C#-like properties in native C++? - c++

In C# / .NET you can do something like this:
someThing.text = "blah";
String blah = someThing.text;
However, the above code does not actually interact with the someThing's text String directly, it uses a get and set property. Similarly, read-only properties can be used.
Is there a way to do something similar in native C++? (not C++ .NET)

WARNING: This is a tongue-in-cheek response and is terrible!!!
Yes, it's sort of possible :)
template<typename T>
class Property
{
private:
T& _value;
public:
Property(T& value) : _value(value)
{
} // eo ctor
Property<T>& operator = (const T& val)
{
_value = val;
return *this;
}; // eo operator =
operator const T&() const
{
return _value;
}; // eo operator ()
};
Then declare your class, declaring properties for your members:
class Test
{
private:
std::string _label;
int _width;
public:
Test() : Label(_label)
, Width(_width)
{
};
Property<std::string> Label;
Property<int> Width;
};
And call C# style!
Test a;
a.Label = "blah";
a.Width = 5;
std::string label = a.Label;
int width = a.Width;

In .NET properties are syntactic sugar for the real get and set functions which are emitted behind the scenes (in fact they are more than syntactic sugar because properties are emitted in the resulting IL and could be used with Reflection). So in C++ you would need to explicitly write those functions as there's no such notion as property.

I warn you, it is not fully compatible native C++: Microsoft-specific C++ only.
The Microsoft compiler allows you to use declspec(property), this way:
struct S {
int i;
void putprop(int j) {
i = j;
}
int getprop() {
return i;
}
// here you define the property and the functions to call for it
__declspec(property(get = getprop, put = putprop)) int the_prop;
};
int main() {
S s;
s.the_prop = 5; // THERE YOU GO
return s.the_prop;
}
cf Microsoft Documentation for more details: declspec(property).

Moo-Juice's answer looks really cool, but has a drawback: you can't use these properties like normal expressions of type T, as you can in C#.
For instance,
a.text.c_str() won't compile (‘class Property<std::basic_string<char> >’ has no member named ‘c_str’)
std::cout << a.text won't compile either (template argument deduction/substitution failed)
I would suggest the following enhancement to template<typename T> class Property:
T& operator() ()
{
return _value;
}
T const& operator() () const
{
return _value;
}
Then you can access the property's members with (), such as:
char const *p = a.text().c_str();
And you can use the property in expressions where the type must be deduced:
std::cout << a.text();

A property in .NET is associated with a get and/or a set member function, so it's really just syntactic sugar. The closest you can get with C++ is to use overloading to give the getter and setter the same name:
const std::string &test() const { return text_; }
void test(const std::string &value) { text_ = value; }
Obviously, you will still have to provide parenthesis for the call:
someThing.text("blah");
String blah = someThing.text();

Yes but it's vendor specific. Microsoft has declspec(property).
C++Builder's implementation is a bit more advanced (via vendor specific __property keyword) in that you could have indexed accessors (which can be of any types you wish).
Also check this out (without relying on vendor specific keywords): http://www.codeproject.com/KB/cpp/cpp_property_indexer.aspx

#include <iostream>
#include <string>
using namespace std;
// ------------------------------------------------------------------
#define PROPERTY_GET_SET(CLASS, NAME, TYPE) GetSetProperty<CLASS, TYPE> NAME() { return GetSetProperty<CLASS, TYPE>(this, &CLASS::get_##NAME, &CLASS::set_##NAME); }
#define PROPERTY_GET(CLASS, NAME, TYPE) GetProperty<CLASS, TYPE> NAME() { return GetProperty<CLASS, TYPE>(this, &CLASS::get_##NAME); }
#define PROPERTY_SET(CLASS, NAME, TYPE) SetProperty<CLASS, TYPE> NAME() { return SetProperty<CLASS, TYPE>(this, &CLASS::set_##NAME); }
template <typename CLASS, typename TYPE>
struct GetSetProperty {
typedef TYPE (CLASS::*Getter_t)() const;
typedef void (CLASS::*Setter_t)(TYPE);
GetSetProperty(CLASS* instance, Getter_t getter, Setter_t setter) : m_instance(instance), m_getter(getter), m_setter(setter) {}
operator TYPE() const { return (this->m_instance->*this->m_getter)(); }
GetSetProperty<CLASS, TYPE>& operator=(TYPE value) { (this->m_instance->*this->m_setter)(value); return *this; }
CLASS* const m_instance;
const Getter_t m_getter;
const Setter_t m_setter;
};
template <typename CLASS, typename TYPE>
struct GetProperty {
typedef TYPE (CLASS::*Getter_t)() const;
GetProperty(CLASS* instance, Getter_t getter) : m_instance(instance), m_getter(getter) {}
operator TYPE() const { return (this->m_instance->*this->m_getter)(); }
CLASS* const m_instance;
const Getter_t m_getter;
};
template <typename CLASS, typename TYPE>
struct SetProperty {
typedef void (CLASS::*Setter_t)(TYPE);
SetProperty(CLASS* instance, Setter_t setter) : m_instance(instance), m_setter(setter) {}
SetProperty<CLASS, TYPE>& operator=(TYPE value) { (this->m_instance->*this->m_setter)(value); return *this; }
CLASS* const m_instance;
const Setter_t m_setter;
};
template <typename CLASS, typename TYPE>
ostream& operator<<(ostream& ostr, const GetSetProperty<CLASS, TYPE>& p) { ostr << (p.m_instance->*p.m_getter)(); return ostr; }
template <typename CLASS, typename TYPE>
ostream& operator<<(ostream& ostr, const GetProperty<CLASS, TYPE>& p) { ostr << (p.m_instance->*p.m_getter)(); return ostr; }
// ------------------------------------------------------------------
class Dummy
{
public:
Dummy() : m_value1(42) {}
PROPERTY_GET_SET(Dummy, Value1, int);
PROPERTY_GET_SET(Dummy, Value2, const string&);
protected:
virtual int get_Value1() const { return this->m_value1; }
virtual void set_Value1(int value) { this->m_value1 = value; }
virtual const string& get_Value2() const { return this->m_value2; }
virtual void set_Value2(const string& value) { this->m_value2 = value; }
private:
int m_value1;
string m_value2;
};
int main(int argc, char* argv[]) {
Dummy d;
cout << d.Value1() << endl;
d.Value1() = 3;
cout << d.Value1() << endl;
cout << d.Value2() << endl;
d.Value2() = "test";
cout << d.Value2() << endl;
return 0;
}
// ------------------------------------------------------------------

