user52880: I'm trying to take a sketch and turn it into a class for a library I'm making, but I'm getting compile issues.
This is not a duplicate as suggested. as i explain in the comment and my answer. Thank you all for your help - as I am new to creating libraries - figured I would attempt something correctly for a change :)
Here is the original sketch: Gotten here
#include <ros.h>
#include <sensor_msgs/BatteryState.h>

#define K           0.00472199
#define CELLS       6
#define MAX_CELLS   12

double cell_const[MAX_CELLS] = 
{
  1.0000,
  2.1915,
  2.6970,
  4.1111,
  4.7333,
  6.6000,
  6.6000,
  7.8293,
  8.4667,
  9.2353,
  11.0000,
  11.0000
};

ros::NodeHandle nh;
sensor_msgs::BatteryState batt_state;
ros::Publisher batteryState("battery_state", &batt_state);

void setup() 
{
  // Initialize the ROS node.
  nh.initNode();
  nh.advertise(batteryState);

  // Populate battery parameters.
  batt_state.design_capacity = 2200;      // mAh
  batt_state.power_supply_status = 2;     // discharging
  batt_state.power_supply_health = 0;     // unknown
  batt_state.power_supply_technology = 3; // LiPo
  batt_state.present = 1;                 // battery present

  batt_state.location = "Crawler";        // unit location
  batt_state.serial_number = "ABC_0001";  // unit serial number
  batt_state.cell_voltage = new float[CELLS];

}

void loop() 
{
  // Battery status.
  double battVoltage = 0.0;
  double prevVoltage = 0.0;

  // Populate battery state message.
  for (int i = 0; i < CELLS; i++)
  {
     // Read raw voltage from analog pin.
    double cellVoltage = analogRead(i) * K;

    // Scale reading to full voltage.
    cellVoltage *= cell_const[i];
    double tmp = cellVoltage;

    // Isolate current cell voltage.
    cellVoltage -= prevVoltage;
    battVoltage += cellVoltage;
    prevVoltage = tmp;

    // Set current cell voltage to message.
    batt_state.cell_voltage[i] = (float)cellVoltage;

    // Check if battery is attached.
    if (batt_state.cell_voltage[i] >= 2.0)
      batt_state.present = 1;
    else
      batt_state.present = 0;
  }

  // Update battery health.
  batt_state.voltage = (float)battVoltage;

  if (batt_state.voltage > CELLS * 4.2)
    batt_state.power_supply_health = 4; // overvoltage
  else if (batt_state.voltage < CELLS * 3.0)
    batt_state.power_supply_health = 3; // dead
  else
    batt_state.power_supply_health = 1; // good

  // Publish data to ROSSERIAL.
  batteryState.publish( &batt_state );
  nh.spinOnce();
  delay(1000);

}

Here is my header file:
#ifndef battery_h
#define battery_h


#if ARDUINO >= 100
#include "Arduino.h"
#else
#include "WProgram.h"
#endif

#include <ros.h>
#include <sensor_msgs/BatteryState.h>

#define K           0.00472199
#define CELLS       6
#define MAX_CELLS   12
#define CRITICAL    0.30

double cell_const[MAX_CELLS] = 
{
  1.0000, 2.1915, 2.6970, 4.1111,
  4.7333, 6.6000, 6.6000, 7.8293,
  8.4667, 9.2353, 11.0000, 11.0000
};

ros::NodeHandle nh;
sensor_msgs::BatteryState batt_state;

ros::Publisher batteryState("/battery/info", &batt_state);

class Battery
{
  private:
   void updateBatteryHealth();
   double battVoltage;
   double prevVoltage; 
  public:
    void begin();
    void updateBatteryState();
};

#endif

This is my cpp file:
#include "battery.h"


void Battery::begin(){

  // Launch ROS node and set parameters.
  nh.initNode();

  // Setup publishers/subscribers.
  nh.advertise(batteryState);

