namespace boost { namespace serialization {
template<class Archive>
void save(Archive & ar, const my_class & t, unsigned int version)
{
....
}
template<class Archive>
void load(Archive & ar, my_class & t, unsigned int version)
{
....
}
}}
I need to use this code within a class but I'm getting errors due to the namespaces. Any help? From the docs: https://www.boost.org/doc/libs/1_47_0/libs/serialization/doc/serialization.html#splittingfreefunctions
Thanks in advance!
You are confusing intrusive serialization (member function) with unintrusive (free functions).
The coffee you post is for free functions (which can be used eg when you cannot add serialization code to a class (it might be from a third party header).
Inside a class definition you should take the member functions approach: https://www.boost.org/doc/libs/1_72_0/libs/serialization/doc/serialization.html#member
If you also need to split save/load functions you can do that as member functions too: https://www.boost.org/doc/libs/1_72_0/libs/serialization/doc/serialization.html#splittingmemberfunctions
The answer was to just place the entirety of the namespace outside of the class.
Related
As cppreference indicates:
std::enable_if can be used as an additional function argument (not applicable to operator overloads), as a return type (not applicable to constructors and destructors), or as a class template or function template parameter.
Is that because it doesn't make any difference where exactly enable_if is used in a template class or template function - the only thing that matters is the fact that it IS used in a template class or template function (and will remove an instantiation from an overload resolution set)?
Could it be also used this way for example
template<typename T>
class X {
public:
void someFunc() {
enable_if<is_integral<T>::value, int>::type dummy;
}
};
to achieve the same effect as when being used as cppreference indicates?
I am aware of the lack of reflection and basic template mechanics in C++ so the example below can't work. But maybe there's a hack to achieve the intended purpose in another way?
template <typename OwnerClass>
struct Template
{
OwnerClass *owner;
};
struct Base
{
virtual void funct ()
{
Template <decltype(*this)> temp;
// ...
}
};
struct Derived : public Base
{
void whatever ()
{
// supposed to infer this class and use Template<Derived>
// any chance some macro or constexpr magic could help?
funct();
}
};
In the example, Derived::whatever() calls virtual method Base::funct() and wants it to pass its own class name (Derived) to a template. The compiler complains "'owner' declared as a pointer to a reference of type 'Base &'". Not only does decltype(*this) not provide a typename but a reference, the compiler also can't know in advance that funct is called from Derived, which would require funct() to be made a template.
If funct() was a template however, each derived class needs to pass its own name with every call, which is pretty verbose and redundant.
Is there any hack to get around this limitation and make calls to funct() infer the typename of the calling class? Maybe constexpr or macros to help the compiler infer the correct type and reduce verbosity in derived classes?
You should use CRTP Pattern (Curiously Recurring Template Pattern) for inheritance.
Define a base class:
struct CBase {
virtual ~CBase() {}
virtual void function() = 0;
};
Define a prepared to CRTP class:
template<typename T>
struct CBaseCrtp : public CBase {
virtual ~CBaseCrtp() {}
void function() override {
using DerivedType = T;
//do stuff
}
};
Inherit from the CRTP one:
struct Derived : public CBaseCrtp<Derived> {
};
It should work. The only way to know the Derived type is to give it to the base!
Currently, this can't be done. Base is a Base and nothing else at the time Template <decltype(*this)> is instantiated. You are trying to mix the static type system for an inheritance hierarchy inherently not resolved before runtime. This very same mechanism is the reason for not calling virtual member functions of an object during its construction.
At some point, this limitation might change in the future. One step towards this is demonstrated in the Deducing this proposal.
How to restrict a template argument of Wrapper to be a Wrapper<Any,MyArray<AnyT>> elegantly?
Don't break content assist (Visual Studio).
High readability. Not use a hacky approach.
For some reasons, most solution love to hack.
Make it obvious at the very first line by using C++ syntax rule. (not just green comment)
As far as I know, there are many solutions, but every solution doesn't meet the criteria.
Workaround 1 (template specialization, fail 1)
template<class T> class MyArray{};
template<class T,class T2> class Wrapper;
template<class T,class T2> class Wrapper<T,MyArray<T2>>{
using Test=int;
};
class B{};
class C{};
int main() {
Wrapper<C,MyArray<B>> wrapper;
return 0;
}
This code is modified from https://stackoverflow.com/a/43518221 (#max66).
Context clue / syntax highlighting of IDE will be confused.
