How to mock an external function in GTest - c++

Question

I have a function called tt_init_device(). I have written a TSET_F() test case to verify it.
But inside tt_init_device(), I have an external function wfdEnumerateDevices(NULL, 0, NULL);...I need to mock this function to get return value as true or false...Need help in to do the same.
TEST_F(FusaAppTh,Sample)
{
FusaTelltaleClientAppTh AppThObj(1,"abc");
EXPECT_EQ( WFD_ERROR_NONE,AppThObj.tt_init_device());
}
WFDErrorCode FusaTelltaleClientAppTh::tt_init_device(void)
{
.
.
value = wfdEnumerateDevices(NULL, 0, NULL);
.
.
}
Thanks in advance!

Answer 1

You should create a wrapper for the function that you want to mock. Here is an example.
First you create an interface for the wrapper, then you create two inherited versions. One for production code, which calls the original wfdEnumerateDevices function, and one for testing, which is used for mocking that function.
Here is a sample implementation:
// This is your original function.
int wfdEnumerateDevices(int* p1, int v1, int* p2) { return 0; }
// Create a wrapper interface for it.
class WrapperInterface {
public:
virtual int wfdEnumerateDevices(int* p1, int v1, int* p2) = 0;
};
// An inherited wrapper that calls your original function. Use this for
// your production code.
class ProductionWrapper : public WrapperInterface {
public:
int wfdEnumerateDevices(int* p1, int v1, int* p2) {
return ::wfdEnumerateDevices(p1, v1, p2);
}
};
// An inherited wrapper used only for testing.
class MockWrapper : public WrapperInterface {
public:
MOCK_METHOD(int, wfdEnumerateDevices, (int*, int, int*), (override));
};
class FusaTelltaleClientAppTh {
public:
// Your class should now have a wrapper member which can either be the
// production version for production or the test version for testing.
WrapperInterface* wrapper_;
// Inject the wrapper in constructor.
FusaTelltaleClientAppTh(int v1, std::string s1, WrapperInterface* wrapper)
: wrapper_(wrapper) {}
int tt_init_device(void) {
// Call the wrapper function instead of the original one. Depending on the
// wrapper_ type, either the production or the test version will be
// called.
return wrapper_->wfdEnumerateDevices(nullptr, 0, nullptr);
}
};
// Tests mocking the function to true.
TEST(FusaAppTh, Sample1) {
MockWrapper mockWrapper;
FusaTelltaleClientAppTh AppThObj(1, "abc", &mockWrapper);
ON_CALL(mockWrapper, wfdEnumerateDevices)
.WillByDefault(testing::Return(true));
EXPECT_EQ(1, AppThObj.tt_init_device());
}
// Tests mocking the function to false.
TEST(FusaAppTh, Sample2) {
MockWrapper mockWrapper;
FusaTelltaleClientAppTh AppThObj(1, "abc", &mockWrapper);
ON_CALL(mockWrapper, wfdEnumerateDevices)
.WillByDefault(testing::Return(false));
EXPECT_EQ(0, AppThObj.tt_init_device());
}
Here is a live example: https://godbolt.org/z/vfbKf6oT8