I'm trying to do the following (only relevant parts of code below):
template<typename ContainerType>
struct IsContainerCheck : is_container<ContainerType>
{
static constexpr char* err_value = "Type is not a container model";
};
namespace _check_concept {
template<typename ResultType>
struct run {
constexpr static int apply() {
static_assert(false, IsContainerCheck<ResultType>::err_value)
return 0;
}
};
template<>
struct run<true_t> {
constexpr static int apply() {
return 0;
}
};
}
This fails because the static_assert allows only literals to be printed. The same is with BOOST_STATIC_ASSERT_MSG macro.
So my question is - is there any way to output a constexpr string during compilation?
If there is a gcc extension providing this functionality that would also be great.
Used compiler gcc 4.8.1
GCC does not provide such a mechanism as you want. However you will not need
it if you are able to refactor your code somewhat as illustrated in the
following program. (I have filled in a few gaps so as to give us a
compilable example):
#include <type_traits>
#include <vector>
template<typename ContainerType>
struct is_container
{
static bool const value = false;
};
template<>
struct is_container<std::vector<int>>
{
static bool const value = true;
};
template<typename ContainerType>
struct IsContainerCheck // : is_container<ContainerType> <- Uneccessary
{
static_assert(is_container<ContainerType>::value,
"Type is not a container model");
};
namespace _check_concept {
template<typename ResultType>
struct run {
constexpr static int apply() {
return (IsContainerCheck<ResultType>(),0);
}
};
// No such specialization is necessary. Delete it.
// template<>
// struct run<true_t> {
// constexpr static int apply() {
// return 0;
// }
//};
}
using namespace _check_concept;
int main(int argc, char **argv)
{
auto verdict0 = run<std::vector<int>>::apply();
(void)verdict0;
// The following line will static_assert: "Type is not a container model"
auto verdict1 = run<float>::apply();
(void)verdict1;
return 0;
}
In your specialization _check_concept::struct run<true_t> I presume that
true_t is not an alias or equivalent of std::true_type, but rather
just a place-holder for some ResultType that is a container type. As
the test program shows, no such specialization is now necessary, because
IsContainerCheck<ResultType>() will static_assert, or not, depending
on ResultType, in the unspecialized run<ResultType>::apply().
I had some time (and a good liqueur to come along with it) to think more about the problem. This is what I came up with:
namespace _details {
struct PassedCheck {
constexpr static int printError () {
return 0; //no error concept check passed
}
};
template<template<typename> class ConceptCheck, typename ...ModelTypes>
struct check_concept_impl;
template<template<typename> class ConceptCheck, typename FirstType, typename ...ModelTypes>
struct check_concept_impl<ConceptCheck, FirstType, ModelTypes...> : mpl::eval_if< typename ConceptCheck<FirstType>::type,
check_concept_impl<ConceptCheck, ModelTypes...>,
mpl::identity<ConceptCheck<FirstType>>>
{ };
template<template<typename> class ConceptCheck, typename LastType>
struct check_concept_impl<ConceptCheck, LastType> : mpl::eval_if<typename ConceptCheck<LastType>::type,
mpl::identity<PassedCheck>,
mpl::identity<ConceptCheck<LastType>>>
{ };
}
template<template<typename> class ConceptCheck, typename ...ModelTypes>
struct check_concept {
private:
typedef typename _details::check_concept_impl<ConceptCheck, ModelTypes...>::type result_type;
public:
// the constexpr method assert produces shorter, fixed depth (2) error messages than a nesting assert in the trait solution
// the error message is not trahsed with the stack of variadic template recursion
constexpr static int apply() {
return result_type::printError();
}
};
template<typename ContainerType>
struct IsContainerCheck : is_container<ContainerType>
{
template<typename BoolType = false_t>
constexpr static int printError () {
static_assert(BoolType::value, "Type is not a container model");
return 0;
}
};
and the usage:
check_concept<IsContainerCheck, std::vector<int>, std::vector<int>, float, int>::apply();
The solution is probably not the most elegant one but I it keeps the assert message short:
In file included from ../main.cpp:4:0:
../constraint.check.hpp: In instantiation of ‘static constexpr int IsContainerCheck::printError() [with BoolType = std::integral_constant; ContainerType = float]’:
../constraint.check.hpp:61:34: required from ‘static constexpr int check_concept::apply() [with ConceptCheck = IsContainerCheck; ModelTypes = {std::vector >, std::vector >, float, int}]’
../main.cpp:25:83: required from here
../constraint.check.hpp:74:3: error: static assertion failed: Type is not a container model
static_assert(BoolType::value, "Type is not a container model");
The assert is issued in a constexpr method after the check_concept template specialization has been done. Embedding the static assert directly into the template class definition would drag the whole check_concept_impl recursion stack into the error message.