In my case, it marks some correct types as error e.g. :-
class ShowError : public Wrapper<B,MyArray<C>>{
Test n=0; //<-- unknown "Test" (intellisense)
};
Workaround 2 (some hacky field/typedef, fail 2)
template<class T> class MyArray{
public: using MyArrayT=T;
};
template<class T,class T2> class Wrapper{
public: using myT=typename T2::MyArrayT;
//^ assert at compile time
};
This idea come from a comment in https://stackoverflow.com/a/43518295 (#Jarod42)
The class declaration doesn't mention about MyArray, it just uses a hacky (less readable) way (MyArrayT) to enforce that T2 is MyArray.
Workaround 3 (base class, fail 2)
class MyArrayBase{};
template<class T> class MyArray : public MyArrayBase{ };
template<class T,class T2> class Wrapper{
//check something around MyArrayBase *object = new T2();
// or "is_base_of"
};
The code is modified from Restrict C++ Template Parameter to Subclass and C++ templates that accept only certain types.
It has same disadvantage as workaround 2.
It is not obvious for common user.
Workaround 4 (SNIFAE, fail 1)
By adding std::enable_if on the template class declaration (Wrapper), I can get a working hack.
Unfortunately, content assist hate it.
Reference
Here are the other links that I read :-
http://www.informit.com/articles/article.aspx?p=376878 (template template parameter)
restrict a template function, to only allow certain types (not related to template type as a parameter)
You can write a custom type trait is_specialization, as follows:
template<class Type, template<class...> class Template>
struct is_specialization
: std::false_type {};
template<template<class...> class Template, class... TArgs>
struct is_specialization<Template<TArgs...>, Template>
: std::true_type {};
Then you just need to static_assert that is_specialization is true for the given template argument:
template<class T,class T2>
class Wrapper {
static_assert(is_specialization<T2, MyArray>::value, "T2 must be a specialization of MyArray");
};
I searched many pages, and I think I have known how to write the std::hash. But I don't know where to put it.
An example is presented here http://en.cppreference.com/w/cpp/utility/hash .
However, I defined my type Instance in namespace ca in file instance_management.h. I want to use unordered_set<Instance> in the same file in another class InstanceManager. So I write the following code:
namespace std
{
template <> struct hash<ca::Instance>
{
size_t operator()(const ca::Instance & instance) const
{
std::size_t seed = 0;
// Some hash value calculation here.
return seed;
}
};
} // namespace std
But where should I put it? I tried many locations but all failed.
I am using visual studio 2013. I tried to put the previous code in some locations but all failed to compile it.
// location 1
namespace ca
{
class Instance {...}
class InstanceManager
{
// ... some other things.
private unordered_set<Instance>;
}
}
// location 2
There are several ways.
Specializing std::hash
In your code make sure that your std::hash<Instance> specialization is preceded immediately by the Instance class definition, and followed by the use of the unordered_set container that uses it.
namespace ca
{
class Instance {...};
}
namespaces std {
template<> hash<Instance> { ... };
}
namespace ca {
class InstanceManager
{
// ... some other things.
private unordered_set<Instance>;
}
}
One drawback is that you can have funny name lookup interference when passing a std::hash<ca::Instance> to other functions. The reason is that the associated namespace (ca) of all the template arguments of std::hash can be used during name lookup (ADL). Such errors are a bit rare, but if they occur they can be hard to debug.
See this Q&A for more details.
Passing your hash to unordered_set
struct MyInstanceHash { ... };
using MyUnorderedSet = std:unordered_set<Instance, MyInstanceHash>;
Here, you simply pass your own hash function to the container and be done with it. The drawback is that you have to explicitly type your own container.
Using hash_append
Note, however, there is the N3980 Standard proposal is currently pending for review. This proposal features a much superior design that uses a universal hash function that takes an arbitrary byte stream to be hashed by its template parameter (the actual hashing algorithm)
template <class HashAlgorithm>
struct uhash
{
using result_type = typename HashAlgorithm::result_type;
template <class T>
result_type
operator()(T const& t) const noexcept
{
HashAlgorithm h;
using std::hash_append;
hash_append(h, t);
return static_cast<result_type>(h);
}
};
A user-defined class X then has to provide the proper hash_append through which it presents itself as a byte stream, ready to be hashed by the univeral hasher.
class X
{
std::tuple<short, unsigned char, unsigned char> date_;
std::vector<std::pair<int, int>> data_;
public:
// ...
friend bool operator==(X const& x, X const& y)
{
return std::tie(x.date_, x.data_) == std::tie(y.date_, y.data_);
}
// Hook into the system like this
template <class HashAlgorithm>
friend void hash_append(HashAlgorithm& h, X const& x) noexcept
{
using std::hash_append;
hash_append(h, x.date_);
hash_append(h, x.data_);
}
}
For more details, see the presentation by the author #HowardHinnant at CppCon14 (slides, video). Full source code by both the author and Bloomberg is available.