By using std::function you can get pretty close. Featurewise everything is here.
First create the templated Property class:
#include <functional>
template<class T>
class Property
{
std::function<T (void)> _get;
std::function<void(const T&)> _set;
public:
Property(
std::function<T (void)> get,
std::function<void(const T&)> set)
: _get(get),
_set(set)
{ }
Property(
std::function<T(void)> get)
: _get(get),
_set([](const unsigned int&){})
{ }
operator T () const { return _get(); }
void operator = (const T& t) { _set(t); }
};
Use the Property in a class by creating a get and a set method similar to what you would in do C#:
class Test
{
private:
std::string _label;
public:
Property<std::string> Label = Property<std::string>
(
[this]()->std::string
{
return this->_label;
},
[this](const std::string& value)
{
this->_label = value;
}
);
Property<unsigned int> LabelSize = Property<unsigned int>
(
[this]()->unsigned int
{
return this->_label.size();
}
);
};
Testing this code:
Test test;
test.Label = "std functional";
std::cout << "label = " << std::string(test.Label) << std::endl
<< "label size = " << int(test.LabelSize) << std::endl;
will output
label = std functional
label size = 14
I think this is as syntactic-sugar-coated as you can get it in c++ :)

Probably the best option currently is to use the microsoft's __declspec( property( get=get_func_name, put=put_func_name ) ) PropertyType PropertyName attribute.
it is also supported by clang,
it is converted into your getter/setter when compiled (won't add any new variables),
in use, it is the closest thing to a real property (can access property of a property...).
But if you're using other compilers, you could use macros:
#define PROPERTY_GEN(Class, Type, Name, GetMethod, SetMethod) \
class Property_##Name { \
public: \
Property_##Name(Class* parent) : _parent(parent) { } \
Type operator = (Type value) \
{ \
_parent->SetMethod(value); \
return _parent->GetMethod(); \
} \
operator Type() const \
{ \
return static_cast<const Class*>(_parent)->GetMethod(); \
} \
Property_##Name& operator =(const Property_##Name& other) \
{ \
operator=(other._parent->GetMethod()); return *this; \
}; \
Property_##Name(const Property_##Name& other) = delete; \
private: \
Class* _parent; \
} Name { this };
// PROPERTY - Declares a property with the default getter/setter method names.
#define PROPERTY(Class, Type, Name) \
PROPERTY_GEN(Class, Type, Name, get_##Name, set_##Name)
Then use them like:
class SomeClass
{
public:
PROPERTY(SomeClass, int, Value)
int get_Value() const { return _value; }
void set_Value(int value) { _value = value; }
private:
int _value = 0;
};
int main()
{
SomeClass s, c;
s.Value = 5;
c.Value = 3 * s.Value;
s.Value = c.Value;
}
You could also add other macro variants for read-only, write-only properties and read-only non-const getters. To be able to access sub-properties via ->, you could add operator-> overloads to the macro.
Compared to microsoft's __declspec(property(...)), getter and setter methods can be made private but this isn't a real advantage since client might need to take the address of a getter/setter sometimes.
There is also a disadvantage of having an additional _parent variable for every property, and you would need to explicitly define copy constructors for parent classes if they are used.

