Why doesn't shared_ptr permit direct assignment

Question

So when using shared_ptr<Type> you can write:

shared_ptr<Type> var(new Type());

I wonder why they didn't allow a much simpler and better (imo):

shared_ptr<Type> var = new Type();

Instead to achieve such functionality you need to use .reset():

shared_ptr<Type> var;
var.reset(new Type());

I am used to OpenCV Ptr class that is a smart pointer that allows direct assignment and everything works fine

Answer 1

The syntax:

shared_ptr<Type> var = new Type();

Is copy initialization. This is the type of initialization used for function arguments.

If it were allowed, you could accidentally pass a plain pointer to a function taking a smart pointer. Moreover, if during maintenance, someone changed void foo(P*) to void foo(std::shared_ptr<P>) that would compile just as fine, resulting in undefined behaviour.

Since this operation is essentially taking an ownership of a plain pointer this operation has to be done explicitly. This is why the shared_ptr constructor that takes a plain pointer is made explicit - to avoid accidental implicit conversions.

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The safer and more efficient alternative is:

auto var = std::make_shared<Type>();