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Setting Up Mocks

Returning Values

The most common scenario is returning a value when a method is invoked on the mock object. This can be done like so:

MockClass mockClass;
When(mockClass.Add).Return(123);

assert(mockClass.Add(1, 2) == 123);

By default a zero initialized value will be returned from the method (i.e. return {}) if nothing has been setup. Also, there is no error/warning if you setup a method and it is not called. If you want to return a value for specific arguments, then you can use argument matchers:

When(mockClass.Add).With(1, 2).Return(3);

assert(mockClass.Add(1, 2) == 3);

The arrange methods setup is searched in reverse order, so the most recent call to When will take priority. This enables you to setup a default value and then to provide a more specific one:

When(mockClass.Add).Return(5);
When(mockClass.Add).With(1, 2).Return(3);

assert(mockClass.Add(1, 2) == 3);
assert(mockClass.Add(2, 2) == 123);

It's also possible to setup return values over multiple calls (notice that the last value in the list will be used for all subsequent calls):

When(mockClass.Add).Return({1, 2});

assert(mockClass.Add(1, 2) == 1);
assert(mockClass.Add(1, 2) == 2);
assert(mockClass.Add(1, 2) == 2);

Performing Actions

Instead of returning a value from a method, you can execute some arbitrary code that calculates the value:

When(mockClass.Add).Do([](int a, int b) { return a * b; });

assert(mockClass.Add(2, 3) == 6);

The same rules with regards to argument matching and setup order that apply to Return also apply to Do (i.e. the most recent call to Do will be checked if the arguments match first). Although it's possible to throw an exception with the above, there's a simpler method available:

When(mockClass.Add).Throws<std::out_of_range>("Expected");

try
{
    mockClass.Add(1, 2);
    assert(false)
}
catch (const std::out_of_range& ex)
{
    assert(std::strcmp(ex.what(), "Expected") == 0);
}

Using the Do method also allows you to set arguments that have been passed as references (a.k.a output parameters), like the following:

When(mockClass.TryGetValue).Do([](int& out) { out = 123; return true; });

int output = 0;
assert(mockClass.TryGetValue(output) == true);
assert(output == 123);