Check if the returned object is of expected subclass in C++ unit test

Question

I have a function that returns object of abstract class.

AbstractClass some_function(int argument);

I have an assumption that if argument == 1 then some_function should return object of DerivedClassOne and if argument == 2 it should be DerivedClassTwo. I want to check these assumptions with a simple unit test.

What's the best (simple, readable, reliable and independent of any third-party libraries and tools) way to check that?

Answer 1

In your example, some_function() is returning an AbstractClass by value, which means the returned object gets sliced and will always be just a AbstractClass by itself. Polymorphism only works with pointers and references. Either way, you can use dynamic_cast for your validation check.

For example:

AbstractClass* some_function(int argument);

...

AbstractClass *obj;

obj = some_function(1);
if (dynamic_cast<DerivedClassOne*>(obj) != NULL)
    // returned the correct type...
else
    // returned the wrong type...

obj = some_function(2);
if (dynamic_cast<DerivedClassTwo*>(obj) != NULL)
    // returned the correct type...
else
    // returned the wrong type...

Or:

AbstractClass& some_function(int argument);

...

try {
    dynamic_cast<DerivedClassOne&>(some_function(1));
    // returned the correct type...
}
catch (const std::bad_cast&) {
    // returned the wrong type...
}

try {
    dynamic_cast<DerivedClassTwo&>(some_function(2));
    // returned the correct type...
}
catch (const std::bad_cast&) {
    // returned the wrong type...
}