# Strange Linking Error

**URL:** <https://community.particle.io/t/strange-linking-error/31339>\
**Category:** Firmware\
**Created:** [April 4, 2017, 9:49pm UTC](https://community.particle.io/t/strange-linking-error/31339 "2017-04-04T21:49:20Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![BulldogLowell](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bulldoglowell/32/4305_2.png) [@BulldogLowell](https://community.particle.io/u/BulldogLowell)\
**Post date:** [April 4, 2017, 9:49pm UTC](https://community.particle.io/t/strange-linking-error/31339/1 "2017-04-04T21:49:20Z")

</div>

Maybe I am missing something obvious, I’m struggling trying to convert a little template class from a library I built for another project/platform. If I put the implementation of the three functions into the header, it compiles. If they are in the .cpp file, they will not.

Here is a stripped down version of my class that shows the errors:

ino:

```cpp
#include "RingBuffer.h"

RingBuffer <int> ringBuffer(10);
//Timer timer(100, readSensor);

void setup(void)
{
  Serial.begin(9600);
}

void loop(void)
{
  static uint32_t lastSerialUpdateMillis = 0;
  if(millis() - lastSerialUpdateMillis > 1000)
  {
    Serial.print(ringBuffer.getAverage());
    lastSerialUpdateMillis = millis();
  }
  //timer.start();
}

void readSensor(void)
{
  //read sensor here
  ringBuffer.addValue(1.0f);
}

```

header:

```cpp
#ifndef RINGBUFFER_H
#define RINGBUFFER_H

#include "Particle.h"

template < class Any_Type > class RingBuffer{
  public:
    RingBuffer(const size_t &size);
    void addValue(const Any_Type &value);
    Any_Type getAverage(void) const;
  private:
    Any_Type* myArray;
    size_t arraySize;
    uint32_t index = 0;
};

#endif

```

implementation:

```cpp
#include "RingBuffer.h"

template < class Any_Type > RingBuffer < Any_Type >::RingBuffer(const size_t &size)
{
  arraySize = size;
  myArray = new Any_Type[size];
}

template < class Any_Type > void RingBuffer < Any_Type >::addValue(const Any_Type &value)
{
  myArray[index++] = value;
  index %= arraySize;
}

template < class Any_Type > Any_Type RingBuffer < Any_Type >::getAverage(void) const
{
  Any_Type sum = 0;
  for(int i = 0; i < arraySize; i++)
  {
    sum += myArray[i];
  }
  return (sum / arraySize);
}

```

error

```bash
Compiling code for photon

Including:
    RingBuffer.h
    CircularBuffer.ino
    RingBuffer.cpp
attempting to compile firmware 
Compile failed. Exiting.
../../../build/target/user/platform-6/libuser.a(CircularBuffer.o): In function `loop':
CircularBuffer.cpp:16: undefined reference to `RingBuffer<int>::getAverage() const'
../../../build/target/user/platform-6/libuser.a(CircularBuffer.o): In function `__static_initialization_and_destruction_0':
CircularBuffer.cpp:3: undefined reference to `RingBuffer<int>::RingBuffer(unsigned int const&)'
collect2: error: ld returned 1 exit status
make: *** [3f90c7d659bb94af3242458c08c6d4b27c51b71bfee3cf281671dbffbdb1.elf] Error 1

```

Admittedly, i was distracted by [this](https://community.particle.io/t/averaging-analog-signal/31311/10) post… and now regret spending any time on this!

---

<div class="post-metadata">

**Author:** ![woodwhcker](https://avatars.discourse-cdn.com/v4/letter/w/a698b9/32.png) [@woodwhcker](https://community.particle.io/u/woodwhcker)\
**Post date:** [April 5, 2017, 1:07am UTC](https://community.particle.io/t/strange-linking-error/31339/2 "2017-04-05T01:07:22Z")

</div>

C++ doesn’t permit splitting the implementation from the declaration for template types.

---

<div class="post-metadata">

**Author:** ![BulldogLowell](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/bulldoglowell/32/4305_2.png) [@BulldogLowell](https://community.particle.io/u/BulldogLowell)\
**Post date:** [April 5, 2017, 1:17am UTC](https://community.particle.io/t/strange-linking-error/31339/3 "2017-04-05T01:17:56Z")

</div>

makes sense, it’s really not a class, it’s a template!

Thank you.

---

<div class="post-metadata">

**Author:** ![KyleG](https://sea2.discourse-cdn.com/flex026/user_avatar/community.particle.io/kyleg/32/11077_2.png) [@KyleG](https://community.particle.io/u/KyleG)\
**Post date:** [April 5, 2017, 1:22am UTC](https://community.particle.io/t/strange-linking-error/31339/4 "2017-04-05T01:22:46Z")

</div>

Thanks for the help @woodwhcker!