Linker Error: Undefined reference to `ThreadPool::ThreadPool()' - c++

Question

I have a class threadpool. I have a header file threadpool.h and the implementation threadpool.cc. When trying to I initialize the threadpool like so: ThreadPool *tp = new ThreadPool(); I get the following linker error:
/usr/bin/ld: ../src/libmapreducelib.a(mapreduce_impl.cc.o): in function `Master::Master(MapReduceSpec const&, std::vector<FileShard, std::allocator<FileShard> > const&)':
/aos-pr4/src/master.h:43: undefined reference to `ThreadPool::ThreadPool()'
collect2: error: ld returned 1 exit status
make[2]: *** [test/CMakeFiles/mrdemo.dir/build.make:127: bin/mrdemo] Error 1
make[1]: *** [CMakeFiles/Makefile2:330: test/CMakeFiles/mrdemo.dir/all] Error 2
make: *** [Makefile:130: all] Error 2
Here is my project source hierarchy:
src/
>CMakeLists.txt
>threadpool.h
>threadpool.cc
>otherfiles
CMakeLists.txt
Below is my threadpool.h, threadpool.cc and CMakeLists.txt file.
// in theadpool.h
#ifndef THREADPOOL_H
#define THREADPOOL_H
#pragma once
class ThreadPool {
public:
using Task = std::function<void()>;
// constructor, init static number of workers to be added to queue
ThreadPool()
ThreadPool(int numOfWorkers);
virtual ~ThreadPool();
....
}
#endif
// in threadpool.cc
#include "threadpool.h"
ThreadPool::ThreadPool(){
// commented out
}
....
I even ensured my cmakelist library includes the threadpool.h.
//CMakeLists.txt
project(mapreducer)
include(GenerateProtos.cmake)
add_library(
mapreducelib #library name
mapreduce.cc mapreduce_impl.cc #sources
master.h mapreduce_spec.h file_shard.h threadpool.h) #headers
target_link_libraries(mapreducelib p4protolib)
target_include_directories(mapreducelib PUBLIC ${MAPREDUCE_INCLUDE_DIR})
add_dependencies(mapreducelib p4protolib)
I believe it should be fine and I've used my threadpool in another project. It must be the project hierarchy.