Re: Unit Testing Frameworks (was Re: Singletons)

[Please do not mail me a copy of your followup]

Ian Collins <ian-news@this.is.invalid> spake the secret code
<al3diaF2gb5U1@mid.individual.net> thusly:


Yes, I agree completely. The difficulty of Singletons isn't that they
are a singleton per se, but that the SUT has lots of code like this:

class Single
{
public:
    static Single *instance();
    // other public methods but Single is not a pure virtual base
    void doSomething();
    void doOtherThing();

private:
    Single();
    ~Single();
};

class SUT
{
public:
     void someMethod();
     // ...
};

void SUT::someMethod()
{
     Single::instance()->doSomething();
     // ...
     Single::instance()->doOtherThing();
     // ...
}

....which means I'm left only with link-time substitution as a way to
mock out the singleton if I leave this code unchanged. Consider this
minor refactoring, however:

class Single
{
public:
    virtual ~Single() { }

    // pure virtual base defining interface to Single
    virtual void doSomething() = 0;
    virtual void doOtherThing() = 0;
};

class ProductionSingle : public Single
{
public:
    static Single *instance();

private:
    ProductionSingle();
    virtual ~ProductionSingle();
};

class SUT
{
public:
    void someMethod() { someMethod(ProductionSingle::instance()); }
    void someMethod(Single *single);
};

void SUT::someMethod(Single *single)
{
     single->doSomething();
     // ...
     single->doOtherThing();
     // ...
}

Now I can test SUT::someMethod without it being directly coupled to
the production singleton; in fact, there's NOTHING in SUT that
requires it's collaborator to be a singleton, but the choice of making
that collaborator a singleton "leaked" into the implementation of SUT
making it harder to test. Extracting an interface over Single and
using DI (at the method level) makes testing SUT::someMethod *much*
easier and I don't have to resort to link-time substitution in order
to test the method. Existing code that calls SUT::someMethod()
doesn't have to change.
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