How do I unit test a protected method in C++?

Question

How do I unit test a protected method in C++?

In Java, I'd either create the test class in the same package as the class under test or create an anonymous subclass that exposes the method I need in my test class, but neither of those methods are available to me in C++.

I am testing an unmanaged C++ class using NUnit.

Solution 1

Assuming you mean a protected method of a publicly-accessible class:

In the test code, define a derived class of the class under test (either directly, or from one of its derived classes). Add accessors for the protected members, or perform tests within your derived class . "protected" access control really isn't very scary in C++: it requires no co-operation from the base class to "crack into" it. So it's best not to introduce any "test code" into the base class, not even a friend declaration:

// in realclass.h
class RealClass {
    protected:
    int foo(int a) { return a+1; }
};

// in test code
#include "realclass.h"
class Test : public RealClass {
    public:
    int wrapfoo(int a) { return foo(a); }
    void testfoo(int input, int expected) {
        assert(foo(input) == expected);
    }
};

Test blah;
assert(blah.wrapfoo(1) == 2);
blah.testfoo(E_TO_THE_I_PI, 0);

Solution 2

You can also use using keyword to expose public block (using .

// in realclass.h
class RealClass {
    protected:
    int foo(int a) { return a+1; }
    int foo(string a) { return a.length(); } // Overload works too
    template<class T> int foo(const T& a) { return 1; } // Templates work too
};

// in test code
#include "realclass.h"
class RealClassExposed : public RealClass {
    public:
        using RealClass::foo;
};

RealClassExposed blah;
assert(blah.foo(1) == 2);
assert(blah.foo("test") == 4);
assert(blah.foo(blah) == 1);

See: http://en.cppreference.com/w/cpp/language/using_declaration

Solution 3

I use CxxTest and have the CxxTest derive from the class that contains the protected member function. If you're still searching around for your favorite C++ Unit Testing framework, take a look at this article.

Solution 4

Declare a friend class MyClass_UnitTest; in your MyClass. You can then define MyClass_UnitTest elsewhere in your unit test program that has full access to MyClass internals, but you don't have to provide an implementation in your release application. See CppUnit documentation for a good example of how this is done.

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Comments

[arkon]

Friend classes are not your friend.

[arkon]

This is the true object oriented solution to this problem. Inheritance exists for a reason!

[Rob K]

Then nothing is your friend. Every feature of every language can be misused and abused. Just because idiots misuse the feature doesn't mean I should never use it for its proper purpose.

[arkon]

Agreed, but some features are easier to abuse than others. More often than not, I see friend functions used to circumvent encapsulation.

[Langley]

In response to the -1... I know in "theory" you should be testing the public interface. However, in practice during the development cycle this can be an invaluable trick. My advice... use it judiciously.