How do I access the contents of a std::shared_ptr? - c++

Question

Here is my code.
std::shared_ptr<WSUStudent> WSUStudent::registerStudent(
std::string lastName,
std::string firstName
)
{
auto result = std::shared_ptr<WSUStudent>(new WSUStudent(lastName, firstName));
s_allStudents.insert(&result);
return result;
}
I have successfully managed to change the function so it returns a shared_ptr instead of a normal pointer. I have successfully encapsulated the 'new' statement with a shared pointer, as per the assignment (I think), but the line of code below 'auto' didn't work without the &, and it doesn't work WITH the &. I receive an error stating that there is no matching function call, with or without the &. That line of code is attempting to insert the new student (or a pointer to the new student?) into the list of all students. However the 'insert' method is not locally overridden, so I'm not quite sure what to do here. Error printed below.
/mnt/hgfs/Data Structures and Algorithms/HW04/WSUStudent.cpp:146:32: error: no matching function for call to ‘std::set<WSUStudent*>::insert(std::shared_ptr<WSUStudent>*)’
s_allStudents.insert(&result);
The point of this assignment is to fix memory leaks ('new' statements that don't get deleted with their pointers) by turning normal pointers into weak pointers and shared pointers. The original code is as follows.
WSUStudent *WSUStudent::registerStudent(
std::string lastName,
std::string firstName
)
{
auto result = new WSUStudent(lastName, firstName);
s_allStudents.insert(result);
return result;
}
Am I going about this wrong? I can't get the s_allStudents line to run.

Answer 1

Given the type of s_allStudents, you can use:
s_allStudents.insert(result.get());
However, a better option will be to change type of s_allStudents.
static std::set<std::shared_ptr<WSUStudent>> s_allStudents;
and use:
s_allStudents.insert(result);
Update
The default operator<() of shared_ptr is such that the objects in s_allStudents will be sorted by pointer value. If you would like to sort the objects using a different criterion, you'll need to define a custom functor/function as a parameter of the template.
struct MyCompare
{
bool operator<(shared_ptr<WSUStudent> const& lhs,
shared_ptr<WSUStudent> const& rhs) const
{
// Implement the logic ...
}
};
and use it as:
static std::set<std::shared_ptr<WSUStudent>, MyCompare> s_allStudents;

Answer 2

If you are going to return a std::shared_ptr<WSUStudent> then you are returning ownership rights to the object you created - meaning someone else will be trying to delete it at some point.
Unless you also keep ownership that means your pointer could get deleted before you are finished with it. So you need to also store std::shared_ptr in your static set:
I am guessing at how you are using this class but what I mean goes something like this:
class WSUStudent
{
// you really need to store shared pointers in here
static std::set<std::shared_ptr<WSUStudent>> s_allStudents;
std::string lastName;
std::string firstName;
// only the static factory function can make students
WSUStudent(
const std::string& lastName, // passing by const& is more usual (idiomatic)
const std::string& firstName)
: lastName(lastName)
, firstName(firstName)
{
}
public:
static std::shared_ptr<WSUStudent> registerStudent(
const std::string& lastName,
const std::string& firstName);
};
std::shared_ptr<WSUStudent> WSUStudent::registerStudent(
const std::string& lastName,
const std::string& firstName
)
{
auto result = std::shared_ptr<WSUStudent>(new WSUStudent(lastName, firstName));
// put the shared student in your set
s_allStudents.insert(result);
return result;
}
// define your set
std::set<std::shared_ptr<WSUStudent>> WSUStudent::s_allStudents;
int main ()
{
// make students
auto s = WSUStudent::registerStudent("bill", "bob");
// all deletions should be in order
}