Question

I am trying to mock a templated method.

Here is the class containing the method to mock :

class myClass
{
public:
    virtual ~myClass() {}

    template<typename T>
    void myMethod(T param);
}

How can I mock the method myMethod using Google Mock?

Answer 1

In previous version of Google Mock you can only mock virtual functions, see the documentation in the project s page.

More recent versions allowed to mock non-virtual methods, using what they call hi-perf dependency injection.

As user @congusbongus states in the comment below this answer:

Google Mock relies on adding member variables to support method mocking, and since you can t create template member variables, it s impossible to mock template functions

A workaround, by Michael Harrington in the googlegroups link from the comments, is to make specialized the template methods that will call a normal function that can be mocked. It doesn t solve the general case but it will work for testing.

struct Foo
{
    MOCK_METHOD1(GetValueString, void(std::string& value));

    template <typename ValueType>
    void GetValue(ValueType& value); 

    template <>
    void GetValue(std::string& value) {
        GetValueString(value);
    } 
};

Answer 2

Here is the original post again with comments to aid in understanding:

    struct Foo 
    { 
        // Our own mocked method that the templated call will end up calling.
        MOCK_METHOD3(GetNextValueStdString, void(const std::string& name, std::string& value, const unsigned int streamIndex)); 

        // If we see any calls with these two parameter list types throw and error as its unexpected in the unit under test.
        template< typename ValueType > 
        void GetNextValue( const std::string& name, ValueType& value, const unsigned int streamIndex ) 
        { 
            throw "Unexpected call."; 
        } 
        template< typename ValueType > 
        void GetNextValue( const std::string& name, ValueType& value ) 
        { 
            throw "Unexpected call."; 
        } 

        // These are the only two templated calls expected, notice the difference in the method parameter list. Anything outside
        // of these two flavors is considerd an error.
        template<> 
        void GetNextValue< std::string >( const std::string& name, std::string& value, const unsigned int streamIndex ) 
        { 
            GetNextValueStdString( name, value, streamIndex ); 
        } 
        template<> 
        void GetNextValue< std::string >( const std::string& name, std::string& value ) 
        { 
            GetNextValue< std::string >( name, value, 0 ); 
        } 
    };

Answer 3

Here a solution that doesn t require to implement each template instantiation manually, this is especially helpful if there are a lot of different template instantiations.


struct Foo
{
  template <typename T>
  struct FooImpl
  {
    MOCK_METHOD(void, myMethod, (const T& value), ());
  };

  template <typename T>
  void myMethod(const T& value)
  {
    GetMock<T>()->myMethod(value);
  }

  template <typename T>
  std::shared_ptr<FooImpl<T>> GetMock()
  {
    std::shared_ptr<FooImpl<T>> result;
    std::shared_ptr<void> voidValue;
    if (!(voidValue = mCachedImpls[typeid(T).name()])
    {
      result = std::make_shared<FooImpl<T>>();
      mCachedImpls[typeid(T).name()] = std::reinterpret_pointer_cast<void>(result);
    }
    else
    {
      result = std::reinterpret_pointer_cast<FooImpl<T>>(voidValue);
    }

    return result;
  }

private:
  std::unordered_map<std::string, std::shared_ptr<void>> mCachedImpls;
};

Mock can then be prepared like this:

Foo foo;
EXPECT_CALL(*(foo.GetMock<int>()), myMethod(_)).Times(1);

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