# Undefined reference to function defined in another HPP file

**URL:** https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710
**Category:** Code
**Tags:** os:linux
**Created:** [October 23, 2022, 10:44am UTC](https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710 "2022-10-23T10:44:26Z")
**Posts on this page:** 4
**Page:** 1

<div class="post-metadata">

### Author: ![AD2605](https://discourse.cmake.org/user_avatar/discourse.cmake.org/ad2605/32/1856_2.png) [@AD2605](https://discourse.cmake.org/u/AD2605)
#### Post date: [October 23, 2022, 10:44am UTC](https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710/1 "2022-10-23T10:44:26Z")

</div>

Hello All,  
My project directory is as follows -

```auto
DIR
    * main.cpp
    * test.hpp
    * test.cpp

```

And code in the above mentioned files is as follows -  
`main.cpp` -

```auto
#include <opencv2/opencv.hpp>
#include "test.hpp"

int main()
{

   std::vector<float> A{10, 100};
   std::vector<float> B{10, 100};
   std::vector<float> C{10, 100};

   add(A, B, C);
}

```

`test.hpp` -

```auto
#ifndef TEST_HPP
#define TEST_HPP
#pragma once

#include <iostream>
#include <vector>
#include <cassert>

template<typename T>
void add(std::vector<T> A, std::vector<T> B, std::vector<T> C);

#endif

```

and `test.cpp` -

```auto
#include "test.hpp"

template<typename T>
void add(std::vector<T> A, std::vector<T> B, std::vector<T> C)
{
    assert(A.size() == B.size());
    assert(B.size() == C.size());

    for(int i=0; i < 100; i++)
        C[i] = A[i] + B[i];
}

```

`CMakeLists.txt` -

```auto
cmake_minimum_required(VERSION 3.15)

project(cudaTest CXX)

set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CUDA_STANDARD 14)

find_package(CUDAToolkit REQUIRED)
find_package(OpenCV REQUIRED)

#add_library(cudaLib STATIC kernels.cu kernels.cuh)
#set_target_properties(cudaLib PROPERTIES CUDA_ARCHITECTURES "75")
#target_link_libraries(cudaLib CUDA::cudart ${OpenCV_LIBS})

add_executable(exe main.cpp test.hpp test.cpp)
#add_dependencies(exe cudaLib)
target_link_libraries(exe ${OpenCV_LIBS})

```

However, upon compilation, I get the following linker error,

```auto
/usr/bin/ld: CMakeFiles/exe.dir/main.cpp.o: in function `main':
main.cpp:(.text+0x1c8): undefined reference to `void add<float>(std::vector<float, std::allocator<float> >, std::vector<float, std::allocator<float> >, std::vector<float, std::allocator<float> >)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/exe.dir/build.make:181: exe] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/exe.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

```

I did go through other similar posts, but was unable to figure out the issue. A few rudimentary checks -

- its a .hpp file, thus `extern C` is not required
- same function definitions and declaration
- All the required files are added in `add_executable`

What exactly am I doing wrong,

TIA

---

<div class="post-metadata">

### Author: ![marc.chevrier](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/m/ecb155/32.png) [@marc.chevrier](https://discourse.cmake.org/u/marc.chevrier)
#### Post date: [October 23, 2022, 11:17am UTC](https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710/2 "2022-10-23T11:17:13Z")

</div>

This is not at all a `CMake` problem, but a wrong usage of `C++` templates.

To enable the compiler to instantiate the templates when needed (i.e. in your case during compilation of `main.cpp`), the definition of templates must be available. So, the definition of `add` must be in `test.hpp` file rather than in the `test.cpp` file.

---

<div class="post-metadata">

### Author: ![hsattler](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/h/59ef9b/32.png) [@hsattler](https://discourse.cmake.org/u/hsattler)
#### Post date: [October 23, 2022, 7:20pm UTC](https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710/3 "2022-10-23T19:20:36Z")

</div>

The other option is an explicit template instantiation for float in test.cpp with an extern declaration in test.hpp. Not the usual template usage, however.

---

<div class="post-metadata">

### Author: ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)
#### Post date: [October 24, 2022, 10:00am UTC](https://discourse.cmake.org/t/undefined-reference-to-function-defined-in-another-hpp-file/6710/4 "2022-10-24T10:00:00Z")

</div>

There’s an in-depth explanation of this in a [canonical question on StackOverflow](https://stackoverflow.com/q/495021/1782465)