So changing the IsContainerCheck trait to something like (rest of the changes omitted for readibility):
template<typename ContainerType>
struct IsContainerCheck
{
static_assert(is_container<ContainerType>::type::value, "Type is not a container model");
};
would yield an error
../constraint.check.hpp: In instantiation of ‘struct IsContainerCheck’:
../constraint.check.hpp:36:9: required from ‘struct _details::check_concept_impl’
/usr/include/boost/mpl/eval_if.hpp:38:31: required from ‘struct boost::mpl::eval_if, _details::check_concept_impl, boost::mpl::identity > > >’
../constraint.check.hpp:36:9: required from ‘struct _details::check_concept_impl >, float, int>’
/usr/include/boost/mpl/eval_if.hpp:38:31: required from ‘struct boost::mpl::eval_if, _details::check_concept_impl >, float, int>, boost::mpl::identity > > >’
../constraint.check.hpp:36:9: required from ‘struct _details::check_concept_impl >, std::vector >, float, int>’
../constraint.check.hpp:53:84: required from ‘struct check_concept >, std::vector >, float, int>’
../main.cpp:25:81: required from here
../constraint.check.hpp:72:2: error: static assertion failed: Type is not a container model
static_assert(is_container::type::value, "Type is not a container model");
As you can see each recursive eval_if call is emended in the error description which is bad because it makes the error message dependent from the amount and type of template parameters.
Related
I have a C11 std::vector of structures. The structure contains an iterator to another structure of the same type, describing a tree.
Using a forward declaration doesn't work, because vector needs to have the full definition of the structure, but vector can't have the full definition of the structure until definition is complete.
#include <vector>
template <class Payload>
class Tree
{
public:
typedef struct _Node Node;
struct _Node
{
Payload t_payload;
//error: invalid use of incomplete type '_Value_type {aka struct _Node}'
std::vector<Node>::iterator pst_father;
};
std::vector<Node> gast_tree;
};
int main()
{
Tree<int> my_tree;
return 0;
}
in instantiation of 'void std::_Destroy(_ForwardIterator, _ForwardIterator) [with _ForwardIterator = _Node*]':
required from 'void std::_Destroy(_ForwardIterator, _ForwardIterator, std::allocator<_T2>&) [with _ForwardIterator = _Node*; _Tp = _Node]'
required from 'std::vector<_Tp, _Alloc>::~vector() [with _Tp = _Node; _Alloc = std::allocator<_Node>]'
required from here
error: invalid use of incomplete type '_Value_type {aka struct _Node}'
I want std::vector to serve as container for the Node structure, and I want Node(s) to link with each others to build a tree. It would be trivial using int indexes instead of iterators and resolving the reference later, but I'm trying to learn std::vector<>::iterators. This compiles just fine:
#include <vector>
template <class Payload>
class Tree
{
public:
typedef struct _Node
{
Payload t_payload;
//use a dumb index
int s32_father_index;
} Node;
std::vector<Node> gast_tree;
};
int main()
{
Tree<int> my_tree;
return 0;
}
I tried several ways, but I can't get the iterator to compile. Is it possible to have an iterator to an object inside the object?
SOLUTION
The keyword typedef is redundant in C++.
The keyword typename can be used in this scenario where the definition for the template is still incomplete. Details.
#include <vector>
template <class Payload>
class Tree
{
public:
struct Node
{
Payload t_payload;
typename std::vector<Node>::iterator pst_father;
};
std::vector<Node> gast_tree;
};
int main()
{
Tree<int> my_tree;
return 0;
}
SOLUTION
The keyword typedef is redundant in C++.
