I'm trying to learn to use BOOST_CONCEPT_CHECK for custom iterators,
ranges, and containers. I've built a simple linked list class with an
iterator (and const_iterator) that, as far as I can tell, satisfies the
forward traversal iterator requirements. However, g++ gives the
following errors:
concept.cpp:81: error: `*' cannot appear in a constant-expression
concept.cpp:81: error: a call to a constructor cannot appear in a
constant-expression
concept.cpp:81: error: template argument 1 is invalid
concept.cpp:81: error: template argument 1 is invalid
concept.cpp:82: error: `*' cannot appear in a constant-expression
concept.cpp:82: error: a call to a constructor cannot appear in a
constant-expression
concept.cpp:82: error: template argument 1 is invalid
concept.cpp:82: error: template argument 1 is invalid
concept.cpp:83: error: `*' cannot appear in a constant-expression
concept.cpp:83: error: a call to a constructor cannot appear in a
constant-expression
concept.cpp:83: error: template argument 1 is invalid
concept.cpp:83: error: template argument 1 is invalid
concept.cpp:84: error: `*' cannot appear in a constant-expression
concept.cpp:84: error: a call to a constructor cannot appear in a
constant-expression
concept.cpp:84: error: template argument 1 is invalid
concept.cpp:84: error: template argument 1 is invalid
Compiling with -E, lines 81-84 are as follows:
typedef ::boost::concepts::detail::instantiate<
&::boost::concepts::requirement_::failed> boost_concept_check81;
typedef ::boost::concepts::detail::instantiate<
&::boost::concepts::requirement_::failed> boost_concept_check82;
typedef ::boost::concepts::detail::instantiate<
&::boost::concepts::requirement_)>::failed> boost_concept_check83;
typedef ::boost::concepts::detail::instantiate<
&::boost::concepts::requirement_)>::failed> boost_concept_check84;
That tells me that, regardless of why g++ is having trouble, my iterator
fails to satisfy the concept somehow. So I wrote some code (see
my_naive_check, below) to try to figure out which requirement is
failing, but my check code is missing the same thing as the iterator,
because the check code runs fine.
Any pointers much appreciated.
// concept.cpp
// What's wrong with the iterators?
#include <cassert>
#include <cstdlib>
#include <iostream>
#include <string>
#include
#include
#include
#include
#include
template <typename T>
class List {
struct Node {
Node(const T& value_, Node *next_): value(value_), next(next_) {}
T value;
Node *next;
} *head;
public:
List(): head(0) {}
~List() {
Node *node = head;
while (node) {
Node *tmp = node->next;
delete node;
node = tmp;
}
}
void push_front(const T& value) { head = new Node(value, head); }
class iterator:
public boost::iterator_facade
{
public:
iterator(): node(0) {}
private:
iterator(Node * const node_): node(node_) {}
friend class List;
friend class boost::iterator_core_access;
T& dereference() const { return node->value; }
bool equal(const iterator& rhs) const { return node == rhs.node; }
void increment() { node = node->next; }
Node *node;
};
iterator begin() { return iterator(head); }
iterator end() { return iterator(); }
class const_iterator:
public boost::iterator_facade
{
public:
const_iterator(): node(0) {}
const_iterator(const iterator& old): node(old.node) {}
private:
friend class boost::iterator_core_access;
const T& dereference() const { return node->value; }
bool equal(const const_iterator& rhs) const { return node ==
rhs.node; }
void increment() { node = node->next; }
const Node *node;
};
const_iterator begin() const { return const_iterator(head); }
const_iterator end() const { return const_iterator(); }
BOOST_CONCEPT_ASSERT((boost::concepts::ForwardTraversalIterator
));
BOOST_CONCEPT_ASSERT((boost::concepts::WritableIterator<iterator>));
BOOST_CONCEPT_ASSERT((boost::concepts::ForwardTraversalIterator));
BOOST_CONCEPT_ASSERT((boost::concepts::ReadableIterator)
);
};
/*
ForwardTraversalIterator must be
SinglePassIterator
IncrementableIterator
Assignable
Copy Constructible
&r == &++r
r++
*r++
==
!=
difference_type
DefaultConstructible
iterator_traversal<X>::type => forward_traversal_tag
*/
void my_naive_check(Liststd::string& words) {
typedef Liststd::string::iterator It;
It it1 = words.end();
it1 = words.begin();
&it1 == &++it1;
It it2 = it1++;
std::string word = *it2++;
assert(it1 == it2);
assert(words.begin() != words.end());
It::difference_type distance = 0;
It it3;
boost::forward_traversal_tag tag =
boost::iterator_traversal<It>::type();
}
int main() {
Liststd::string words;
words.push_front("world");
words.push_front("Hello");
my_naive_check(words);
boost::for_each(words, std::cout << boost::lambda::_1 << "\n");
return EXIT_SUCCESS;
}
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