[RESOLVED] boost shared_ptr (allocation question)

[John E]

Sometimes I'll see a shared_ptr declared like this:-

shared_ptr<type> p;

and other times I'll see this;-

shared_ptr<type> p(new type);

AFAIK the first option allocates an object of type 'type' using new - so do the two declarations do anything different?

[2kaud]

I don't use boost so can't comment upon boost's implementation. But for c++11,

shared_ptr<type> p;

defines p using the default constructor The object p is empty (owns no pointer, use count of zero).

shared_ptr<type> p(new type);

defines p using the constructor that takes a pointer (returned by new type) whose ownership is taken over by the object and sets the use count to 1.

See http://www.cplusplus.com/reference/m...tr/shared_ptr/

PS. It looks like from the boost documentation that boost shared_ptr<> is the same.

default constructor

shared_ptr(); // never throws
shared_ptr(std::nullptr_t); // never throws

Effects: Constructs an empty shared_ptr.

Postconditions: use_count() == 0 && get() == 0.

Throws: nothing.

PPS It is more efficient to use make_shared(). See http://en.cppreference.com/w/cpp/mem...tr/make_shared

[wolle]

But for c++11,

shared_ptr<type> p;

defines p using the default constructor The object p is empty (owns no pointer, use count of zero).

This particular emptiness is known as the nullptr.

[John E]

Good explanations guys, thanks