The keyword typename can be used in this scenario where the definition for the template is still incomplete.
#include <vector>
template <class Payload>
class Tree
{
public:
struct Node
{
Payload t_payload;
typename std::vector<Node>::iterator pst_father;
};
std::vector<Node> gast_tree;
};
int main()
{
Tree<int> my_tree;
return 0;
}
I have seen an idiom for using Derived type traits in the base class of a CRTP pattern that looks like this:
template<typename Derived>
struct traits;
template<typename Derived>
struct Base {
using size_type = typename traits<Derived>::size_type;
};
template <typename T>
struct Derived1 : Base<Derived1<T>>{
using size_type = size_t;
void print(){ std::cout << "Derived1" << std::endl; }
};
template <typename T>
struct traits<Derived1<T>> {
using size_type = size_t;
};
int main()
{
using T = float;
Derived1<T> d1;
d1.print();
}
My understanding is that the purpose of the idiom is to delay the instantiation of the Base class's size_type. What I am confused by is the fact that this pattern only seems to work if the derived class is itself templated. For instance, if we change the code to:
template<typename Derived>
struct traits;
template<typename Derived>
struct Base {
using size_type = typename traits<Derived>::size_type;
};
struct Derived1 : Base<Derived1>{
using size_type = size_t;
void print(){ std::cout << "Derived1" << std::endl; }
};
template <>
struct traits<Derived1> {
using size_type = size_t;
};
int main()
{
Derived1 d1;
d1.print();
}
then we get the error
prog.cc: In instantiation of 'struct Base<Derived1>':
prog.cc:21:19: required from here
prog.cc:18:58: error: invalid use of incomplete type 'struct traits<Derived1>'
using size_type = typename traits<Derived>::size_type;
^
prog.cc:14:8: note: declaration of 'struct traits<Derived1>'
struct traits;
^~~~~~
prog.cc: In function 'int main()':
prog.cc:33:9: error: 'Derived1' is not a template
Derived1<float> d1;
Could somebody give me an explanation indicating why the templated derived class compiles, but the untemplated class does not?
The issue you're seeing has nothing to do with CRTP.
Here's what the standard mentions.
If a class template has been declared, but not defined, at the point of instantiation (13.7.4.1),
the instantiation yields an incomplete class type (6.7). [Example:
template<class T> class X; X<char> ch; // error: incomplete type
X<char>
Your traits has only been declared at the point of instantiation of Base<Derived>, hence as per the standard(see above extraction from the standard), struct traits<Derived> yields an incomplete type.
You should reorder the code so that it sees the traits<Derived> specialization when Base<Derived> gets instantiated.
The compilation error you are seeing has nothing to do with CRTP, it's just a bit of a mish-mash of dependencies.
In the code without the templation, your "Base" struct needs the definition of the specialized "traits" struct but it only appears afterwards, so it tries to use the incomplete type it saw in the declaration above.
To get the code to work you need to have the "traits" specialization before the Base declaration, which requires you to also add a declaration of Derived 1, here is a compiling code:
class Derived1;
template<typename Derived>
struct traits;
template <>
struct traits<Derived1> {
using size_type = size_t;
};
template<typename Derived>
struct Base {
using size_type = typename traits<Derived>::size_type;
};
struct Derived1 : Base<Derived1>{
using size_type = size_t;
void print(){ std::cout << "Derived1" << std::endl; }
};
int main()
{
Derived1 d1;
d1.print();
}
Consider the following code:
#include <boost/iterator/iterator_facade.hpp>
#include <map>
// Class implements an stl compliant iterator to access the "sections" stored within a configuration.
template < typename _Iterator, typename _Reference >
class Section
: public boost::iterator_facade<
Section< _Iterator, _Reference >,
_Iterator,
boost::random_access_traversal_tag,
_Reference
>
{
private:
// Define the type of the base class:
typedef boost::iterator_facade<
Section< _Iterator, _Reference >,
_Iterator,
boost::random_access_traversal_tag,
_Reference
> base_type;
public:
// The following type definitions are common public typedefs:
typedef Section< _Iterator, _Reference > this_type;
typedef typename base_type::difference_type difference_type;
typedef typename base_type::reference reference;
typedef _Iterator iterator_type;
public:
explicit Section( const iterator_type it )
: m_it( it )
{ }
// Copy constructor required to construct a const_iterator from an iterator:
template < typename _U >
Section( const Section< _U, _Reference > it )
: m_it( it.m_it )
{ }
private:
// The following classes are friend of this class to ensure access onto the private member:
friend class boost::iterator_core_access;
template < typename _Iterator, typename _Reference > friend class Section;
void increment( ){ ++m_it; } // Advance by one position.