Do not specialise std::hash, instead write your own hash function object (see Edge_Hash below) and declare your unordered_set with two template arguments.
#include <unordered_set>
#include <functional>
namespace foo
{
// an edge is a link between two nodes
struct Edge
{
size_t src, dst;
};
// this is an example of symmetric hash (suitable for undirected graphs)
struct Edge_Hash
{
inline size_t operator() ( const Edge& e ) const
{
static std::hash<size_t> H;
return H(e.src) ^ H(e.dst);
}
};
// this keeps all edges in a set based on their hash value
struct Edge_Set
{
// I think this is what you're trying to do?
std::unordered_set<Edge,Edge_Hash> edges;
};
}
int main()
{
foo::Edge_Set e;
}
Related posts are, eg:
Inserting in unordered_set using custom hash function
Trouble creating custom hash function unordered_map
Thanks to everyone.
I have found the reason and solved the problem somehow: visual studio accepted the InstanceHash when I was defining instances_. Since I was changing the use of set to unordered_set, I forgot to specify InstanceHash when I tried to get the const_iterator, so this time the compiler tried to use the std::hash<> things and failed. But the compiler didn't locate the line using const_iterator, so I mistakenly thought it didn't accept InstanceHash when I was defining instances_.
I also tried to specialize the std::hash<> for class Instance. However, this specialization requires at least the declaration of class ca::Instance and some of its member functions to calculate the hash value. After this specialization, the definition of class ca::InstanceManage will use it.
I now generally put declarations and implementations of almost every classes and member functions together. So, the thing I need to do is probably to split the ca namespace scope to 2 parts and put the std{ template <> struct hash<ca::Instance>{...}} in the middle.
I am trying to replace Poco::AutoPtr with some alternative in boost. Here is what I have discovered so far:
What I have: below classess are being used with Poco::AutoPtr. They need to implement reference counted method with implementing duplicate() and release() methods.
I am using above referece_counted.h and Poco::AutoPtr in a complex class hierarchy with multiple inheritance and c++ diamond problems.
A simplified version of classes would look something like this
class A : virtual public ReferenceCounted
{
...
}
class B : public A
{
...
}
class C : public A
{
...
}
class D : public A, B
{
...
}
and the list goes on for few more level deep. I know that this needs to be refactored with a simplified hierarchy but I wanna remove Poco::AutoPtr first with proper replacement in boost:
What I have found so far:
I have found that boost::intrusive_ptr is the closest smart pointer that can be a good replacement of Poco::AutoPtr.
I am however not able to implement the proper solution with this because the intrusive_ptr requires intrusive_ptr_add_ref and intrusive_ptr_release methods created specifically for each class with which I need to use the pointer. I tried using templates but still not having proper solution at hand.
Also one more issue is that I need to typecast from base to derived class many times.
is intrusive_ptr is the correct smart pointer for this usage ? and if yes.. can anybody give me suggestion regarding how to use the same ?
I am however not able to implement the proper solution with this
because the intrusive_ptr requires intrusive_ptr_add_ref and
intrusive_ptr_release methods created specifically for each class with
which I need to use the pointer.
No-no. It is should not be hard. As Boost documentation says:
On compilers that support argument-dependent lookup,
intrusive_ptr_add_ref and intrusive_ptr_release should be defined in
the namespace that corresponds to their parameter; otherwise, the
definitions need to go in namespace boost.
Try this: main.cpp (built ok with "g++ main.cpp -o main -lboost_system")
#include <boost/intrusive_ptr.hpp>
class MyObject
{
public:
void duplicate(){
// ...
}
void release(){
// ...
}
};
namespace boost {
template <class T>
void intrusive_ptr_add_ref( T * po ) {
po->duplicate(); // your internal realization
}
template <class T>
void intrusive_ptr_release( T * po ) {
po->release();
}
}
int main(int argc, char **argv)
{
// ...
boost::intrusive_ptr<MyObject> ptr( new MyObject );
boost::intrusive_ptr<MyObject> ptr2 = ptr; // should work
}