turtle/build/boost/doc/motivation.qbk

[section Motivation]

Consider a (very) simple calculator class :

 class calculator
 {
 public:
     int add( int a, int b );
 };

Obviously writing unit tests for such a class is trivial, one of them could be :

 BOOST_AUTO_TEST_CASE( zero_plus_zero_is_zero )
 {
     calculator c;
     BOOST_CHECK_EQUAL( 0, c.add( 0, 0 ) );
 }

What now if the calculator class looks more like this :

 class view
 {
 public:
     virtual void display( int result ) = 0;
 };
 
 class calculator
 {
 public:
     calculator( view& v );
 
     void add( int a, int b ); // the result will be sent to the view 'v'
 };

Writing unit tests becomes a bit more tedious and requires some boiler-plate code, for instance :

 class my_view : public view
 {
 public:
     my_view()
         : called( false )
     {}
     virtual void display( int result )
     {
         called = true;
         value = result;
     }
     bool called;
     int value;
 };
 
 BOOST_AUTO_TEST_CASE( zero_plus_zero_is_zero )
 {
     my_view v;
     calculator c( v );
     c.add( 0, 0 );
     BOOST_REQUIRE( v.called );
     BOOST_CHECK_EQUAL( 0, v.value );
 }

Mock objects main purpose is to alleviate the user from the burden of writing all this boiler-plate code.

Here is how the last test can be rewritten using a mock object :

 MOCK_BASE_CLASS( mock_view, view ) // declare a 'mock_view' class implementing 'view'
 {
     MOCK_METHOD( display, 1 )      // implement the 'display' method from 'view' (taking 1 argument)
 };
 
 BOOST_AUTO_TEST_CASE( zero_plus_zero_is_zero )
 {
     mock_view v;
     calculator c( v );
     MOCK_EXPECT( v.display ).once().with( 0 ); // expect the 'display' method to be called once (and only once) with a parameter value equal to 0
     c.add( 0, 0 );
 }

and all the checks are automatically handled by the library.

[endsect]