I realize it's this question is probably too old to add another answer but to expand on Moo-Juice's answer, I've come up with a pretty neat and simple solution:
/// Utility for functions get, set & ptr.
template<typename TVal>
using GetFn = std::function<const TVal& (void)>;
template<typename TVal>
using SetFn = std::function<void(const TVal&)>;
template<typename TVal>
using PtrFn = std::function<TVal* (void)>;
/// The property class and each specialization utility.
template<typename TVal, bool Delegate, bool ReadOnly>
class Property;
template<typename TVal>
using PropertyGetSet = Property<TVal, false, false>;
template<typename TVal>
using PropertyDelGetSet = Property<TVal, true, false>;
template<typename TVal>
using PropertyGet = Property<TVal, false, true>;
template<typename TVal>
using PropertyDelGet = Property<TVal, true, true>;
/// <summary>
/// Property get-set.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, false, false>
{
public:
typedef TVal Value;
Property(const TVal& val)
: m_value(val)
{}
inline const TVal& Get() const { return m_value; }
inline void Set(const TVal& val) { m_value = val; }
inline TVal* Ptr() { return &m_value; }
private:
TVal m_value;
};
/// <summary>
/// Property delegate get-set.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, true, false>
{
public:
typedef TVal Value;
Property(GetFn<TVal> getFn, SetFn<TVal> setFn, PtrFn<TVal> ptrFn)
: m_getFn(getFn)
, m_setFn(setFn)
, m_ptrFn(ptrFn)
{}
inline const TVal& Get() const { return m_getFn(); }
inline void Set(const TVal& val) { m_setFn(val); }
inline TVal* Ptr() { return m_ptrFn(); }
private:
GetFn<TVal> m_getFn;
SetFn<TVal> m_setFn;
PtrFn<TVal> m_ptrFn;
};
/// <summary>
/// Property get.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, false, true>
{
public:
typedef TVal Value;
Property(const TVal& val)
: m_value(val)
{}
inline const TVal& Get() const { return m_value; }
inline TVal* Ptr() { return &m_value; }
private:
TVal m_value;
};
/// <summary>
/// Property delegate get.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, true, true>
{
public:
typedef TVal Value;
Property(GetFn<TVal> getFn, PtrFn<TVal> ptrFn)
: m_getFn(getFn)
, m_ptrFn(ptrFn)
{}
inline const TVal& Get() const { return m_getFn(); }
inline TVal* Ptr() { return m_ptrFn(); }
private:
GetFn<TVal> m_getFn;
PtrFn<TVal> m_ptrFn;
};
And then to use it:
PropertyGetSet<std::string> strGetSet = PropertyGetSet<std::string>("GetSet");
std::string m_strGetSet = "DelGetSet";
PropertyDelGetSet<std::string> strDelGetSet =
PropertyDelGetSet<std::string>(
[&]() -> const std::string& { return m_strGetSet; },
[&](const std::string& val) { m_strGetSet = val; },
[&]() { return &m_strGetSet; /* throw? */ });
// The get (read-only) version is the same but without the set function
Some caveats:
The get function returns a const& so you are not able to use it for changing the value, this is by design as it would allow people to use the reference to set the value instead of the explicit Set which gives the advantage of knowing when the value is set.
There is no syntactic sugar for the get-set-ptr functions, personally, I didn't like using operators cause it made the underlying system more obtuse, so using explicit functions lets the user know that it's a property and not something else. But if you may, you could sprinkle some operator overloads.
All specializations have a Ptr function which will be the pointer of the data. However, when using the delegate version, you can choose to throw so anyone trying to use it will have to work around it. The reason it's there is that in the worst-case scenario you may try to use the pointer for a very particular situation, I would highly advise not to use this tho, so feel free to remove it or make an extra specialization for it.
Lastly, it's a bit verbose, you could wrap the usage in a macro to make the syntax a bit shorter, but personally, I like it the way it is as it's more explicit that way.
EDIT:
You may run into the same issue I had with this design, have a look at the following link for the issue and the solution I've come up with: https://stackoverflow.com/a/68563492/3339838

Another try to enhance Moo-Juice's answer, by defining a Getter class (which the client can only get) and a Getter-Setter class which is also assignable:
template <typename T>
class Getter {
protected:
T &_value;
public:
Getter(T &value) : _value(value) {}
operator const T() const {
return _value;
}
};
template <typename T>
class GetterSetter : public Getter<T> {
using Getter<T>::_value;
using Getter<T>::Getter;
public:
GetterSetter<T> & operator=(const T val) {
_value = val;
return *(this);
}
};
This gives you the option to decide which properties could be changed from outside the class, and which will only change internally.

