C++ Using a variable declared in class definition to declare a different variable in the same class definition

Question

I am doing a network simulation using Omnet++ which is based on C++. I am trying to define a class for a certain module using the following code:

#ifndef __PROJECT_IMS_SLF_H_
#define __PROJECT_IMS_SLF_H_

#include <omnetpp.h>
#include "mDIAMETER_m.h"
#include "IPPacket_m.h"

class SLF : public cSimpleModule
{
  public:
    mDIAMETER *generateDIAMETERmsg(const char* name, long userID, bool registered, std::string server);
    IPPacket *generateIPPacket(int srcIP,int desIP);
  protected:
    virtual void initialize();
    virtual void handleMessage(cMessage *msg);
  private:
    int MyIP;
    int N = par("N");
    struct Registry {
            long UserID;
            bool Registered;
            std::string Server;
        };
    struct Registry MyReg[N];   // Create a Registry Table for all UEs

};



#endif

I am getting the following error: "invalid use of non-static data member SLF::N". I know I'm declaring N inside the class definition as a private variable but I have to do so because N is a parameter related to the module which is defined as an instance of the class SLF and it is read from the configurations of this module using the OMNET++ defined function "par".

If I declareMyReg[N]inside the initialize function it works but the problem is that I need to use this variable inside the other function (handleMessage). Any hints?

Thanks in advance!

Instead of using a C-style array, use a std::vector, then you don't need the N member variable as you can initialize the vector to the correct size in the constructor. — Some programmer dude
– Some programmer dude, Commented Feb 5, 2015 at 8:49
Standard C++ requires that the dimension be a constant. 'N' isn't a constant, and unless 'par("N")' is a constexpr function then I'm not sure it can ever be. Use a std::vector instead. — Richard Corden
– Richard Corden, Commented Feb 5, 2015 at 8:50
par("N") looks like a function call (although not shown in your question) that returns the value at runtime. The compiler needs to value at compile time when you declare MyReg[N]'. That's the reason you need to allocate the Registry objects meomry at runtime. — harper
– harper, Commented Feb 5, 2015 at 9:32

Jim Buck · Accepted Answer · 2015-02-05 08:53:56Z

1

Allocate the array on the heap:

class SLF /* etc. */
{
    /* etc. */

    virtual void initialize()
    {
        /* etc. */

        N = par("N");
        MyReg = new Registry[N];

        /* etc. */
    }

    /* etc. */

    int N;

    /* etc. */

    struct Registry *MyReg;
}

And be sure to delete the memory in a destructor.

edited Feb 5, 2015 at 8:53

answered Feb 5, 2015 at 8:48

Jim Buck

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1 Comment

Jim Buck Over a year ago

Or use a std::vector, but my answer was meant to keep the spirit of your original code.

mshabeeb · Accepted Answer · 2015-02-05 09:35:53Z

0

Problem solved as follows:

First, define a pointer to the instance of this structure:

struct Registry *MyReg;

Now inside the module, within the initialize function, N can be read and the instance of the structure can be declared with its size:

int N = par("N");
this->MyReg = new struct Registry[N];

I also defined a finish module withtin the header file and deleted MyReg inside that function.

answered Feb 5, 2015 at 9:35

mshabeeb

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2 Comments

Rerito Over a year ago

That's exactly what Jim Buck stated in his answer :|

mshabeeb Over a year ago

After I posted the answer I noticed Jims

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