does this struct need a non-default constructor

Question

I have the following struct in C++, and I wondered if I need to define a non-default constructor for it when I use it as follows: boost::shared_ptr<node> p_node = boost:shared_ptr<node>();

struct node
{
    std::string name;
    std::map<std::string, std::vector<variant> > values; // it is possible that nodes contain as a value, key/value pairs so we need a map

    NodeType type;  //Enum

    typedef struct attrib
    {
        std::string key;
        variant value;  //Boost::variant
    };

    std::vector<attrib> attributes;

    boost::shared_ptr<node> childnode;
};

Answer 1

"does this POD need a non-default constructor"... what POD? POD's don't contain complex objects like strings and maps. POD stands for plain old data, which is things like doubles and char arrays.

Whether you need a constructor depends on whether you want to make sure all the data is initialised in some sane state. std::map , std::vector and std::string are all initialised for you to be empty. The other boost::shared_ptr will be NULL . attrib is only a type and you won't initially have any attrib objects, so no worries there. But, your NodeType enum isn't initialised anywhere unless you do it yourself in a constructor. Does that matter? Only you can decide, but technically you MUST make sure you assign to it somewhere before reading from it, otherwise you technically get undefined behaviour.

Answer 2

Note that this is not a POD .

Yes, this struct needs a default ctor, because otherwise the enum member will have an undefined value after default construction. It doesn't matter how you use it -- or, as in your example code boost::shared_ptr<node> p_node = boost:shared_ptr<node>(); do not use it at all, as that just initializes a NULL shared ptr and you could just as well have written boost::shared_ptr<node> p_node;

Answer 3

1-) If you define any constructor (with 0 or 1 or n parameter) in your struct or class then C++ default constructor will not be created automatically.

So every thing is now depend only on your call to constructor.

if you create object Node* x = new Node(); This must call constructor with no parameter and your class must have constructor with no parameter[Read First Line Again].

Lets check if there is any constructor with no parameter in your struct or class. I say it again if you create any constructor with any number of parameter in C++ then default constructor will not be created by C++ interpreter automatically.

Line 1 is very important to understand.