Why doesn't shared_ptr permit direct assignment

Question

So when using shared_ptr you can write:

shared_ptr var(new Type());

I wonder why they didn\'t allow a

Answer 1

I wonder why they didn't allow a much simpler and better...

Your opinion will change as you become more experienced and encounter more badly written, buggy code.

shared_ptr<>, like all standard library objects is written in such as way as to make it as difficult as possible to cause undefined behaviour (i.e. hard to find bugs that waste everyone's time and destroy our will to live).

consider:

#include

struct Foo {};

void do_something(std::shared_ptr pfoo)
{
  // ... some things
}

int main()
{
  auto p = std::make_shared(/* args */);
  do_something(p.get());
  p.reset();  // BOOM!
}

This code cannot compile, and that's a good thing. Because if it did, the program would exhibit undefined behaviour.

This is because we'd be deleting the same Foo twice.

This program will compile, and is well-formed.

#include

struct Foo {};

void do_something(std::shared_ptr pfoo)
{
  // ... some things
}

int main()
{
  auto p = std::make_shared(/* args */);
  do_something(p);
  p.reset();  // OK
}