How to use Struct with a *variable name?

Question

private:
 // data containing character strings
 // shared by Strings when copying/assigning
 struct SharedData
 {
   char *data; // 0-terminated char array
   size_t n;      // number of non-0 characters in string
   size_t count;  // reference count, how many Strings share this object?
 };

 SharedData *shared; //here i don't understand how to use.

How do I use n?

do I just do this

shared -> n = 3;

I keep getting segmentation faults so i don't know what i am doing. I want to grab the length of a string and then pass it to n to be stored

Answer 1

Your class contains a pointer to SharedData. You need to make that pointer point to something, presumably by doing this

shared = new SharedData();
shared->n = 3;
// etc.

Same thing is true of data inside shared

shared->data = new char[100];
strcpy(shared->data, ...);

I have a feeling this is going to be quite a hard exercise for you if you don't have a good grasp of pointers yet.

Answer 2

You should allocate some memory first:

SharedData *shared = new SharedData;
shared->n = 3;

Better use something like this:

struct SharedData {
    SharedData(size_t Capacity) : data(0), capacity(Capacity), length(0), count(1) {
        data = new char[capacity];
    }
    ~SharedData() {
        if (--count <= 0) delete [] data;
    }
    char *data; // 0-terminated char array
    size_t length;      // number of non-0 characters in string
    size_t capacity;
    size_t count;  // reference count, how many Strings share this object?
};
SharedData* shared = new SharedData(1000);

Edit: Of course this short example lacks of copy constructor and handle reference counting in appropriate manner. But this is not subject of this answer.

Answer 3

Modifying an un-allocated point in memory is undefined behavior.

You must allocate memory for shared or set it to a valid address.

class Class
{
private:

  SharedData *shared;

  ...

public:

  Class() : shared(new SharedData()) {}

  ~Class() { delete shared; }

  void doSomething() { shared->n = 3; }

};

Try not to use bare pointers when you can use smart pointers:

class Class
{
private:

  std::unique_ptr<SharedData> shared;

  or

  std::shared_ptr<SharedData> shared;

  ...

public:

  Class() : shared(new SharedData()) {}

  void doSomething() { shared->n = 3; }

};

Answer 4

Are you using a C++11 compiler? Why don't you make it a bit easier?

struct SharedData
{
   std::string data;
   size_t count;  // reference count, how many Strings share this object?
};

std::shared_ptr<SharedData> shared = std::make_shared<SharedData>();

If you don't need the actual count of the number of referrers, you just want to know if there is anyone referencing your data, simply use a std::shared_ptr<std::string> . It will keep track of references and destroy the string object when all the references are gone.