Question

I’m having trouble figuring out which is better in C++:

I use a struct to manage clients in a message queue, the struct looks like this:

typedef struct _MsgClient {
  int handle;
  int message_class;
  void *callback;
  void *private_data;
  int priority;
} MsgClient;

All of these being POD entities.

Now, I have an array of these structs where I store my clients (I use an array for memory constraints, I have to limit fragmentation). So in my class I have something like this:

class Foo
{
private:
  MsgClient _clients[32]; 

public:
  Foo()
  {
     memset(_clients, 0x0, sizeof(_clients));
  }

}

Now, I read here and there on SO that using memset is bad in C++, and that I’d rather use a constructor for my structure.
I figured something like this:

typedef struct _MsgClient {
  int handle;
  int message_class;
  void *callback;
  void *private_data;
  int priority;
  // struct constructor
  _MsgClient(): handle(0), message_class(0), callback(NULL), private_data(NULL), priority(0) {};
} MsgClient;

…would eliminate the need of the memset. But my fear is that when foo is initialized, the struct constructor will be called 32 times, instead of optimizing it as a simple zero out of the memory taken by the array.

What’s your opinion on this?
I just found this: Can a member struct be zero-init from the constructor initializer list without calling memset? , is it appropriate in my case (which is different: I have an array, not a single instance of the structure)?
Also, according to this post, adding a constructor to my structure will automatically convert it into a non-POD structure, is it right?

Answer 1

On a conforming implementation, it’s perfectly valid to value-initialize an array in the constructor initializer list with an empty member initializer. For your array members, this will have the effect of zero-initializing the members of each array element.

The compiler should be able to make this very efficient and there’s no need for you to add a constructor to your struct.

E.g.

Foo() : _clients() {}