  // Populate battery parameters.
  batt_state.design_capacity          = 2200;  // mAh
  batt_state.power_supply_status      = 2;     // discharging
  batt_state.power_supply_health      = 0;     // unknown
  batt_state.power_supply_technology  = 3;     // LiPo
  batt_state.present                  = 1;     // battery present

  batt_state.location      = "Crawler";        // unit location
  batt_state.serial_number = "ABC_0001";       // unit serial number

  batt_state.cell_voltage = new float[CELLS];  // individual cell health


}  

void Battery::updateBatteryState() {
  battVoltage = 0.0;
  prevVoltage = 0.0;
  // Reset Power Supply Health.
  batt_state.power_supply_health = 0;
    // Populate battery state message.
  for (int i = 0; i < CELLS; i++)
  {
    // Read raw voltage from analog pin.
    double cellVoltage = analogRead(i) * K;

    // Scale reading to full voltage.
    cellVoltage *= cell_const[i];
    double tmp = cellVoltage;

    // Isolate current cell voltage.
    cellVoltage -= prevVoltage;
    battVoltage += cellVoltage;
    prevVoltage = tmp;

    // Set current cell voltage to message.
    batt_state.cell_voltage[i] = (float)cellVoltage;

    // Check if battery is attached.
    if (batt_state.cell_voltage[i] >= 2.0)
    {
      if (batt_state.cell_voltage[i] <= 3.2)
        batt_state.power_supply_health = 5; // Unspecified failure.
      batt_state.present = 1;
    }
    else{
      batt_state.present = 0;
    }
  }
};

void Battery::updateBatteryHealth() {
    // Update battery health.
  if (batt_state.present)
  {
    batt_state.voltage = (float)battVoltage;
    float volt = batt_state.voltage;
    float low  = 3.0 * CELLS;
    float high = 4.2 * CELLS;
    batt_state.percentage = constrain((volt - low) / (high - low), 0.0, 1.0);    
  }
  else 
  {
    batt_state.voltage = 0.0;
    batt_state.percentage = 0.0;
  }

  // Update power supply health if not failed.
  if (batt_state.power_supply_health == 0 && batt_state.present)
  {
    if (batt_state.voltage > CELLS * 4.2)
      batt_state.power_supply_health = 4; // overvoltage
    else if (batt_state.voltage < CELLS * 3.0)
      batt_state.power_supply_health = 3; // dead
    else
      batt_state.power_supply_health = 1; // good 
  }
   // Publish data to ROSSERIAL.
  batteryState.publish( &batt_state );
  nh.spinOnce();
};

this is the error im getting simply by calling the Battery::begin function in my main class:
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `nh'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `batt_state'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `batteryState'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `cell_const'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
/var/folders/m4/lq13_j5j23qc0l2wp8dqmt4h0000gp/T//cczNyzqC.ltrans1.ltrans.o: In function `begin':
/Users/keith.brown/Documents/Arduino/libraries/KBROSRobot/KBROSRobot.cpp:9: undefined reference to `Battery::begin()'
collect2: error: ld returned 1 exit status
exit status 1


user37857: Don't define your global variables in the header file. Only declare them, using external linkage. Then define them in the implementation file
Header (.h):
extern ros::NodeHandle nh;

Implementation (.cpp):
ros::NodeHandle nh;

Try to keep usage of global variables to a minimum. If you don't need them outside of the library, declare them in the implementation file and mark them static.
Or even better: encapsulate them inside of the class.

The reason that you are getting these errors is because each implementation file is compiled separately. You have two implementation files that include your header file, so the header file is compiled twice. This results in two global variables with the name nh. During the linking stage, the linker tries to combine the compiled version of both KBROSRobot.cpp and rob_test.ino.cpp (these are called object files, .o). The linker has no way of knowing which variable is which, because they are both defined twice, so it throws an error.
Multiple declarations are fine, because it just tells the compiler "there is a definition somewhere", and the linker will find that definition eventually. That's why the declaration with external linkage solves your problem.
Multiple definitions are not allowed (in most cases).