No, there is not. You would just create getter and setter functions:
someThing.setText("blah");
std::string blah = someThing.getText();

Related

Invoke a method of templated derived class method from non-template base class pointer

I'm trying to implement a Property system in my project similar to Property system in Qt. We just started with some ideas and are in prototyping stage.
Basically, what I understood from Qt is, client should be able to pass the get function, set function and property type through some macro in the .h file. So I tried to mimic the same.
Following is my sample code:
Abstract getter class. This type of getter class is a member in Property Class
class AbstractFunc
{
public:
template < typename R >
R Invoke ()
{
return (this)->Invoke ();
}
};
Get Function template: Return values can be T , T&, const T& , T* etc..
template < typename R, class T > class GetterFunction : public AbstractFunc
{
typedef R (T::*GetterFunc) ();
public:
GetterFunction (T * obj, GetterFunc func):m_Obj (obj), m_Func (func)
{
}
R Invoke ()
{
return m_Obj->*(m_Func) ();
}
public:
T * m_Obj;
GetterFunc m_Func;
};
Property Class:
class Property
{
public:
Property (string name, AbstractFunc* getter):m_name (name), m_getter (getter)
{
}
template < typename R > R GetValue ()
{
return m_getter->Invoke < R > ();
}
private:
string m_name;
AbstractFunc* m_getter;
};
Some Window Class:
class Window
{
public:
};
Example window class
class CheckBox :public Window
{
public:
int GetChecked ()
{
return m_checked;
}
void SetChecked (int nChecked)
{
m_checked = nChecked;
}
void AddProperty (string name)
{
m_prop = new Property (name, new GetterFunction< int, Checked >(this, &Checked::GetChecked));
}
int m_checked;
Property *m_prop;
};
main function:
int main ()
{
CheckBox cc;
cc.AddProperty ("Hello");
cout<<"value:"<< cc.m_prop->GetValue<int>();
return 0;
}
PROBLEM:
Getter function is remembered as AbstractFunc in Property Class. I want to call 'Invoke' on AbstractFunc* instance and it should invoke the member function and return correct return type. The above code throws error at AbstractFunc::Invoke.
see live
Your AbstractFunc isn't abstract at all: its Invoke isn't virtual. So even though GetterFunction also has a method named Invoke, that method doesn't actually override AbstractFunc::Invoke; it just hides it. When you try to call Invoke through the AbstractFunc*, it calls AbstractFunc::Invoke, which goes into infinite recursion and thus produces UB.
I would follow #n.m.'s suggestion to make a class hierarchy like so:
class AbstractFunc {
// lock down construction
AbstractFunc() = default;
public:
template<typename R>
R Invoke();
template<typename R>
bool HasType() const noexcept;
virtual ~AbstractFunc() = default; // need to have SOME virtual method so that we have runtime type info; also a virtual destructor is required anyway
template<typename R>
friend class TypedFunc;
};
template<typename R>
struct TypedFunc : AbstractFunc { // the ONLY instances of AbstractFunc are also instances of specializations of TypedFunc
virtual R InvokeTyped() = 0;
};
// one kind of TypedFunc applies a getter on an object
template<typename R, typename T>
struct GetterFunc : TypedFunc<R> {
// you never see a GetterFunc in the interface anyway... don't see a need to hide these
T *obj; // have you considered std::shared_ptr?
R (T::*getter)();
GetterFunc(T *obj, R (T::*getter)()) : obj(obj), getter(getter) { }
R InvokeTyped() override { return (obj->*getter)(); }
};
template<typename R, typename T>
std::unique_ptr<GetterFunc<R, T>> MakeGetterFunc(T *obj, R (T::*getter)()) {
return std::make_unique<GetterFunc<R, T>>(obj, getter);
}
// another kind applies a functor, etc.
template<typename R, typename F>
struct FunctorFunc : TypedFunc<R> {
F func;
template<typename... G>
FunctorFunc(G&&... args) : func(std::forward<G>(args)...) { }
R InvokeTyped() override { return func(); }
};
This is already usable: if you have an AbstractFunc* or an AbstractFunc&, you can dynamic_cast it down to a TypedFunc of the expected type (e.g. TypedFunc<int>). If that succeeds (you get a nonnull pointer or there is no std::bad_cast exception), then you just call InvokeTyped without having to know what kind of GetterFunc/FunctorFunc/whatever you are actually dealing with. The functions Invoke and HasType declared in AbstractFunc are just sugar to help do this.
template<typename R>
bool AbstractFunc::HasType() const noexcept {
return dynamic_cast<TypedFunc<R> const*>(this);
}
template<typename R>
R AbstractFunc::Invoke() {
return dynamic_cast<TypedFunc<R>&>(*this).InvokeTyped();
// throws std::bad_cast if cast fails
}
Done.
class Property {
std::string name;
std::unique_ptr<AbstractFunc> getter;
public:
Property(std::string name, std::unique_ptr<AbstractFunc> getter) : name(std::move(name)), getter(std::move(getter)) { }
template<typename R>
bool HasType() const noexcept { return getter->HasType<R>(); }
template<typename R>
R GetValue() const { return getter->Invoke<R>(); }
std::string const &GetName() const noexcept { return name; }
};
struct Window {
virtual ~Window() = default;
// doesn't really make sense to add/remove these from outside...
virtual std::vector<Property> GetProperties() = 0;
};
class CheckBox : public Window {
int checked = 0;
public:
int GetChecked() /*const*/ noexcept { return checked; }
void SetChecked(int checked) noexcept { this->checked = checked; }
std::vector<Property> GetProperties() override {
std::vector<Property> ret;
ret.emplace_back("Boxes Checked", MakeGetterFunc(this, &CheckBox::GetChecked));
return ret;
}
};
int main() {
CheckBox cb;
cb.SetChecked(5);
for(auto const &prop : cb.GetProperties()) std::cout << prop.GetName() << ": " << prop.GetValue<int>() << "\n";
}
You could then add e.g. a virtual std::type_info const& GetType() const or similar to AbstractFunc if you want to be able to directly get at the type, etc.

