How do you do static constants right?

[Cruz]

I'm just not getting this right somehow. I would like to define a couple of static constants for a class, just like the static final variables in java. This is how I did it:

class SerialCommunication
{
	private:
		static const qint64 RX_BUFFER_SIZE = 128;
};
 
void SerialCommunication::receiveMsg()
{
	char receiveBuffer[RX_BUFFER_SIZE];
	int bytesRead = port->read(receiveBuffer, qMin(port->bytesAvailable(), RX_BUFFER_SIZE));
}

class SerialCommunication
{
	private:
		static const qint64 RX_BUFFER_SIZE = 128;
};

void SerialCommunication::receiveMsg()
{
	char receiveBuffer[RX_BUFFER_SIZE];
	int bytesRead = port->read(receiveBuffer, qMin(port->bytesAvailable(), RX_BUFFER_SIZE));
}

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For the second call to RX_BUFFER_SIZE in the read() command I get a compiler error "undefined reference to SerialCommunication::RX_BUFFER_SIZE". How come the first one is ok then? What am I not doing right?