52880: <p>I'm trying to take a sketch and turn it into a class for a library I'm making, but I'm getting compile issues.</p>

<p>This is not a duplicate as suggested. as i explain in the comment and my answer. Thank you all for your help - as I am new to creating libraries - figured I would attempt something correctly for a change :)</p>

<p>Here is the original sketch: <a href="https://www.instructables.com/id/1S-6S-Battery-Voltage-Monitor-ROS/" rel="nofollow noreferrer">Gotten here</a></p>

<pre><code>#include &lt;ros.h&gt;
#include &lt;sensor_msgs/BatteryState.h&gt;

#define K           0.00472199
#define CELLS       6
#define MAX_CELLS   12

double cell_const[MAX_CELLS] = 
{
  1.0000,
  2.1915,
  2.6970,
  4.1111,
  4.7333,
  6.6000,
  6.6000,
  7.8293,
  8.4667,
  9.2353,
  11.0000,
  11.0000
};

ros::NodeHandle nh;
sensor_msgs::BatteryState batt_state;
ros::Publisher batteryState("battery_state", &amp;batt_state);

void setup() 
{
  // Initialize the ROS node.
  nh.initNode();
  nh.advertise(batteryState);

  // Populate battery parameters.
  batt_state.design_capacity = 2200;      // mAh
  batt_state.power_supply_status = 2;     // discharging
  batt_state.power_supply_health = 0;     // unknown
  batt_state.power_supply_technology = 3; // LiPo
  batt_state.present = 1;                 // battery present

  batt_state.location = "Crawler";        // unit location
  batt_state.serial_number = "ABC_0001";  // unit serial number
  batt_state.cell_voltage = new float[CELLS];

}

void loop() 
{
  // Battery status.
  double battVoltage = 0.0;
  double prevVoltage = 0.0;

  // Populate battery state message.
  for (int i = 0; i &lt; CELLS; i++)
  {
     // Read raw voltage from analog pin.
    double cellVoltage = analogRead(i) * K;

    // Scale reading to full voltage.
    cellVoltage *= cell_const[i];
    double tmp = cellVoltage;

    // Isolate current cell voltage.
    cellVoltage -= prevVoltage;
    battVoltage += cellVoltage;
    prevVoltage = tmp;

    // Set current cell voltage to message.
    batt_state.cell_voltage[i] = (float)cellVoltage;

    // Check if battery is attached.
    if (batt_state.cell_voltage[i] &gt;= 2.0)
      batt_state.present = 1;
    else
      batt_state.present = 0;
  }

  // Update battery health.
  batt_state.voltage = (float)battVoltage;

  if (batt_state.voltage &gt; CELLS * 4.2)
    batt_state.power_supply_health = 4; // overvoltage
  else if (batt_state.voltage &lt; CELLS * 3.0)
    batt_state.power_supply_health = 3; // dead
  else
    batt_state.power_supply_health = 1; // good

  // Publish data to ROSSERIAL.
  batteryState.publish( &amp;batt_state );
  nh.spinOnce();
  delay(1000);

}
</code></pre>

<p>Here is my header file:</p>

<pre><code>#ifndef battery_h
#define battery_h


#if ARDUINO &gt;= 100
#include "Arduino.h"
#else
#include "WProgram.h"
#endif

#include &lt;ros.h&gt;
#include &lt;sensor_msgs/BatteryState.h&gt;

#define K           0.00472199
#define CELLS       6
#define MAX_CELLS   12
#define CRITICAL    0.30

double cell_const[MAX_CELLS] = 
{
  1.0000, 2.1915, 2.6970, 4.1111,
  4.7333, 6.6000, 6.6000, 7.8293,
  8.4667, 9.2353, 11.0000, 11.0000
};

ros::NodeHandle nh;
sensor_msgs::BatteryState batt_state;

ros::Publisher batteryState("/battery/info", &amp;batt_state);

class Battery
{
  private:
   void updateBatteryHealth();
   double battVoltage;
   double prevVoltage; 
  public:
    void begin();
    void updateBatteryState();
};

#endif
</code></pre>

<p>This is my cpp file:</p>

<pre><code>#include "battery.h"


void Battery::begin(){

  // Launch ROS node and set parameters.
  nh.initNode();

  // Setup publishers/subscribers.
  nh.advertise(batteryState);

  // Populate battery parameters.
  batt_state.design_capacity          = 2200;  // mAh
  batt_state.power_supply_status      = 2;     // discharging
  batt_state.power_supply_health      = 0;     // unknown
  batt_state.power_supply_technology  = 3;     // LiPo
  batt_state.present                  = 1;     // battery present

  batt_state.location      = "Crawler";        // unit location
  batt_state.serial_number = "ABC_0001";       // unit serial number

  batt_state.cell_voltage = new float[CELLS];  // individual cell health


}  

void Battery::updateBatteryState() {
  battVoltage = 0.0;
  prevVoltage = 0.0;
  // Reset Power Supply Health.
  batt_state.power_supply_health = 0;
    // Populate battery state message.
  for (int i = 0; i &lt; CELLS; i++)
  {
    // Read raw voltage from analog pin.
    double cellVoltage = analogRead(i) * K;

