c++ - Basic syntax question for shared_ptr -

i new shared_ptr. have few questions regarding syntax of c++0x shared_ptr below:

//first question shared_ptr<classa>ptr(new classa()); //works shared_ptr<classa>ptr; ptr = ?? //how create new object assign shared pointer?   //second question shared_ptr<classa>ptr2; //could tested null if statement below shared_ptr<classa> ptr3(new classa()); ptr3 = ?? //how assign null again ptr3 if statement below becomes true?  if(!ptr3) cout << "ptr equals null"; 

shared_ptr<classa> ptr; ptr = shared_ptr<classa>(new classa(params)); // or: ptr.reset(new classa(params)); // or better: ptr = make_shared<classa>(params);  ptr3 = shared_ptr<classa>(); // or ptr3.reset(); 

edit: summarize why make_shared preferred explicit call new:

1. some (all?) implementation use 1 memory allocation object constructed , counter/deleter. increases performance.

2. it's exception safe, consider function taking 2 shared_ptrs:

   f(shared_ptr<a>(new a), shared_ptr<b>(new b)); 

   since order of evaluation not defined, possible evaluation may be: construct a, construct b, initialize share_ptr<a>, initialize shared_ptr<b>. if b throws, you'll leak a.

3. separation of responsibility. if shared_ptr responsible deletion, make responsible allocation too.