void decrement( ){ --m_it; } // Retreat by one position.
void advance( const difference_type& n ){ m_it += n }; // Advance by n positions.
bool equal( const this_type& rhs ) const{ return m_it == rhs.m_it; } // Compare for equality with rhs.
reference dereference( ) const { return m_it->second; } // Access the value referred to.
difference_type distance_to( const this_type& rhs ) const{ return rhs.m_it - m_it; } // Measure the distance to rhs.
private:
// Current "section" iterator:
iterator_type m_it;
};
struct Data
{
void f( ) const
{ }
};
typedef std::map< int, Data > map_type;
typedef Section< const map_type::const_iterator, const Data& > iterator_type;
map_type g_map;
iterator_type begin( )
{
return iterator_type( g_map.begin( ) );
}
void main( )
{
iterator_type i = begin( );
// i->f( ); // <--- error C2039: 'f' : is not a member of 'std::_Tree_const_iterator<_Mytree>'
( *i ).f( );
}
So the iterator facade shall return a reference to Data type. This works well when dereference operator is called but compile fails when operator->() is called. So I am a bit confused because operator->() tries to return a std::map::iterator. Any ideas ?
The iterator returns an iterator on dereference. To get the f part, you need to dereference twice.
It looks a lot like you misunderstood the meaning of the template arguments to iterator_facade. The second argument is not supposed to be any iterator type (this is what causes all your trouble). Instead you should use it to name your value_type.¹
From the way you specified the dereference operation (and Ref) and wanted to use it in main (i->f()) it looks like you just wanted to iterate the map's values. So, I'd rewrite the whole thing using more descriptive names as well, and here it is, working:
Live On Coliru
#include <boost/iterator/iterator_facade.hpp>
#include <map>
// Class implements an stl compliant iterator to access the "sections" stored within a configuration.
template <typename Map, typename Value = typename Map::mapped_type>
class MapValueIterator : public boost::iterator_facade<MapValueIterator<Map>, Value, boost::random_access_traversal_tag, Value const&> {
private:
// Define the type of the base class:
typedef Value const& Ref;
typedef boost::iterator_facade<MapValueIterator<Map>, Value, boost::random_access_traversal_tag, Ref> base_type;
public:
// The following type definitions are common public typedefs:
typedef MapValueIterator<Map> this_type;
typedef typename base_type::difference_type difference_type;
typedef typename base_type::reference reference;
typedef typename Map::const_iterator iterator_type;
public:
explicit MapValueIterator(const iterator_type it) : m_it(it) {}
// Copy constructor required to construct a const_iterator from an iterator:
template <typename U, typename V> MapValueIterator(const MapValueIterator<U,V> it) : m_it(it.m_it) {}
private:
// The following classes are friend of this class to ensure access onto the private member:
friend class boost::iterator_core_access;
template <typename U, typename V> friend class MapValueIterator;
void increment() { std::advance(m_it); } // Advance by one position.
void decrement() { std::advance(m_it, -1); } // Retreat by one position.
void advance(const difference_type &n) { std::advance(m_it, n); } // Advance by n positions.
bool equal(const this_type &rhs) const { return m_it == rhs.m_it; } // Compare for equality with rhs.
reference dereference() const { return m_it->second; } // Access the value referred to.
difference_type distance_to(const this_type &rhs) const { return rhs.m_it - m_it; } // Measure the distance to rhs.
private:
// Current iterator:
iterator_type m_it;
};
#include <iostream>
struct Data {
void f() const {
std::cout << __PRETTY_FUNCTION__ << "\n";
}
};
typedef std::map<int, Data> map_type;
template <typename Map>
MapValueIterator<Map> map_value_iterator(Map const& m) {
return MapValueIterator<Map>(m.begin());
}
int main() {
map_type g_map;
auto i = map_value_iterator(g_map);
i->f();
}
Which prints the output
void Data::f() const
as you'd expect.
