What are the advantages of using boost.any library ? Could you please give me some real life examples ? Why the same functionality couldn't be achieved by having some generic type in the root of object's hierarchy and creating containers with that base type ?
boost::any will happily store ints and floats, types that clearly have no base classes. A real-life example where you can use it is a virtual machine for a high-level interpreted language. Your "function" objects will need an array of arguments. This can easily be implemented with a std::list<boost::any> behind the scenes.
I consider that Boost.Variant should always be preferred as it's non-intrusive and still calls for very structured programming.
But i guess the main idea behind boost.any is to provide the equivalent of java and c# object types. It's a way of saying "yes we can" ! :-)
We've used it in a property map, (std::map<std::string, boost::any>), to store a lot of things dynamically in a simple, flat dataspace.
Mostly we either stored smart-ptr-to-scriptable-objects or strings, but some entries where other types (floats, vec3f, matrices, and other non-standard objects).
It works pretty well for adding more dynamic capabilities to c++, or wherever you want some type-erasure to just add any type of data to an object.
Why the same functionality couldn't be achieved by having some generic type in the root of object's hierarchy and creating containers with that base type ?
That calls an object hierarchy -- a construct you are injecting in artificially in to the design for solving a peripheral problem. Further, such a construct is easy to get wrong and a wrong implementation can wreak havoc. Boost.Any is a community reviewed safe, well-tested alternative.
Could you please give me some real life examples ?
TinyJSON uses boost.Any.
What are the advantages of using boost.any library ?
I refer the introductory documentation.
We use boost.any as the carrier type for a type-safe tagged variadic container. Here's what that means:
We have a "raft" object, which travels through a set of filters. When a filter wants to add data to the raft, it can do something like this:
raft.addTaggedData<ETag1>(3.0);
raft.addTaggedData<ETag2>("a string")`;
std::string str = raft.getTaggedData<ETag2>();
int a = raft.getTaggedData<ETag1>(); // <-- Compile error
Where ETag1 and ETag2 are members of an enum, and we use a traits template to map tags to types.
The raft class is using a list of pair<ETagType, boost::any> as a backing store. Boost.any saved us the pain of managing raw buffers for various types.
Related
I'm looking for a data structure which behaves similar to boost::property_tree but (optionally) leaves the get/set implementation for each value item to the developer.
You should be able to do something like this:
std::function<int(void)> f_foo = ...;
my_property_tree tree;
tree.register<int>("some.path.to.key", f_foo);
auto v1 = tree.get<int>("some.path.to.key"); // <-- calls f_foo
auto v2 = tree.get<int>("some.other.path"); // <-- some fallback or throws exception
I guess you could abuse property_tree for this but I haven't looked into the implementation yet and I would have a bad feeling about this unless I knew that this is an intended use case.
Writing a class that handles requests like val = tree.get("some.path.to.key") by calling a provided function doesn't look too hard in the first place but I can imagine a lot of special cases which would make this quite a bulky library.
Some extra features might be:
subtree-handling: not only handle terminal keys but forward certain subtrees to separate implementations. E.g.
tree.register("some.path.config", some_handler);
// calls some_handler.get<int>("network.hostname")
v = tree.get<int>("some.path.config.network.hostname");
search among values / keys
automatic type casting (like in boost::property_tree)
"path overloading", e.g. defaulting to a property_tree-implementation for paths without registered callback.
Is there a library that comes close to what I'm looking for? Has anyone made experiences with using boost::property_tree for this purpose? (E.g. by subclassing or putting special objects into the tree like described here)
After years of coding my own container classes I ended up just adopting QVariantMap. This way it pretty much behaves (and is as flexible as) python. Just one interface. Not for performance code though.
If you care to know, I really caved in for Qt as my de facto STL because:
Industry standard - used even in avionics and satellite software
It has been around for decades with little interface change (think about long term support)
It has excellent performance, awesome documentation and enormous user base.
Extensive feature set, way beyond the STL
Would an std::map do the job you are interested in?
Have you tried this approach?
I don't quite understand what you are trying to do. So please provide a domain example.
Cheers.
I have some home-cooked code that lets you register custom callbacks for each type in GitHub. It is quite basic and still missing most of the features you would like to have. I'm working on the second version, though. I'm finishing a helper structure that will do most of the job of making callbacks. Tell me if you're interested. Also, you could implement some of those features yourself, as the code to register callbacks is already done. It shouldn't be so difficult.
Using only provided data structures:
First, getters and setters are not native features to c++ you need to call the method one way or another. To make such behaviour occur you can overload assignment operator. I assume you also want to store POD data in your data structure as well.
So without knowing the type of the data you're "get"ting, the only option I can think of is to use boost::variant. But still, you have some overloading to do, and you need at least one assignment.
You can check out the documentation. It's pretty straight-forward and easy to understand.
http://www.boost.org/doc/libs/1_61_0/doc/html/variant/tutorial.html
Making your own data structures:
Alternatively, as Dani mentioned, you can come up with your own implementation and keep a register of overloaded methods and so on.
