NULL pointer with boost::shared_ptr?

Question

What's the equivalent to the following:

std::vector<Foo*> vec;
vec.push_back(NULL);

when dealing with boost::shared_ptr? Is it the following code?

std::vector< boost::shared_ptr<Foo> > vec;
vec.push_back(boost::shared_ptr<Foo>());

Note: I may push back a lot of such objects. Should I declare a global static nullPtr object somewhere? That way only one of them would have to be constructed:

boost::shared_ptr<Foo> nullPtr;

Answer 1

Your suggestion (calling the shared_ptr<T> constructor with no argument) is correct. (Calling the constructor with the value 0 is equivalent.) I don't think that this would be any slower than calling vec.push_back() with a pre-existing shared_ptr<T>, since construction is required in both cases (either direct construction or copy-construction).

But if you want "nicer" syntax, you could try the following code:

class {
public:
    template<typename T>
    operator shared_ptr<T>() { return shared_ptr<T>(); }
} nullPtr;

This declares a single global object nullPtr, which enables the following natural syntax:

shared_ptr<int> pi(new int(42));
shared_ptr<SomeArbitraryType> psat(new SomeArbitraryType("foonly"));

...

pi = nullPtr;
psat = nullPtr;

Note that if you use this in multiple translation units (source files), you'll need to give the class a name (e.g. _shared_null_ptr_type), move the definition of the nullPtr object to a separate .cpp file, and add extern declarations in the header file where the class is defined.