Undefined reference after using CMAKE to generate a makefile - c++

Question

I can generate my CMake project, but I can't build it.
I have a fairly straightforward project structure:
├── bin
├── build
├── CMakeLists.txt
├── include
│   └── project
│   ├── base_socket.h
│   ├── tcp_client.h
│   ├── tcp_server.h
│   ├── udp_client.h
│   └── udp_server.h
├── LICENSE.txt
├── README.md
└── src
└── project
├── base_socket.cpp
├── CMakeLists.txt
├── main.cpp
├── tcp_client.cpp
├── tcp_server.cpp
├── udp_client.cpp
└── udp_server.cpp
And a fairly straight forward CMakeLists.txt file in the top directory:
# Project initialization
cmake_minimum_required(VERSION 2.7.2 FATAL_ERROR)
project("cpp_sockets")
# Add include directory
include_directories(include)
# Add subdirectories
add_subdirectory(src/project)
And also a fairly simple one in src/project/CmakeLists.txt:
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR}/bin)
set(HEADER_DIR ${CMAKE_SOURCE_DIR}/include/project)
set(HEADER_FILES
${HEADER_DIR}/base_socket.h
${HEADER_DIR}/tcp_client.h
${HEADER_DIR}/tcp_server.h
${HEADER_DIR}/udp_client.h
${HEADER_DIR}/udp_server.h
)
add_executable(cpp_sockets main.cpp ${HEADER_FILES})
When I cd into /build and type cmake .. it works fine.
However, when I follow that with make, I get many terrible errors about undefined reference.
main.cpp:(.text+0x2c6): undefined reference to `UDPClient::UDPClient(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x2eb): undefined reference to `UDPClient::send_message(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x359): undefined reference to `UDPServer::UDPServer(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x380): undefined reference to `UDPServer::socket_bind()'
main.cpp:(.text+0x38c): undefined reference to `UDPServer::listen()'
main.cpp:(.text+0x400): undefined reference to `TCPClient::TCPClient(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x40c): undefined reference to `TCPClient::make_connection()'
main.cpp:(.text+0x42b): undefined reference to `TCPClient::send_message(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x492): undefined reference to `TCPServer::TCPServer(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
main.cpp:(.text+0x4b9): undefined reference to `TCPServer::socket_bind()'
CMakeFiles/cpp_sockets.dir/main.cpp.o: In function `UDPClient::~UDPClient()':
main.cpp:(.text._ZN9UDPClientD2Ev[_ZN9UDPClientD5Ev]+0x14): undefined reference to `Socket::~Socket()'
CMakeFiles/cpp_sockets.dir/main.cpp.o: In function `UDPServer::~UDPServer()':
main.cpp:(.text._ZN9UDPServerD2Ev[_ZN9UDPServerD5Ev]+0x14): undefined reference to `Socket::~Socket()'
CMakeFiles/cpp_sockets.dir/main.cpp.o: In function `TCPClient::~TCPClient()':
main.cpp:(.text._ZN9TCPClientD2Ev[_ZN9TCPClientD5Ev]+0x14): undefined reference to `Socket::~Socket()'
CMakeFiles/cpp_sockets.dir/main.cpp.o: In function `TCPServer::~TCPServer()':
main.cpp:(.text._ZN9TCPServerD2Ev[_ZN9TCPServerD5Ev]+0x14): undefined reference to `Socket::~Socket()'
collect2: error: ld returned 1 exit status
src/project/CMakeFiles/cpp_sockets.dir/build.make:94: recipe for target '../bin/cpp_sockets' failed
make[2]: *** [../bin/cpp_sockets] Error 1
CMakeFiles/Makefile2:85: recipe for target 'src/project/CMakeFiles/cpp_sockets.dir/all' failed
make[1]: *** [src/project/CMakeFiles/cpp_sockets.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
main.cpp file looks like:
#include "project/udp_server.h"
#include "project/udp_client.h"
#include "project/tcp_client.h"
Why would the linker not be able to find the references like this?

Answer 1

It looks like your src/project/CMakeLists.txt neglects to specify the other cpp files, like udp_server.cpp and udp_client.cpp, as dependencies of the executable cpp_sockets. You need to do this in order for CMake to compile those source files; it won't automatically figure out to compile them just because you have included their corresponding headers. In fact, the add_executable declaration doesn't need to say anything about the headers, because the include_directories command takes care of that.
Given the header files that your main.cpp depends on, its add_executable line should look like this:
add_executable(cpp_sockets main.cpp udp_server.cpp udp_client.cpp tcp_client.cpp)