Best
Is there a container who can store different types (yes, I really need to work with different kind of types) in Qt? I must create a new class to do this? If so, could you give me a hint to create it?
Most of containers in Qt are template based, then you can use them for different static types.
Another option is to use use QVariant, for example: QVector<QVariant> vec;.
A more dynamic solution is to use polymorphism, you can store pointers to a base class and so on.. .
PS: As a general rule, you should avoid this patterns. From Effective C++, by Scott Meyers:
Anytime you find yourself writing code of the form "if the object is
of type T1, then do something, but if it's of type T2, then do
something else," slap yourself.
I am trying to make a toy language in c++. I have used boost spirit for the grammar, and hopefully for parser/lexer. The idea is a toy language where 'everything is an object' like javascript and some implementation of prototype based inheritance. I want to know how to implement the 'object' type for the language in c++. I saw source codes of engine spidermonkey but mostly it is done using structures, also getting more complex at later stages. As structures are more or less equivalent to classes in C++, I hope I could manage with the stdlib itself. All I want is a solid idea of how the basic object has to be implemented and how properties are created/modified/destroyed. I tried to take a look at V8, but its really confusing me a lot!
Have each class have pointers to parent classes and implement properties and methods in STL containers like <string,pointer_fun> so that you can add/remove dynamically methods.
Then you could just lookup a method in an obj, if there isn't then follow the ptr to parent and lookup there till you find one or fail non-existant method.
For properties you could have a template to wrap them in the STL container so that they share a common ancestor and you can store pointers like <string,property<type>* > where property makes created type inherit from common type.
With this approach and some runtime checks you can support dynamically anything, just need to have clear which are the lookup rules for a method when you call it in an object.
So essentially every obj instance in your system could be:
class obj{
type_class parent*;
string type;
std::map<string,pointer_fun> methods;
std::map<string,property_parent_class> properties;
}
And have constructors/destructor be normal methods with special names.
Then in obj creation you could just lookup for type_name in type_objs and copy the member and properties from the type to the impl obj.
EDIT:
About function objects, you can use functors inheriting from a common one to use the container_of_pointers approach.
For lists I'd create a simple class object that implements metods like __add__() or __len__() or __get__() like in python for example, then when you parse the language you'd substitute list_obj[3] for your_list_obj.method['__get__'] after checking that it exists of course.
A big reason why I use OOP is to create code that is easily reusable. For that purpose Java style interfaces are perfect. However, when dealing with C++ I really can't achieve any sort of functionality like interfaces... at least not with ease.
I know about pure virtual base classes, but what really ticks me off is that they force me into really awkward code with pointers. E.g. map<int, Node*> nodes; (where Node is the virtual base class).
This is sometimes ok, but sometimes pointers to base classes are just not a possible solution. E.g. if you want to return an object packaged as an interface you would have to return a base-class-casted pointer to the object.. but that object is on the stack and won't be there after the pointer is returned. Of course you could start using the heap extensively to avoid this but that's adding so much more work than there should be (avoiding memory leaks).
Is there any way to achieve interface-like functionality in C++ without have to awkwardly deal with pointers and the heap?? (Honestly for all that trouble and awkardness id rather just stick with C.)
You can use boost::shared_ptr<T> to avoid the raw pointers. As a side note, the reason why you don't see a pointer in the Java syntax has nothing to do with how C++ implements interfaces vs. how Java implements interfaces, but rather it is the result of the fact that all objects in Java are implicit pointers (the * is hidden).
Template MetaProgramming is a pretty cool thing. The basic idea? "Compile time polymorphism and implicit interfaces", Effective C++. Basically you can get the interfaces you want via templated classes. A VERY simple example:
template <class T>
bool foo( const T& _object )
{
if ( _object != _someStupidObject && _object > 0 )
return true;
return false;
}
So in the above code what can we say about the object T? Well it must be compatible with '_someStupidObject' OR it must be convertible to a type which is compatible. It must be comparable with an integral value, or again convertible to a type which is. So we have now defined an interface for the class T. The book "Effective C++" offers a much better and more detailed explanation. Hopefully the above code gives you some idea of the "interface" capability of templates. Also have a look at pretty much any of the boost libraries they are almost all chalk full of templatization.
Considering C++ doesn't require generic parameter constraints like C#, then if you can get away with it you can use boost::concept_check. Of course, this only works in limited situations, but if you can use it as your solution then you'll certainly have faster code with smaller objects (less vtable overhead).
Dynamic dispatch that uses vtables (for example, pure virtual bases) will make your objects grow in size as they implement more interfaces. Managed languages do not suffer from this problem (this is a .NET link, but Java is similar).
I think the answer to your question is no - there is no easier way. If you want pure interfaces (well, as pure as you can get in C++), you're going to have to put up with all the heap management (or try using a garbage collector. There are other questions on that topic, but my opinion on the subject is that if you want a garbage collector, use a language designed with one. Like Java).
