How does shared_ptr work in if condition

Question

In C++, I can write something like:

shared_ptr a_sp = someFunctionReturningSharedPtr();
if (a_sp) {
    cout << a_sp->someData << end         


        

Answer 1

shared_ptr has operator bool(), which returns true if it is not empty.

For example, this is Microsoft implementation of shared_ptr::operator bool(): http://msdn.microsoft.com/en-us/library/bb982901.aspx

shared_ptr::operator boolean-type - Tests if an owned resource exists.

Answer 2

shared_ptr has an operator unspecified-bool-type() const that allows it to be used in boolean contexts. The unspecified-bool-type is typically defined as a pointer to function, or pointer to member-function, to disallow accidental matching to bool function overloads.

In C++0x the idiom is to use explicit operator bool() const;, which disallows implicit conversions (such as function calls, conversions to int for arithmetic, and so on), but still allows the shared_ptr to be converted to bool in boolean contexts.