()\n"; }
};

struct TraceByVal
{
    std::shared_ptr<Base> p;

    TraceByVal(std::shared_ptr<Base> p_in) : p(std::move(p_in))
    {
        std::cout << "  TraceByVal::TraceByVal()\n"
                  << "  t.p.get() = " << p.get()
                  << ", t.p.use_count() = " << p.use_count() << '\n';
    }
};
 
struct TraceByLVRef
{
    std::shared_ptr<Base> p;

    TraceByLVRef(std::shared_ptr<Base>& p_in) : p(std::move(p_in))
    {
        std::cout << "  TraceByLVRef::TraceByLVRef()\n"
                  << "  t.p.get() = " << p.get()
                  << ", t.p.use_count() = " << p.use_count() << '\n';
    }
};
 
struct TraceByConstLVRef
{
    std::shared_ptr<Base> p;

    TraceByConstLVRef(const std::shared_ptr<Base>& p_in) : p(std::move(p_in))
    {
        std::cout << "  TraceByConstLVRef::TraceByConstLVRef()\n"
                  << "  t.p.get() = " << p.get()
                  << ", t.p.use_count() = " << p.use_count() << '\n';
    }
};
 
struct TraceByRVal
{
    std::shared_ptr<Base> p;

    TraceByRVal(std::shared_ptr<Base>&& p_in) : p(std::move(p_in))
    {
        std::cout << "  TraceByRVal::TraceByRVal()\n"
                  << "  t.p.get() = " << p.get()
                  << ", t.p.use_count() = " << p.use_count() << '\n';
    }
};
 
int main()
{
    {
        // By value
        std::shared_ptr<Base> p = std::make_shared<Base>();
     
        std::cout << "Created a shared Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n';

        TraceByVal t(std::move(p));

        std::cout << "Created a TraceByVal containing Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n'
                  << "  t.p.get() = " << t.p.get()
                  << ", t.p.use_count() = " << t.p.use_count() << '\n';
    }

    {
        // By lvalue reference
        std::shared_ptr<Base> p = std::make_shared<Base>();
     
        std::cout << "Created a shared Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n';

        TraceByLVRef t(p);

        std::cout << "Created a TraceByLVRef containing Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n'
                  << "  t.p.get() = " << t.p.get()
                  << ", t.p.use_count() = " << t.p.use_count() << '\n';
    }

    {
        // By const lvalue ref
        std::shared_ptr<Base> p = std::make_shared<Base>();
     
        std::cout << "Created a shared Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n';

        TraceByConstLVRef t(std::move(p));

        std::cout << "Created a TraceByConstLVRef containing Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n'
                  << "  t.p.get() = " << t.p.get()
                  << ", t.p.use_count() = " << t.p.use_count() << '\n';
    }

    {
        // By rvalue ref
        std::shared_ptr<Base> p = std::make_shared<Base>();
     
        std::cout << "Created a shared Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n';

        TraceByRVal t(std::move(p));

        std::cout << "Created a TraceByRVal containing Base\n"
                  << "  p.get() = " << p.get()
                  << ", p.use_count() = " << p.use_count() << '\n'
                  << "  t.p.get() = " << t.p.get()
                  << ", t.p.use_count() = " << t.p.use_count() << '\n';
    }

}