shared_ptr and const&

[stereomatching]

void test(std:shared_ptr<some_type> A) {.....}

would make a copy of a shared_ptr
what if we pass a shared_ptr into a function like

void test(std:shared_ptr<some_type> const &A) {.....}

Is this safe?

Thanks a lot

[vijayan121]

> Is this safe?

Yes.

[mike_2000_17]

It's the analog of:

void test(some_type* const & A)

So, yes, it is perfectly fine.

[stereomatching]

Thanks
I discover one thing recently
There are one thing called "make_shared"
http://www.boost.org/doc/libs/1_43_0/libs/smart_ptr/make_shared.html
say this is a better choice than the semantic like this

shared_ptr<type_A> A(new type_A)

So we should choose make_shared over the old semantic anyway?
I am not really understand the way of make_shared work

[vijayan121]

std::make_shared<> is just a wrapper over std::allocate_shared<> with the allocator defaulted to std::allocator<>.

Assuming there is a constructor A::A( int, const char*, double ) for A,

auto pa = std::make_shared<A>( 1, "hello", 22.8 ) ;

is more efficient when compared to

std::shared_ptr<A> pa( new A( 1, "hello", 22.8 ) ) ;

It requires only one memory allocation instead of two; storage for the objct and its shared reference count is allocated together.

[stereomatching]

std::shared_ptr<A> pa = std::make_shared<A>( 1, "hello", 22.8 ) ;

This is more efficient, how about this?

std::shared_ptr<A>  pa = std::make_shared<A>() ;

;
also more efficient than old semantic?

if pass by const reference is safe

void test(std:shared_ptr<some_type> const &A) {.....}

why most of the examples are pass by value?
is it because the cause to copy the object of shared_ptr is very cheap?

Thanks

[vijayan121]

> how about this?
> std::shared_ptr<A> pa = std::make_shared<A>() ;
> also more efficient than old semantic?

Yes, and for the same reason. Requires A to have an accessible default constructor, though.

> is it because the cause to copy the object of shared_ptr is very cheap?

shared_ptr<> hs move constructors and move assignment operators; it is extremely efficient when the source of the copy or assignment is a temporary or is no longer needed. In many other cases, the compiler would be able to apply copy-elision.

Even if a copy has to be made, it is quite cheap (copy a pointer and increment a reference count) if concurrency is not involved. If not, there would be a quality of implementation issue.

[stereomatching]

std::shared_ptr<type> create()
{
  return std::make_shared<type>();
}

Is this a better choice?
Thanks