gtest: undefined reference to `operator+' - googletest

Question

This is just an observation, but I don't understand where is the problem. I implement vector addition operator and everything is work pretty well in the main function. But when I tried to test it by Google Test I got the compilation error
In function `ZN15Basic_Dirs_Test8TestBodyEv':
C:/Users/home/Documents/clion/example/test/main.cpp:10: undefined reference
to `operator+(std::vector<int, std::allocator<int> > const&,
std::vector<int, std::allocator<int> > const&)'
It compiles, however, when I remove src/class.cpp file. Could you tell me how it works? :)
The code is below.
src/class.h
#ifndef EXAMPLE_CLASS_H
#define EXAMPLE_CLASS_H
#include <vector>
std::vector<int> operator+(const std::vector<int>& a, const std::vector<int>& b);
#endif //EXAMPLE_CLASS_H
src/class.cpp
#include "class.h"
std::vector<int> operator+(const std::vector<int>& a, const std::vector<int>& b) {
if (a.size() != b.size()) {
std::cerr << "vectors are not of the same size!\n";
exit(1);
}
std::vector<int> res(a.size());
for (unsigned i = 0; i < a.size(); ++i) {
res[i] = a[i] + b[i];
}
return res;
}
test/main.cpp
#include "class.h"
#include <gtest/gtest.h>
using namespace std;
TEST(Basic, Dirs) {
std::vector<int> a = {1, 1, 0};
std::vector<int> b = {2, 1, 0};
std::vector<int> c = a+b;
EXPECT_EQ(c[0], 3);
EXPECT_EQ(c[1], 2);
EXPECT_EQ(c[2], 0);
}
./CMakeLists
cmake_minimum_required(VERSION 3.7)
project(example)
set(CMAKE_CXX_STANDARD 11)
set(SOURCE_FILES src/main.cpp src/class.cpp)
add_subdirectory(test)
add_executable(example ${SOURCE_FILES})
test/CMakeLists
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")
add_subdirectory("../lib/googletest" "${CMAKE_CURRENT_BINARY_DIR}/gtest_build")
include_directories(../lib/googletest/googletest/include)
include_directories(../src/)
add_executable(runtest main.cpp)
target_link_libraries(runtest gtest gtest_main)

Answer 1

You forgot to link your runtest gtest executable against your example:
./CMakeLists
cmake_minimum_required(VERSION 3.7)
project(example)
set(CMAKE_CXX_STANDARD 11)
set(SOURCE_FILES src/main.cpp src/class.cpp)
add_executable(example ${SOURCE_FILES})
add_subdirectory(test)
test/CMakeLists
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")
add_subdirectory("../lib/googletest" "${CMAKE_CURRENT_BINARY_DIR}/gtest_build")
include_directories(../lib/googletest/googletest/include)
include_directories(../src/)
add_executable(runtest main.cpp)
target_link_libraries(runtest gtest gtest_main example)
Also not that it might be better to actually define your example as a shared library instead of executable (just remove the main function - you probably don't need it anyway):
add_library(example SHARED ${SOURCE_FILES})