Trying to define a static constant variable in a c

问题:

I am defining a variable adc_cmd[9] as a static const unsigned char in my class ADC under private. Since it is a constant I thought I would just define it inside the class it self, but that didn't work apparently:

#pragma once

class ADC{
private:
    static const unsigned char adc_cmd[9] = { 0x87, 0xC7, 0x97, 0xD7, 0xA7, 0xE7, 0xB7, 0xF7, 0x00 };
//...
};

Error:

error: a brace-enclosed initializer is not allowed here before '{' token
error: invalid in-class initialization of static data member of non-integral type 'const unsigned char [9]'

...

So I tried bringing the line out of the class with: static const unsigned char ADC::adc_cmd[9] = { 0x87, 0xC7, 0x97, 0xD7, 0xA7, 0xE7, 0xB7, 0xF7, 0x00 };, but that yielded this error:

error: 'static' may not be used when defining (as opposed to declaring) a static data member
error: 'const unsigned char ADC::adc_cmd [9]' is not a static member of 'class ADC'

I obviously am not declaring this properly. What is the correct way to declare this?

回答1:

You declare it inside the class body:

class ADC{
private:
    static const unsigned char adc_cmd[9];
//...
};

and define (and initialize) it outside (just once, as for any external-linkage definition):

const unsigned char ADC::adc_cmd[9] = { 0x87, 0xC7, 0x97, 0xD7, 0xA7, 0xE7, 0xB7, 0xF7, 0x00 };

without writing static, as specified by the error message.

(don't ask me for an explanation of why here static is forbidden, I always found the various "repetition of qualifiers" rules completely illogic)

回答2:

In C++03, static data member definitions go outside of the class definition.

Header:

#pragma once

class ADC {
private:
    static unsigned char const adc_cmd[9];
};

In one .cpp file:

#include "headername"

unsigned char const ADC::adc_cmd[9] = { 0x87, 0xC7, 0x97, 0xD7, 0xA7, 0xE7, 0xB7, 0xF7, 0x00 };

回答3:

Combine the two:

class ADC{
private:
    static const unsigned char adc_cmd[9];
    //...
};

//.cpp
const unsigned char ADC::adc_cmd[9] = { 0x87, 0xC7, 0x97, 0xD7, 0xA7, 0xE7, 0xB7, 0xF7, 0x00 };

回答4:

Just to contribute to Matteo's answer:

We must point out that the static qualifier is indeed a bit confusing in C++. To my knowledge, it does three different things depending on where it is used:

1. In front of class member attributes: makes this attribute the same for all instances of that class (same as Java).
2. In front of a global variable: reduces the scope of the variable to the current source file only (same as C).
3. In front of a local variable within a method / function: makes this variable the same for all calls to that method / function (same as C, may be useful for the singleton design pattern).