[RESOLVED] Pointer to an array of pointers to a structure

[aryajur]

Hi,
I have an abstract class like this:

class Obj
{
    public:
       virtual int someFunc()=0;
    protected:
        struct A
        {
            int Index;
            std::string Name;
            std::string Hier;
        };
        A **a;
};

And then I derive a class from this like this:

class derObj : public Obj
{
    public:
        int someFunc();
    private:
        struct A a[2] = {
          { 1, "name","Top" }, { NULL, NULL, NULL}
        }
};

This doesn't work out however and I get error messages:

error: a brace-enclosed initializer is not allowed here before '{' token
error: ISO C++ forbids initialization of member 'a'
error: making 'a' static|
error: invalid in-class initialization of static data member of non-integral type 'Obj::A [2]'

Please can anybody help me determine what is causing this error. Any help would be appreciated. Thanks.

[Amleto]

you cant initialise in a class def like that - use a constructor.

you're also calling someting in derObj 'a' - the same name as A** member in Obj. Did you mean to do this?

[aryajur]

Thanks for the reply. I need a const array that needs to be initialized in the derived class but it needs to be accessible from the function defined in the parent class. So how can I do this?

[laserlight]

hmm... I don't know if this is a good or bad idea, but something like this comes to mind:

#include <string>

class Obj
{
public:
    virtual ~Obj() {}
    virtual int someFunc() = 0;
protected:
    struct A
    {
        int Index;
        std::string Name;
        std::string Hier;
    };
private:
    virtual const A& getA(int i) const = 0;
};

class DerObj : public Obj
{
public:
    virtual int someFunc()
    {
        return 0;
    }
private:
    const static Obj::A a[2];

    virtual const A& getA(int i) const
    {
        return a[i];
    }
};

const Obj::A DerObj::a[2] = {
    {1, "name", "Top"}, {0, std::string(), std::string()}
};

[Amleto]

seems a very bad idea unless it's going to be a singleton, surely?

[aryajur]

I tried this out:

class Obj
{
protected:
  struct A
  {
    int Index;
    std::string Name;
    std::string Hier;
  };

  explicit Obj(const A a[]) : a_(a) {}

  const A *a_;
};

class derObj : public Obj
{
  static const A a_init_[];

public:
  derObj() : Obj(a_init_)
  {
  }
};

const Obj::A derObj::a_init_[] = {
  { 1, "name","Top"  },
  { 0, NULL, NULL }
};

It compiled fine but the program crashes when I execute it. It works fine if I remove this line:

const Obj::A derObj::a_init_[] = {
  { 1, "name","Top"  },
  { 0, NULL, NULL }
};

[aryajur]

The above scheme worked nicely. The problem was initializing the string variables with NULL. I initialized them with empty strings "" and then it worked fine.

[Paul McKenzie]

The above scheme worked nicely. The problem was initializing the string variables with NULL. I initialized them with empty strings "" and then it worked fine.

To explain your problem, assigning NULL to a std::string produces undefined behaviour. The string constructor does not take NULL's.

You must construct a string correctly, and assigning NULL to a string is incorrect. The reason is that NULL will be converted to a 0, and then internally the string class will call the constructor, but which constructor? No constructor of std::string has a single argument that takes an int. So the closest constructor is the one that takes a const char*. You are now initializing std::string using the const char* constructor, and that constructor requires that whatever is pointed to be terminated with a NULL. The string class may now attempt to dereference a NULL pointer when performing this operation, and doing that is undefined behaviour.

So the string should be initialized to an empty string, not NULL.

Regards,

Paul McKenzie

[laserlight]

seems a very bad idea unless it's going to be a singleton, surely?

We're dealing with an array of const objects, so there is no global state to be encapsulated into a singleton.