I am trying to implement the following class. However, when I try to instantiate an object of the class within its definition and pass "0" as value to initialize the object, i get an error:
"a type specifier is expected".
Can anyone explain how can i remove this error?
class MessageType
{
public:
static const MessageType msgEvent(0);
private:
MessageType();
virtual ~MessageType();
MessageType(int);
};
You need to initialize(define) it outside the class definition in a cpp file.
MessageType const MessageType::msgEvent;
However, Your intent in doing so is not very clear. Are you trying to implement a Singleton Pattern, probably this sample implementation might help, I leave it to you to decide, whether you really need a singleton, inspite of its disadvantages:
//MessageType.h
#include <boost/noncopyable.hpp>
class MessageType: private boost::noncopyable
{
public:
static MessageType* instance();
private:
MessageType();
~MessageType();
static bool g_initialised;
// static initialisation
static MessageType g_instance;
// dynamic initialisation
};
// MessageType.cpp
#include "MessageType.hpp"
#include <ostream>
#include <iostream>
#include <cstring>
bool MessageType::g_initialised;
// static initialisation
MessageType MessageType::g_instance;
// dynamic initialisation
MessageType::MessageType()
{
g_initialised = true;
}
MessageType::~MessageType()
{
g_initialised = false;
}
MessageType* MessageType::instance()
{
return g_initialised ? &g_instance : 0;
}
You can only initialize static member variables in the definition if they are of int type.
class MessageType
{
public:
static int sCount = 0; // That is fine.
static std::string sLogName; // That is fine.
static std::string sLogName("log.txt"); // Fail!
};
There's no way around this rule. If you want to initialize a static member variable, then you have to do it in the cpp:
std::string MessageType::sLogName("log.txt"); // fine, in the cpp.
This same rule applies directly to your MessageType instance, and has nothing to do with the fact that the class is of it's own type.