dynamically call same named function with different return type

I have a situation here...
I want to design a Factory where I can call a function with same name and no parameters but return different data Types. Based on the SubClassName I need to instantiate the Object.
Need help or lead on any design pattern to follow?
EDIT:
An abstract pseudo code...
class parent{
public:
virtual string getName() = 0;
//some virtual function.. not sure how to design. As the return type is dynamic.
*** getValue(){}
};
class A : public parent{
int x;
public:
virtual string getName(){ return "A";}
virtual int getValue(){retun x;}
};
class B : public parent{
string s;
public:
virtual string getName(){ return "B";}
virtual string getValue(){ return s;}
};
void main(){
string callingClass = "B";
parent * arrayPtrs[2];
arrayPtrs[0] = new A;
arrayPtrs[1] = new B;
for (loop through array, through iterator i){
if(arrayPtrs[i]->getName == callingClass ){
cout<<arrayPtrs[i]->getValue;
}
}
}
In C++ a function can only have one return type at a time, and you cannot change that dynamically.
However - as suggested by #mch - you can use template specializations. Keep in mind though, that this method is not dynamic. Your functions will be generated at compile time.
If I understood your question correctly, maybe this can be of help.
class MyObject1
{
//...
};
class MyObject2
{
//...
};
template<typename T>
struct Factory
{
constexpr static T gen();
};
template<>
struct Factory<MyObject1>
{
constexpr static MyObject1 gen()
{
return MyObject1(/*... whatever parameters you see fit ...*/);
}
};
template<>
struct Factory<MyObject2>
{
constexpr static MyObject2 gen()
{
return MyObject2(/*... whatever parameters you see fit ...*/);
}
};
int main()
{
auto myObj = Factory<MyObject1>::gen();
return 0;
}
Although this method seems fairly useless to me. You could simply call the desired constructor instead of this.
But then again, I'm not sure if this is what you thought of. If I made any mistakes please feel free, to correct me. I'll try to edit my answer best as I can.
EDIT:
To keep the virtual functionality too, the only way I can think of is type erasure: see https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Type_Erasure
The closest I could get to what you've asked for is this:
#include <iostream>
#include <string>
#include <any>
class parent {
public:
// you can use this too but I think type checking is more handy
// see in main function
/* virtual std::string getName() const = 0; */
virtual std::any getValue() const = 0;
};
class A : public parent {
public:
typedef int value_type;
private:
value_type x;
public:
A(value_type x) :
x(x)
{}
/* virtual std::string getName() const override { return "A"; } */
virtual std::any getValue() const override
{ return this->x; }
};
class B : public parent {
public:
typedef std::string value_type;
private:
value_type s;
public:
B(const value_type& s) :
s(s)
{}
/* virtual std::string getName() const override { return "B"; } */
virtual std::any getValue() const override
{ return this->s; }
};
int main(){
using callingClass = A;
parent* arrayPtrs[2];
arrayPtrs[0] = new A(42);
arrayPtrs[1] = new B("my string");
for (unsigned i = 0; i < sizeof(arrayPtrs) / sizeof(parent*); ++i)
{
// Note:
// dynamic cast will return nullptr if $callingClass
// is not a derived class
if (dynamic_cast<callingClass*>(arrayPtrs[i]))
std::cout << std::any_cast<callingClass::value_type>(arrayPtrs[i]->getValue()) << std::endl;
}
return 0;
}
I hope this one helps.
