unit testing an object that takes a “traits” template parameter
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11-12-2019 - |
Question
I have a Visual Studio 2008 C++03 project where I would like to unit test a class that uses a traits template parameter that exposes static methods (Policy-based design, strategy pattern). I am using Google Test and Google Mock frameworks.
For example:
/// the class under test
template< typename FooTraits >
class Foo
{
public:
void DoSomething()
{
FooTraits::handle_type h = FooTraits::Open( "Foo" );
/* ... */
FooTraits::Close( h );
};
};
/// a typical traits structure
struct SomeTraits
{
typedef HANDLE handle_type;
static handle_type Open( const char* name ) { /* ... */ };
static void Close( handle_type h ) { /* ... */ };
};
/// mocked traits that I would like to use for testing
struct MockTraits
{
typedef int handle_type;
static MOCK_METHOD1( Open, handle_type( const char* ) );
static MOCK_METHOD1( Close, void( handle_type ) );
};
/// the test function
TEST( FooTest, VerifyDoSomethingWorks )
{
Foo< MockTraits > foo_under_test;
// expect MockTraits::Open is called once
// expect MockTraits::Close is called once with the parameter returned from Open
foo_under_test.DoSomething();
};
Obviously this won't work as-is. Google Mock can't mock static methods and I would need to create an instance of the Mocked class in the test to set its behavior and expectations.
So what is the correct way to unit test a class that accepts a template policy using Google Test / Google Mock?
Solution
Can you make a class with non-static methods, create a global instance of it (or static in your traits), and have your traits class defer to it?
So, to clarify the idea that was inspired by Rob's comment:
struct FunnyDoodad
{
FunnyDoodad();
~FunnyDoodad();
MOCK_METHOD1( Open, HANDLE( const char* ) );
MOCK_METHOD1( Close, void( handle_type ) );
};
struct FunnyGlobal {
FunnyGlobal() : pimpl() {}
~FunnyGlobal() { delete pimpl; }
// You'd want some protection here rather than just dereferencing.
// it's the whole point. I'd suggest using whatever unit test assertion
// mechanism your framework uses and make it a fatal check.
handle_type Open(char const* name) { return pimpl->Open(name); }
void Close(handle_type h) { pimpl->Close(h); }
private:
FunnyDoodad * pimpl;
friend struct FunnyDoodad;
void register(FunnyDoodad* p) { pimpl = p; }
void deregister() { pimpl = 0; }
};
FunnyGlobal funnyGlobal;
FunnyDoodad::FunnyDoodad() { funnyGlobal.register(this); }
FunnyDoodad::~FunnyDoodad() { funnyGlobal.deregister(); }
struct TestTraits
{
typedef HANDLE handle_type;
static handle_type Open( const char* name ) { return funnyGlobal.Open(name); };
static void Close( handle_type h ) { funnyGlobal.Close(h); };
};
TEST_CASE(blah)
{
FunnyDoodad testDoodad;
...
}
I imagine the above could be templated and almost turned into a pattern...maybe.