Note that there are numerous places where I implemented the member functions using standard library facilities. Note as well, the iterator "mimics" random access, but it won't have the expected performance characteristics (increment is O(n)).
Final note: I'd recommend against having the implicit conversion constructor. I think you can do without it.
¹ The reference-type should typically be the same (but ref-qualified) except in rare cases where you actually "proxy" the values. This is an advanced topic and rarely should be used.
I am trying to create an stl container of a move-only type that uses its own allocator in VStudio 2012.
The trouble is: it seems as though I have to provide a construct function for the allocator which in turn needs access to a public copy constructor on the contained type.
I either get:
error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
or
error C2039: 'construct' : is not a member of 'MyAllocator'
The same code works in clang so I suspect the problem is due to Microsoft but can anyone suggest a possible work around?
This is my code for minimal reproduction
#include <memory>
#include <vector>
using namespace std;
template< typename T>
struct MyAllocator
{
typedef T value_type;
typedef value_type* pointer;
typedef value_type& reference;
typedef const value_type* const_pointer;
typedef const value_type& const_reference;
typedef size_t size_type;
typedef ptrdiff_t difference_type;
template<class t_other>
struct rebind
{
typedef MyAllocator<t_other> other;
};
MyAllocator():m_id(0) {}
MyAllocator(int id):m_id(id){}
template <class T>
MyAllocator(const MyAllocator<T>& other)
:m_id(other.getId())
{
}
T* allocate(std::size_t n)
{
return reinterpret_cast<T*>(malloc(sizeof(T) * n));
}
void deallocate(T* p, std::size_t n)
{
free(p);
}
int getId() const{ return m_id;}
//Have to add these although should not be necessary
void construct(pointer mem, const_reference value)
{
std::_Construct(mem, value);
}
void destroy(pointer mem)
{
std::_Destroy(mem);
}
private:
int m_id;
};
template <class T1, class U>
bool operator==(const MyAllocator<T1>& lhs, const MyAllocator<U>& rhs)
{
return lhs.getId() == rhs.getId() ;
}
template <class T1, class U>
bool operator!=(const MyAllocator<T1>&, const MyAllocator<U>&)
{
return lhs.getId() != rhs.getId();
}
//define a move only type
typedef unique_ptr<uint32_t> MyIntPtr;
//define a container based on MyIntPtr and MyAllocator
typedef vector<MyIntPtr, MyAllocator<MyIntPtr> > MyVector;
int main(int argc, char* argv[])
{
MyAllocator<MyIntPtr> alloc1(1);
MyVector vec(alloc1);
uint32_t* rawPtr = new uint32_t;
*rawPtr = 18;
vec.emplace_back(rawPtr);
return 0;
}
The error you get is because you try to construct a std::unique_ptr from a constant reference to a std::unique_ptr of the same type - and there is no such constructor.
You can rework your construct method to take an an rvalue reference and then everything compiles nicely:
void construct(pointer mem, value_type&& value)
{
std::_Construct(mem, std::move(value));
}
Consider this:
struct TestStruct
{
uint16_t m_a : 8;
uint16_t m_b : 8;
};
template<typename T>
struct some_trait
{
constexpr static const TestStruct value = {0,0};
};
template<>
struct some_trait<int>
{
constexpr static const TestStruct value = {1,1};
};
template<class T>
class Obj
{
public:
Obj(TestStruct t = some_trait<T>::value) : m_t(t)
{
}
TestStruct m_t;
};
int main(int argc, const char * argv[])
{
// Linker error here -> Undefined symbol for some_trait<int>::value
Obj<int> o;
TestStruct t = some_trait<int>::value;
Obj<int> o1(t); // -> This works
}
The following produces a linker error, complaining that the some_trait is not defined. I have two questions:
Why is this happening? I'm guessing it has to do with either the constexpr specifier or the non-POD type of TestStruct ?
Is there a way to make it work, while still keeping the default value in the constructor?
Thanks!