Category: Tech

At this week’s ACCU Conference I went to an excellent talk by Sven Rosvall entitled “Unit Testing Beyond Mock Objects”.

The talk covered the newer Java and C# unit testing frameworks that allow inserting mock objects even where legacy code is using some undesired dependency directly, meaning there is no seam where you can insert a different implementation.

These tools solve a real problem, and could be useful.

However, I want to move the discussion in a different direction: can we avoid mocking altogether, and end up with better code?

Sven gave us an example of some legacy code a little bit like this. (I translated into C++ just for fun.)

// Return true if it's between 12:00 and 14:00 now.
bool is_it_lunch_time_now()
{
    system_clock::time_point now = system_clock::now();
    time_t tt = system_clock::to_time_t( now );
    tm local_tm = ( *localtime( &tt ) );
    int hour = local_tm.tm_hour;

    return (
        12 <= hour      &&
              hour < 14
    );
}

To test this code, we would have something like this:

TEST_CASE( "Now: It's lunchtime now (hopefully)" )
{
    // REQUIRE( is_it_lunch_time_now() ); // NOTE: only  run at lunch time!
    REQUIRE( !is_it_lunch_time_now() );   // NOTE: never run at lunch time!
}

Which is agony. But the production code is fine:

if ( is_it_lunch_time_now() )
{
    eat( sandwich );
}

So, the normal way to allow mocking out a dependency like this would be to add an interface, making our lunch-related code take in a TimeProvider. To avoid coupling the choice of which TimeProvider is used to the calling code, we pass it into the constructor of a LunchCalculator that we plan to make early on:

class TimeProvider
{
public:
    virtual tm now_local() const = 0;
};

class RealTimeProvider : public TimeProvider
{
public:
    RealTimeProvider()
    {
    }
    virtual tm now_local() const
    {
        system_clock::time_point now = system_clock::now();
        time_t tt = system_clock::to_time_t( now );
        return ( *localtime( &tt ) );
    }
};

class HardCodedHourTimeProvider : public TimeProvider
{
public:
    HardCodedHourTimeProvider( int hour )
    {
        tm_.tm_hour = hour;
    }
    virtual tm now_local() const
    {
        return tm_;
    }
private:
    tm tm_;
};

class LunchCalc
{
public:
    LunchCalc( const TimeProvider& prov )
    : prov_( prov )
    {
    }
    bool is_it_lunch_time()
    {
        int hour = prov_.now_local().tm_hour;
        return (
            12 <= hour      &&
                  hour < 14
        );
    }
private:
    const TimeProvider& prov_;
};

and now we can write tests like this:

TEST_CASE( "TimeProvider: Calculate lunch time when it is" )
{
    HardCodedHourTimeProvider tp( 13 ); // 1pm (lunch time!)
    REQUIRE( LunchCalc( tp ).is_it_lunch_time() );
}

TEST_CASE( "TimeProvider: Calculate lunch time when it isn't" )
{
    HardCodedHourTimeProvider tp( 10 ); // 10am (not lunch :-( )
    REQUIRE( ! LunchCalc( tp ).is_it_lunch_time() );
}

Innovatively, these tests will pass at all times of day.

However, look at the price we’ve had to pay: 4 new classes, inheritance, and class names ending with unKevlinic words like “Provider” and “Calculator”. (Even worse, in a vain attempt to hide my embarrassment I abbreviated to “Calc”.)

We’ve paid a bit of a price in our production code too:

    // Near the start of the program:
    config.time_provider = RealTimeProvider();

    // Later, with config passed via PfA
    if ( LunchCalc( config.time_provider ).is_it_lunch_time() )
    {
        eat( sandwich );
    }

The code above where we create the RealTimeProvider is probably not usefully testable, and the class RealTimeProvider is also probably not usefully testable (or safely testable in the case of some more dangerous dependency). The rest of this code is testable, but there is a lot of it, isn’t there?

The advantage of this approach is that we have been driven to a better structure. We can now switch providers on startup by providing config, and even code way above this stuff can be fully exercised safe in the knowledge that no real clocks were poked at any time.

But. Sometimes don’t you ever think this might be better?

tm tm_local_now()  // Not testable
{
    system_clock::time_point now = system_clock::now();
    time_t tt = system_clock::to_time_t( now );
    return ( *localtime( &tt ) );
}

bool is_lunch_time( const tm& time )  // Pure functional - eminently testable
{
    int hour = time.tm_hour;
    return (
        12 <= hour      &&
              hour < 14
    );
}

The tests look like this:

TEST_CASE( "Functional: Calculate lunch time when it is" )
{
    tm my_tm;
    my_tm.tm_hour = 13; // 1pm (lunch time!)
    REQUIRE( is_lunch_time( my_tm ) );
}

TEST_CASE( "Functional: Calculate lunch time when it isn't" )
{
    tm my_tm;
    my_tm.tm_hour = 10; // 10am (not lunch time :-( )
    REQUIRE( ! is_lunch_time( my_tm ) );
}

and the production code looks very similar to what we had before:

if ( is_lunch_time( tm_local_now() ) )
{
    eat( sandwich );
}

Where possible, let’s not mock stuff we can just test as pure, functional code.

Of course, the production code shown here is now untestable, so there may well be work needed to avoid calling it in a test. That work may involve a mock. Or we may find a nicer way to avoid it.