Can changes made to references returned by Google Mock be reflected in the original value?

Question

Can changes made to references returned by Google Mock be reflected in the original value?

Sample test code (using CppUnit as the unit testing framework):

int i = 0;
EXPECT_CALL(mock, GetValue())
    .WillOnce(ReturnRef(i));

...

CPPUNIT_ASSERT_EQUAL( 1, i );

Sample code:

auto x = obj.GetValue();
++x;

My testing shows that this doesn't update the value of i, but Google's description of ReturnRef and ReturnRefOfCopy seems to mean that this should work, i.e.

• ReturnRef(variable) - Return a reference to variable.
• ReturnRefOfCopy(value) - Return a reference to a copy of value; the copy lives as long as the action.

Instead it seems like ReturnRef also returns the reference to a copy. Is there some way to get this behaviour using Google Mock?

Answer 1

auto uses the same type deduction rules as templates. In your case auto x means the same as a template type parameter.

template<typename T>
int function(T t) { 
    T x; // <-- is equivalent to `auto x` outside of a template
};

In your example the type is deduced to an int rather than int& as you were expecting which is why the original value remained the same. You will need to declare the reference explicitly (auto& x) in order to modify the original value.