BOOST_CONCEPT_USAGE macro to declare the function that
exercises all the concept's valid expressions. Note that at this point you
may sometimes need to be a little creative: for example, to check that
- *i++ returns the iterator's value type, we pass both values to
- the same_type member function template, which requires both
- arguments to have the same type, modulo references and cv-qualification.
- It's an imperfect check, but it's better than nothing.
+ *i++ returns something convertible to the iterator's value type,
+ we use it to initialize a variable of type value_type.
You may be wondering why we declared i and v
- as data members in the example above. Why didn't we simply write the
+
You may be wondering why we declared i
+ as a data member in the example above. Why didn't we simply write the
following?
BOOST_CONCEPT_USAGE(InputIterator)
{
- X i; // create the values we need
- value_type v;
- X j(i); // require copy construction
- same_type(*i++,v); // require postincrement-dereference returning value_type
- X& x = ++j; // require postincrement-dereference returning convertible to value_type
+ X& x = ++j; // require preincrement returning X&
}
Unfortunately, that code wouldn't have worked out so well, because it
- unintentionally imposes the requirement that X and its value
- type are both default-constructible. On the other hand, since instances of
+ unintentionally imposes the requirement that X is
+ default-constructible. On the other hand, since instances of
the InputIterator template will never be constructed, the
compiler never has to check how its data members will be constructed (C++
Standard Section 14.7.1 9). For that reason you should always