Note, that I used dynamic_cast to check the correct type. If you know a better solution, you can use that, too. But under these circumstances I couldn't think of any better.
EDIT2:
#include <iostream>
#include <string>
#include <tuple>
class some
{
using id = size_t;
template<typename T>
struct type { static void id() { } };
template<typename T>
static id type_id() { return reinterpret_cast<id>(&type<T>::id); }
template<typename T>
using decay = typename std::decay<T>::type;
template<typename T>
using none = typename std::enable_if<!std::is_same<some, T>::value>::type;
struct base
{
virtual ~base() { }
virtual bool is(id) const = 0;
virtual base *copy() const = 0;
} *p = nullptr;
template<typename T>
struct data : base, std::tuple<T>
{
using std::tuple<T>::tuple;
T &get() & { return std::get<0>(*this); }
T const &get() const& { return std::get<0>(*this); }
bool is(id i) const override { return i == type_id<T>(); }
base *copy() const override { return new data{get()}; }
};
template<typename T>
T &stat() { return static_cast<data<T>&>(*p).get(); }
template<typename T>
T const &stat() const { return static_cast<data<T> const&>(*p).get(); }
template<typename T>
T &dyn() { return dynamic_cast<data<T>&>(*p).get(); }
template<typename T>
T const &dyn() const { return dynamic_cast<data<T> const&>(*p).get(); }
public:
some() { }
~some() { delete p; }
some(some &&s) : p{s.p} { s.p = nullptr; }
some(some const &s) : p{s.p->copy()} { }
template<typename T, typename U = decay<T>, typename = none<U>>
some(T &&x) : p{new data<U>{std::forward<T>(x)}} { }
some &operator=(some s) { swap(*this, s); return *this; }
friend void swap(some &s, some &r) { std::swap(s.p, r.p); }
void clear() { delete p; p = nullptr; }
bool empty() const { return p; }
template<typename T>
bool is() const { return p ? p->is(type_id<T>()) : false; }
template<typename T> T &&_() && { return std::move(stat<T>()); }
template<typename T> T &_() & { return stat<T>(); }
template<typename T> T const &_() const& { return stat<T>(); }
template<typename T> T &&cast() && { return std::move(dyn<T>()); }
template<typename T> T &cast() & { return dyn<T>(); }
template<typename T> T const &cast() const& { return dyn<T>(); }
template<typename T> operator T &&() && { return std::move(_<T>()); }
template<typename T> operator T &() & { return _<T>(); }
template<typename T> operator T const&() const& { return _<T>(); }
};
using any = some;
class parent {
public:
// you can use this too but I think type checking is more handy
/* virtual std::string getName() const = 0; */
virtual any getValue() const = 0;
};
class A : public parent {
public:
typedef int value_type;
private:
value_type x;
public:
A(value_type x) :
x(x)
{}
/* virtual std::string getName() const override { return "A"; } */
virtual any getValue() const override
{ return this->x; }
};
class B : public parent {
public:
typedef std::string value_type;
private:
value_type s;
public:
B(const value_type& s) :
s(s)
{}
/* virtual std::string getName() const override { return "B"; } */
virtual any getValue() const override
{ return this->s; }
};
int main(){
using callingClass = A;
parent* arrayPtrs[2];
arrayPtrs[0] = new A(42);
arrayPtrs[1] = new B("my string");
for (unsigned i = 0; i < sizeof(arrayPtrs) / sizeof(parent*); ++i)
{
// Note:
// dynamic cast will return nullptr if $callingClass
// is not a derived class
if (dynamic_cast<callingClass*>(arrayPtrs[i]))
std::cout << arrayPtrs[i]->getValue()._<callingClass::value_type>() << std::endl;
}
return 0;
}
This snipped is in case you cannot use C++17 features, and is based on:
any class