One big way to ease your heap management pain somewhat is auto pointers. Boost has a nice automatic pointer that does a lot of heap management work for you. The std::auto_ptr works, but it's quite quirky in my opinion.
You might also evaluate whether you really need those pure interfaces or not. Sometimes you do, but sometimes (like some of the code I work with), the pure interfaces are only ever instantiated by one class, and thus just become extra work, with no benefit to the end product.
While auto_ptr has some weird rules of use that you must know*, it exists to make this kind of thing work easily.
auto_ptr<Base> getMeAThing() {
return new Derived();
}
void something() {
auto_ptr<Base> myThing = getMeAThing();
myThing->foo(); // Calls Derived::foo, if virtual
// The Derived object will be deleted on exit to this function.
}
*Never put auto_ptrs in containers, for one. Understand what they do on assignment is another.
This is actually one of the cases in which C++ shines. The fact that C++ provides templates and functions that are not bound to a class makes reuse much easier than in pure object oriented languages. The reality though is that you will have to adjust they manner in which you write your code in order to make use of these benefits. People that come from pure OO languages often have difficulty with this, but in C++ an objects interface includes not member functions. In fact it is considered to be good practice in C++ to use non-member functions to implement an objects interface whenever possible. Once you get the hang of using template nonmember functions to implement interfaces, well it is a somewhat life changing experience. \
We often hear/read that one should avoid dynamic casting. I was wondering what would be 'good use' examples of it, according to you?
Edit:
Yes, I'm aware of that other thread: it is indeed when reading one of the first answers there that I asked my question!
This recent thread gives an example of where it comes in handy. There is a base Shape class and classes Circle and Rectangle derived from it. In testing for equality, it is obvious that a Circle cannot be equal to a Rectangle and it would be a disaster to try to compare them. While iterating through a collection of pointers to Shapes, dynamic_cast does double duty, telling you if the shapes are comparable and giving you the proper objects to do the comparison on.
Vector iterator not dereferencable
Here's something I do often, it's not pretty, but it's simple and useful.
I often work with template containers that implement an interface,
imagine something like
template<class T>
class MyVector : public ContainerInterface
...
Where ContainerInterface has basic useful stuff, but that's all. If I want a specific algorithm on vectors of integers without exposing my template implementation, it is useful to accept the interface objects and dynamic_cast it down to MyVector in the implementation. Example:
// function prototype (public API, in the header file)
void ProcessVector( ContainerInterface& vecIfce );
// function implementation (private, in the .cpp file)
void ProcessVector( ContainerInterface& vecIfce)
{
MyVector<int>& vecInt = dynamic_cast<MyVector<int> >(vecIfce);
// the cast throws bad_cast in case of error but you could use a
// more complex method to choose which low-level implementation
// to use, basically rolling by hand your own polymorphism.
// Process a vector of integers
...
}
I could add a Process() method to the ContainerInterface that would be polymorphically resolved, it would be a nicer OOP method, but I sometimes prefer to do it this way. When you have simple containers, a lot of algorithms and you want to keep your implementation hidden, dynamic_cast offers an easy and ugly solution.
You could also look at double-dispatch techniques.
HTH
My current toy project uses dynamic_cast twice; once to work around the lack of multiple dispatch in C++ (it's a visitor-style system that could use multiple dispatch instead of the dynamic_casts), and once to special-case a specific subtype.
Both of these are acceptable, in my view, though the former at least stems from a language deficit. I think this may be a common situation, in fact; most dynamic_casts (and a great many "design patterns" in general) are workarounds for specific language flaws rather than something that aim for.
It can be used for a bit of run-time type-safety when exposing handles to objects though a C interface. Have all the exposed classes inherit from a common base class. When accepting a handle to a function, first cast to the base class, then dynamic cast to the class you're expecting. If they passed in a non-sensical handle, you'll get an exception when the run-time can't find the rtti. If they passed in a valid handle of the wrong type, you get a NULL pointer and can throw your own exception. If they passed in the correct pointer, you're good to go.
This isn't fool-proof, but it is certainly better at catching mistaken calls to the libraries than a straight reinterpret cast from a handle, and waiting until some data gets mysteriously corrupted when you pass the wrong handle in.
Well it would really be nice with extension methods in C#.
For example let's say I have a list of objects and I want to get a list of all ids from them. I can step through them all and pull them out but I would like to segment out that code for reuse.
so something like
List<myObject> myObjectList = getMyObjects();
List<string> ids = myObjectList.PropertyList("id");
would be cool except on the extension method you won't know the type that is coming in.
So
public static List<string> PropertyList(this object objList, string propName) {
var genList = (objList.GetType())objList;
}
would be awesome.
It is very useful, however, most of the times it is too useful: if for getting the job done the easiest way is to do a dynamic_cast, it's more often than not a symptom of bad OO design, what in turn might lead to trouble in the future in unforeseen ways.