How do you do static constants right?

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:

Code:

---

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?

Code:

---

class SerialCommunication
{
	private:
		static const qint64 RX_BUFFER_SIZE;
};
 
const qint64 SerialCommunication::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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Kinda sucks though that you have to declare and initialize in two different places.

Or you can just define it outside SerialCommunication in the .cpp file.

You mean...global variables that are not members of the class? *scared*

But if you make it static, it's only visible inside the compilation unit.

or use an enum:

Code:

---

class SerialCommunication
{
    private:
        enum { RX_BUFFER_SIZE = 128 };
};

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Quote:

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Originally Posted by Cruz

Kinda sucks though that you have to declare and initialize in two different places.

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For integral types you don't have to: in-class initialization of const static data members of integral types. I don't know why your first example didn't work.