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?

[jpn]

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));
}

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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[Cruz]

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

[jpn]

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

[Cruz]

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

[jpn]

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

[caduel]

or use an enum:

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

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

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[danadam]

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

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.