[Boost-users] Why must a Single Pass Range be default constructible?

Subject: [Boost-users] Why must a Single Pass Range be default constructible?
From: athor (thorena_at_[hidden])
Date: 2009-04-21 11:31:54

A Range does not require copy semantics according to the boost docs. 
http://www.boost.org/doc/libs/1_38_0/libs/range/doc/range.html

The SinglePassRangeConcept (and all range concepts using it) requires a
range to be default constructible. (There is a line "T a;" in the concept

The default constructible requirement means an abstract base class can never
implement any Range Concept. If a range does not require any copy semantics,
I don't see why it must be default constructible? Are there any good reasons
I don't know about?

A change from "T a;" to "const T& a" in concepts.hpp would do the trick...

Example:
struct AbstractArraySlice 
{
int* begin() const = 0;
int* end() const = 0;
};

// FAILS:
BOOST_CONCEPT_ASSERT((SinglePassRangeConcept<AbstractArraySlice>))

Footnote:
I often use abstract base classes to verify my algorithms do not use more
functionality than necessary and it annoys me that I can't use the range
concept check:

struct TestSum
{
  const AbstractArraySlice& r;
  void Test()
  {
  CalculateSum(r);
  }
};

template<class T>
T CalculateSum(const T& range)
<... function requires SinglePassRangeConcept ...>
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