How do you declare an extern global in a namespace and then define it?

Question

It seems that every Arduino library uses the Arduino Serial library for printing debug messages. I'm currently trying to integrate this library into my project https://github.com/jrowberg/i2cdevlib/tree/master/Arduino/I2Cdev. I am using my own python program to receive messages from my atmega 2560. I want to try out some libraries without changing their source code but because of the Serial library I would have to convert their Serial.print() calls to my own API calls. Since I would have to do that for every library I mess with I decided to create a wrapper for the Serial library in which I have a global variable named Serial. That Serial variable would then be my Serial wrapper defined as an extern.

I have the SerialWrapper in the serial namespace. I want a global variable Serial to be defined as extern, also in the serial namespace.

// SerialWrapper.h
namespace serial {

    enum SerialType {
        HEX,
        DEC
    };

    class SerialWrapper {
    public:
        SerialWrapper(ground::Ground& ground);
        void print(const char *string);
        void print(uint8_t data, SerialType type);
    private:
        ground::Ground& ground_;
    };

    extern SerialWrapper Serial;
}

Then in the source file I define the methods and global variable like so

// SerialWrapper.cpp
#include "SerialWrapper.h"

serial::SerialWrapper Serial = serial::SerialWrapper(ground::Ground::reference());

serial::SerialWrapper::SerialWrapper(ground::Ground& ground) : ground_( ground )
{

}

void serial::SerialWrapper::print(const char *string)
{
    ground_.sendString(string);
}

void serial::SerialWrapper::print(uint8_t data, serial::SerialType type)
{

}

Trying to test out the library, I call this in my main method

// main.cpp
using namespace serial;

int main( void )
{
    Serial.print("Hello World!");
}

To make the libraries compatible with my SerialWrapper, I need it to work this way. However, when I compile I get the error from main.cpp that there is an undefined reference to 'serial::Serial'

You can see all my source code under the mpu branch (soon to be merged to master) here https://github.com/jkleve/quaddrone

Possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix) — user7860670, Jul 15 '17 at 05:55
@rangeme _'Every AVR library uses the Arduino Serial library for printing debug messages.'_ Where have you taken it from? It is **NOT THE TRUTH** — 0___________, Jul 15 '17 at 15:33

minnymauer · Answer 1 · 2017-07-15T15:47:29.890

To get the undefined reference to go away I had to move the declaration outside of the namespace like so

// SerialWrapper.h
namespace serial {

    enum SerialType {
        HEX,
        DEC
    };

    class SerialWrapper {
    public:
        SerialWrapper(ground::Ground& ground);
        void print(const char *string);
        void print(uint8_t data, SerialType type);
    private:
        ground::Ground& ground_;
    };
}

extern serial::SerialWrapper Serial;

I do not know why this has to be done. Maybe someone else can comment on the issue with having the extern declared in the namespace.

I also found this reference https://www.ics.com/designpatterns/book/namespace-example.html

Following the way found in the reference you can also setup the declaration and definition as follows

// SerialWrapper.h
namespace serial {

    enum SerialType {
        HEX,
        DEC
    };

    class SerialWrapper {
    public:
        SerialWrapper(ground::Ground& ground);
        void print(const char *string);
        void print(uint8_t data, SerialType type);
    private:
        ground::Ground& ground_;
    };
    extern SerialWrapper Serial;
}

The definition has to be changed to this.

// SerialWrapper.cpp
#include "SerialWrapper.h"

serial::SerialWrapper serial::Serial = serial::SerialWrapper(ground::Ground::reference());

How do you declare an extern global in a namespace and then define it?

1 Answers1