我应该从静态成员方法返回什么类型的指针

Question

I come mostly from the world of Java, but have recently been writing a bunch of c++ and still don't have a great understanding of how to use pointers or what type of pointers to use where. I will give a brief example of my case, and keep in mind the question I am really asking is, "What type of pointer should I use and why?".

A perfect parallel to my code is to consider a class for a chain link. Each link has a parent link, except for the root link of which there can only be 1.

// Link.hpp
class Link
{
public:
    Link(const std::string &linkName, Link *parentLink);
    Link(const std::string &linkNam);
    static Link* createRootLink(const std::string &linkName);
    static Link* getRootLink();

private:
    static Link *rootLink;
    Link *parentLink;
}

//Link.cpp
Link* Link::rootLink = Link::createRootLink("root");

Link* Link::createRootLink(const std::string &linkName);
{
    // Is raw pointer correct here or should I 
    // use a different pointer type, i.e. smart_pointer, 
    // shared_pointer, etc?
    Link *rootLink = new Link(linkName); 
    return rootLink;
}

Link* getRootLink()
{
     return rootLink;
}

Link::Link(const std::string &linkName)
{
    this->linkName = linkName;
    this->parentLink = NULL; // Root link has no parent link.
}

Link::Link(const std::string &linkName, Link *parentLink)
{
    this->linkName = linkName;
    this->parentLink = parentLink;
}

Hopefully this is clear enough. I Java this type of thing is simple, but in c++ I am not sure how to manage the need for pointers to static objects. Thanks in advance for the help!

Answer 1

一个Link*很好，并且恰好有一个类（ static ）没有关系 - 它仍然只是一个指向Link对象的指针。

Answer 2

I use this (as it is inlinable in the header):

class Link
{
public:
    Link(const std::string &linkName, Link *parentLink);
    Link(const std::string &linkNam);
    static Link* getRootLink() {
        static std::unique_ptr<Link> root(new Link("root"));
        return root.get();
    }
};

In a single threaded environment this is fine, in multithreaded you'll need to protect against multiple threads accessing getRootLink at the same time for the very first time (there's a guard pattern to enable this.)

Note the unique_ptr is there to allow the deconstructor to run at the end of your programme. If you know you don't need the deconstructor, a plain old pointer is fine too.

Answer 3

I'd suggest returning a std::unique_pointer<Link> or std::shared_pointer<Link> or (before C++11) std::auto_ptr<Link> .

You can return raw pointers but you will have to manage the pointers (and lifetime of the objects they point to) separately. Since Java does that cleanup for you - and Java developers often encourage you not to worry about such things - you will often not have the mindset to manage such things sensibly.

Answer 4

I don't see why it is returning a pointer at all. I would expect createRootLink() to return by-value and getRootLink() to return a reference.

class Link
{
public:
    Link(const std::string& name, Link& parent) : name(name), parent(&parent) {}
    Link(const std::string& name) : name(name), parent(nullptr) {}

    static Link createRootLink(const std::string& name)
    {
        return Link(name);
    }

    static Link& getRootLink() 
    {
         static Link rootLink = createRootLink("root");
         return rootLink;
    }

private:
    std::string name;
    Link *parent;
};