passing std::type_info for identifying void *

I have to pass around the void * for supporting types that can't be known at compile time, but I also don't want to go totally insane and left everything on myself so I think to use type_info for type_Checking but since type_info doesn't support copy operation I am getting compiler errors when passing them around
#include <bits/stdc++.h>
struct PropertyClassInterface {
virtual const char * name() = 0;
virtual void set(const char * property_name,std::pair<const void *,std::type_info> new_val) = 0;
virtual std::pair<std::shared_ptr<void>,std::type_info> get(const char * property_name) = 0;
template< typename Type>
static std::pair<std::shared_ptr<void>,std::type_info> get_type_pair(const Type& Value) {
std::shared_ptr<void> t = std::make_shared<Type>();
*static_cast<Type *>(t.get()) = Value;
*return std::make_pair(t,typeid(Type));* // error
}
};
struct PropertyManager {
using base_pointer = std::shared_ptr<PropertyClassInterface>;
void add_object(base_pointer new_member) {
objects.push_back(new_member);
}
template<typename Type>
void set(const char * object_name,const char * property_name,const Type& new_val) {
find_object_orThrow(object_name)->set(property_name,std::make_pair(static_cast<const void *>(&new_val),typeid(new_val)));
}
template<typename Type>
Type get(const char * object_name, const char * property_name) {
auto a = find_object_orThrow(object_name)->get(property_name);
if (typeid(Type).hash_code() != a.second.hash_code())
throw std::runtime_error{"get(): mismatched type"};
return a.first;
}
public:
std::vector<base_pointer> objects;
base_pointer find_object_orThrow(const char * obj_name){
for(auto& o : objects) {
if (!strcmpi(o->name(),obj_name)) {
return o;
}
}
throw std::runtime_error{std::string("no object named \"") + obj_name + "\" found"};
}
};
struct testClass : PropertyClassInterface {
void set(const char * property_name,std::pair<const void *,std::type_info> new_val) {
auto checkTypeInfo = [&new_val](const std::type_info& expected) {
if (new_val.second.hash_code() != expected.hash_code())
throw std::runtime_error{"set(): wrong type"};
};
if (!strcmpi(property_name,"my_number")) {
checkTypeInfo(typeid(decltype(my_number)));
my_number = *static_cast<const decltype(my_number) *>(new_val.first);
}
};
std::pair<std::shared_ptr<void>,std::type_info> get(const char * property_name) {
if (!strcmpi(property_name,"my_number")) {
PropertyClassInterface::get_type_pair(my_number);
}
}
private:
int my_number;
};
int main() {
};
so do I have to use dynamic memory for storing type_info as well
I am limited to c++11 and I know about not using bits headers and am only using for testing
What you want to do is implement an any, or use boost any.
An any isn't hard to write.
namespace details {
struct any_concept;
using pimpl=std::unique_ptr<any_concept>;
struct any_concept {
virtual ~any_concept() {}
virtua pimpl clone() const = 0;
virtual std::type_info const& type() const = 0;
private:
virtual void* unsafe_get() = 0;
virtual void const* unsafe_get() const = 0;
public:
template<class T>
T* get() {
if (typeid(T) != type()) return nullptr;
return static_cast<T*>(unsafe_get());
}
template<class T>
T const* get() const {
if (typeid(T) != type()) return nullptr;
return static_cast<T const*>(unsafe_get());
}
};
template<class T>
struct any_model:any_concept {
T t;
virtual ~any_model() = default;
virtual pimpl clone() const final override {
return pimpl( new any_model(t) );
}
virtual std::type_info const& type() const final override {
return typeid(T);
}
template<class U>
any_model(U&& u):t(std::forward<U>(u)){}
private:
virtual void* unsafe_get() final override { return std::addressof(t); }
virtual void const* unsafe_get() const final override { return std::addressof(t); }
};
}
struct any {
template<class T, typename std::enable_if<!std::is_same<any, typename std::decay<T>::type>::value, bool> =true>
any( T&& t ):
pImpl( new details::any_model<typename std::decay<T>::type>( std::forward<T>(t) ) )
{}
template<class T>
T* get() {
if (!pImpl) return nullptr;
return pImpl->get<T>();
}
template<class T>
T const* get() const {
if (!pImpl) return nullptr;
return const_cast<details::any_concept const&>(*pImpl).get<T>();
}
template<class T>
bool contains()const { return get<T>(); }
explicit operator bool() const {
return (bool)pImpl;
}
any()=default;
any(any&&)=default;
any& operator=(any&&)=default;
~any()=default;
any(any const& o):
pImpl( o.pImpl?o.pImpl->clone():pimpl{} )
{}
any& operator=(any const& o) {
any tmp(o);
std::swap(*this, tmp);
return *this;
}
private:
details::pimpl pImpl;
};
there; a really simple any implementation. Written on a phone, so probably contains typos.
It supports value semantics, but store anything (that can be copied and destroyed). If you know what it stores, you can .get<T>() it. You can ask it if it contains<T>() as well.
This is known as a vocabulary type. It is basically your void* and type info bundled in a way that makes misuse more difficult.

Update class member before access in C++ [duplicate]

