fork download

#include <iostream>
#include <memory>
 
class Base {};
 
class Derived : public Base {};
 
void f(std::shared_ptr<Base>& x)
{
    std::cout << "in function expecting const shared_ptr<Base>& - Use count: " << x.use_count() << std::endl;
}
 
int main(int argc, char const *argv[])
{
    std::cout << "Base class" << std::endl;
    auto a = std::make_shared<Base>();
    std::cout << "Created shared_ptr:  Initial use count: " << a.use_count() << std::endl;
    f(a);
 
    std::cout << "------------------\nChild class" << std::endl;
    auto b = std::make_shared<Derived>();
    std::cout << "Created shared_ptr. Initial use count: " << b.use_count() << std::endl;
    f(b);
 
    return 0;
}

prog.cpp: In function ‘int main(int, const char**)’:
prog.cpp:23:8: error: invalid initialization of non-const reference of type ‘std::shared_ptr<Base>&’ from an rvalue of type ‘std::shared_ptr<Base>’
     f(b);
        ^
In file included from /usr/include/c++/6/memory:82:0,
                 from prog.cpp:2:
/usr/include/c++/6/bits/shared_ptr.h:221:2: note:   after user-defined conversion: std::shared_ptr<_Tp>::shared_ptr(const std::shared_ptr<_Tp1>&) [with _Tp1 = Derived; <template-parameter-2-2> = void; _Tp = Base]
  shared_ptr(const shared_ptr<_Tp1>& __r) noexcept
  ^~~~~~~~~~
prog.cpp:8:6: note:   initializing argument 1 of ‘void f(std::shared_ptr<Base>&)’
 void f(std::shared_ptr<Base>& x)
      ^

Standard output is empty