[Test] Add gtest to project (#547)

* add gtest module

* add gtest

* fix

* Update CMakeLists.txt

* Update README.md

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "third_party/dlpack"]
 	path = third_party/dlpack
 	url = https://github.com/dmlc/dlpack.git
+[submodule "third_party/googletest"]
+	path = third_party/googletest
+	url = https://github.com/google/googletest.git

CMakeLists.txt

@@ -68,3 +68,16 @@ target_link_libraries(dgl ${DGL_LINKER_LIBS} ${DGL_RUNTIME_LINKER_LIBS})
 
 # Installation rules
 install(TARGETS dgl DESTINATION lib${LIB_SUFFIX})
+
+# Testing
+if(BUILD_CPP_TEST)
+  message("Build with unittest")
+  add_subdirectory(./third_party/googletest)
+  enable_testing()
+  include_directories(${gtest_SOURCE_DIR}/include ${gtest_SOURCE_DIR})
+  file(GLOB_RECURSE TEST_SRC_FILES ${PROJECT_SOURCE_DIR}/tests/cpp/*.cc)
+  add_executable(runUnitTests ${TEST_SRC_FILES})
+  target_link_libraries(runUnitTests gtest gtest_main)
+  target_link_libraries(runUnitTests dgl)
+  add_test(UnitTests runUnitTests)
+endif(BUILD_CPP_TEST)

tests/README.md

 Unit test
 ===
 
+## Python Unittest
 The code organization goes as follows:
 
 * `backend`: Additional unified tensor interface for supported frameworks.
@@ -15,3 +16,14 @@ The code organization goes as follows:
   functions).
 * `lint`: Pylint-related files.
 * `scripts`: Automated test scripts for CI.
+
+## C++ Unittest
+Compile with unittest by executing the command below
+```
+# Assume current directory is the root directory of dgl, and googletest submodule is initialized
+mkdir build
+cd build
+cmake .. -DBUILD_CPP_TEST=1
+make -j${nproc}
+./runUnitTests
+```

tests/cpp/graph_index_test.cc

+#include <gtest/gtest.h>
+#include <dgl/graph.h>
+
+TEST(GraphTest, TestNumVertices){
+  dgl::Graph g(false);
+  g.AddVertices(10);
+  ASSERT_EQ(g.NumVertices(), 10);
+};

googletest @ f71fb4f9

+Subproject commit f71fb4f9a912ec945401cc49a287a759b6131026