    // Scale reading to full voltage.
    cellVoltage *= cell_const[i];
    double tmp = cellVoltage;

    // Isolate current cell voltage.
    cellVoltage -= prevVoltage;
    battVoltage += cellVoltage;
    prevVoltage = tmp;

    // Set current cell voltage to message.
    batt_state.cell_voltage[i] = (float)cellVoltage;

    // Check if battery is attached.
    if (batt_state.cell_voltage[i] &gt;= 2.0)
    {
      if (batt_state.cell_voltage[i] &lt;= 3.2)
        batt_state.power_supply_health = 5; // Unspecified failure.
      batt_state.present = 1;
    }
    else{
      batt_state.present = 0;
    }
  }
};

void Battery::updateBatteryHealth() {
    // Update battery health.
  if (batt_state.present)
  {
    batt_state.voltage = (float)battVoltage;
    float volt = batt_state.voltage;
    float low  = 3.0 * CELLS;
    float high = 4.2 * CELLS;
    batt_state.percentage = constrain((volt - low) / (high - low), 0.0, 1.0);    
  }
  else 
  {
    batt_state.voltage = 0.0;
    batt_state.percentage = 0.0;
  }

  // Update power supply health if not failed.
  if (batt_state.power_supply_health == 0 &amp;&amp; batt_state.present)
  {
    if (batt_state.voltage &gt; CELLS * 4.2)
      batt_state.power_supply_health = 4; // overvoltage
    else if (batt_state.voltage &lt; CELLS * 3.0)
      batt_state.power_supply_health = 3; // dead
    else
      batt_state.power_supply_health = 1; // good 
  }
   // Publish data to ROSSERIAL.
  batteryState.publish( &amp;batt_state );
  nh.spinOnce();
};
</code></pre>

<p>this is the error im getting simply by calling the Battery::begin function in my main class:</p>

<pre><code>libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `nh'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `batt_state'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `batteryState'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
libraries/KBROSRobot/KBROSRobot.cpp.o (symbol from plugin): In function `KBROSRobot::KBROSRobot()':
(.text+0x0): multiple definition of `cell_const'
sketch/rob_test.ino.cpp.o (symbol from plugin):(.text+0x0): first defined here
/var/folders/m4/lq13_j5j23qc0l2wp8dqmt4h0000gp/T//cczNyzqC.ltrans1.ltrans.o: In function `begin':
/Users/keith.brown/Documents/Arduino/libraries/KBROSRobot/KBROSRobot.cpp:9: undefined reference to `Battery::begin()'
collect2: error: ld returned 1 exit status
exit status 1
</code></pre>
 37857: <p>Don't define your global variables in the header file. Only declare them, using external linkage. Then define them in the implementation file</p>

<p>Header (.h):</p>

<pre><code>extern ros::NodeHandle nh;
</code></pre>

<p>Implementation (.cpp):</p>

<pre><code>ros::NodeHandle nh;
</code></pre>

<p>Try to keep usage of global variables to a minimum. If you don't need them outside of the library, declare them in the implementation file and mark them static.<br>
Or even better: encapsulate them inside of the class.</p>

<hr>

<p>The reason that you are getting these errors is because each implementation file is compiled separately. You have two implementation files that include your header file, so the header file is compiled twice. This results in two global variables with the name <code>nh</code>. During the linking stage, the linker tries to combine the compiled version of both <code>KBROSRobot.cpp</code> and <code>rob_test.ino.cpp</code> (these are called object files, <code>.o</code>). The linker has no way of knowing which variable is which, because they are both defined twice, so it throws an error.</p>

<p>Multiple <em>declarations</em> are fine, because it just tells the compiler "there is a definition somewhere", and the linker will find that definition eventually. That's why the declaration with external linkage solves your problem.<br>
Multiple <em>definitions</em> are not allowed (in most cases).</p>
