Question

I could really use some help mocking the dependencies in this scenario.

I have a client function that I need to test. It takes a 3rd party API resource and calls a method on it, populating the rest of its arguments with the output:

void ClassA::enumerateConnectedDevices(ThirdPartyAPI::BusManager & busMgr, QList<QByteArray> &devices, QStringList &descriptions)

At some point in this function s flow, it makes a call on the busMgr object whose signature looks like this:

ThirdPartyAPI::Error busMgr.GetNumOfCameras(...)

And the client code that uses that call looks like this:

ThirdPartyAPI::Error err = busMgr.GetNumOfCameras(&numCameras);

if (err != ThirdPartyAPI::ERROR_OK) { // compares against some enum
    cout << "Error: " << err.GetDescription();
}

The problem I ran into is the copy of the error returned from GetNumberOfCameras to the err local variable. Mocking out the BusManager and Error classes works until that copy is made. Since the mock error is a subclass of ThirdPartyAPI::Error, the assignment operator slices the mock away. The third party API constructs valid error objects and returns them. Client code can only copy-construct or assign them.

All I need is to return valid mock error objects from my mocked BusManager for the SUT method to be testable. I m open to refactoring the SUT method as long as the intent remains the same.

Any suggestions will be greatly appreciated!

Cheers,

Alex

Answer 1

It sounds like you re dealing with a classic problem in C++: the slicing problem when dealing with polymorphism. Because C++ doesn t know about your subclass at runtime, when you assign your mocked Error object to an Error reference, only the base class part gets copied (the "slicing").

One workaround for this is to use pointers or smart pointers instead of references. That way, polymorphism is preserved and the "whole" object is assigned, not just the base class part.

So, if it s possible, you can modify your client function to use pointers, and then use polymorphism as intended.

Here is an example of how this might look like:

void ClassA::enumerateConnectedDevices(ThirdPartyAPI::BusManager* busMgr, QList<QByteArray> &devices, QStringList &descriptions)
{
    int numCameras = 0;
    ThirdPartyAPI::Error* err = busMgr->GetNumOfCameras(&numCameras);

    if (*err != ThirdPartyAPI::ERROR_OK) { // compares against some enum
        cout << "Error: " << err->GetDescription();
    }

    // Don t forget to delete your pointers if not using smart pointers!
    delete err;
}

This way, your BusManager mock can return a pointer to a mock Error, and the full mock will be assigned, not just the base class part.

If the API doesn t allow you to use pointers, and you can t modify the API, this might be a bit more challenging. You would then need to find a way to either change the way you re testing, or refactor the code to be more testable. For example, you could wrap the API calls in a new layer of your own classes that does allow for pointers, and test that layer.

Let me know if this helps or if you need more assistance!

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