In C# / .NET you can do something like this:
someThing.text = "blah";
String blah = someThing.text;
However, the above code does not actually interact with the someThing's text String directly, it uses a get and set property. Similarly, read-only properties can be used.
Is there a way to do something similar in native C++? (not C++ .NET)
WARNING: This is a tongue-in-cheek response and is terrible!!!
Yes, it's sort of possible :)
template<typename T>
class Property
{
private:
T& _value;
public:
Property(T& value) : _value(value)
{
} // eo ctor
Property<T>& operator = (const T& val)
{
_value = val;
return *this;
}; // eo operator =
operator const T&() const
{
return _value;
}; // eo operator ()
};
Then declare your class, declaring properties for your members:
class Test
{
private:
std::string _label;
int _width;
public:
Test() : Label(_label)
, Width(_width)
{
};
Property<std::string> Label;
Property<int> Width;
};
And call C# style!
Test a;
a.Label = "blah";
a.Width = 5;
std::string label = a.Label;
int width = a.Width;
In .NET properties are syntactic sugar for the real get and set functions which are emitted behind the scenes (in fact they are more than syntactic sugar because properties are emitted in the resulting IL and could be used with Reflection). So in C++ you would need to explicitly write those functions as there's no such notion as property.
I warn you, it is not fully compatible native C++: Microsoft-specific C++ only.
The Microsoft compiler allows you to use declspec(property), this way:
struct S {
int i;
void putprop(int j) {
i = j;
}
int getprop() {
return i;
}
// here you define the property and the functions to call for it
__declspec(property(get = getprop, put = putprop)) int the_prop;
};
int main() {
S s;
s.the_prop = 5; // THERE YOU GO
return s.the_prop;
}
cf Microsoft Documentation for more details: declspec(property).
Moo-Juice's answer looks really cool, but has a drawback: you can't use these properties like normal expressions of type T, as you can in C#.
For instance,
a.text.c_str() won't compile (‘class Property<std::basic_string<char> >’ has no member named ‘c_str’)
std::cout << a.text won't compile either (template argument deduction/substitution failed)
I would suggest the following enhancement to template<typename T> class Property:
T& operator() ()
{
return _value;
}
T const& operator() () const
{
return _value;
}
Then you can access the property's members with (), such as:
char const *p = a.text().c_str();
And you can use the property in expressions where the type must be deduced:
std::cout << a.text();
A property in .NET is associated with a get and/or a set member function, so it's really just syntactic sugar. The closest you can get with C++ is to use overloading to give the getter and setter the same name:
const std::string &test() const { return text_; }
void test(const std::string &value) { text_ = value; }
Obviously, you will still have to provide parenthesis for the call:
someThing.text("blah");
String blah = someThing.text();
Yes but it's vendor specific. Microsoft has declspec(property).
C++Builder's implementation is a bit more advanced (via vendor specific __property keyword) in that you could have indexed accessors (which can be of any types you wish).
Also check this out (without relying on vendor specific keywords): http://www.codeproject.com/KB/cpp/cpp_property_indexer.aspx
#include <iostream>
#include <string>
using namespace std;
// ------------------------------------------------------------------
#define PROPERTY_GET_SET(CLASS, NAME, TYPE) GetSetProperty<CLASS, TYPE> NAME() { return GetSetProperty<CLASS, TYPE>(this, &CLASS::get_##NAME, &CLASS::set_##NAME); }
#define PROPERTY_GET(CLASS, NAME, TYPE) GetProperty<CLASS, TYPE> NAME() { return GetProperty<CLASS, TYPE>(this, &CLASS::get_##NAME); }
#define PROPERTY_SET(CLASS, NAME, TYPE) SetProperty<CLASS, TYPE> NAME() { return SetProperty<CLASS, TYPE>(this, &CLASS::set_##NAME); }
template <typename CLASS, typename TYPE>
struct GetSetProperty {
typedef TYPE (CLASS::*Getter_t)() const;
typedef void (CLASS::*Setter_t)(TYPE);
GetSetProperty(CLASS* instance, Getter_t getter, Setter_t setter) : m_instance(instance), m_getter(getter), m_setter(setter) {}
operator TYPE() const { return (this->m_instance->*this->m_getter)(); }
GetSetProperty<CLASS, TYPE>& operator=(TYPE value) { (this->m_instance->*this->m_setter)(value); return *this; }
CLASS* const m_instance;
const Getter_t m_getter;
const Setter_t m_setter;
};
template <typename CLASS, typename TYPE>
struct GetProperty {
typedef TYPE (CLASS::*Getter_t)() const;
GetProperty(CLASS* instance, Getter_t getter) : m_instance(instance), m_getter(getter) {}
operator TYPE() const { return (this->m_instance->*this->m_getter)(); }
CLASS* const m_instance;
const Getter_t m_getter;
};
template <typename CLASS, typename TYPE>
struct SetProperty {
typedef void (CLASS::*Setter_t)(TYPE);
SetProperty(CLASS* instance, Setter_t setter) : m_instance(instance), m_setter(setter) {}
SetProperty<CLASS, TYPE>& operator=(TYPE value) { (this->m_instance->*this->m_setter)(value); return *this; }
CLASS* const m_instance;
const Setter_t m_setter;
};
template <typename CLASS, typename TYPE>
ostream& operator<<(ostream& ostr, const GetSetProperty<CLASS, TYPE>& p) { ostr << (p.m_instance->*p.m_getter)(); return ostr; }
template <typename CLASS, typename TYPE>
ostream& operator<<(ostream& ostr, const GetProperty<CLASS, TYPE>& p) { ostr << (p.m_instance->*p.m_getter)(); return ostr; }
// ------------------------------------------------------------------
class Dummy
{
public:
Dummy() : m_value1(42) {}
PROPERTY_GET_SET(Dummy, Value1, int);
PROPERTY_GET_SET(Dummy, Value2, const string&);
protected:
virtual int get_Value1() const { return this->m_value1; }
virtual void set_Value1(int value) { this->m_value1 = value; }
virtual const string& get_Value2() const { return this->m_value2; }
virtual void set_Value2(const string& value) { this->m_value2 = value; }
private:
int m_value1;
string m_value2;
};
int main(int argc, char* argv[]) {
Dummy d;
cout << d.Value1() << endl;
d.Value1() = 3;
cout << d.Value1() << endl;
cout << d.Value2() << endl;
d.Value2() = "test";
cout << d.Value2() << endl;
return 0;
}
// ------------------------------------------------------------------
By using std::function you can get pretty close. Featurewise everything is here.
First create the templated Property class:
#include <functional>
template<class T>
class Property
{
std::function<T (void)> _get;
std::function<void(const T&)> _set;
public:
Property(
std::function<T (void)> get,
std::function<void(const T&)> set)
: _get(get),
_set(set)
{ }
Property(
std::function<T(void)> get)
: _get(get),
_set([](const unsigned int&){})
{ }
operator T () const { return _get(); }
void operator = (const T& t) { _set(t); }
};
Use the Property in a class by creating a get and a set method similar to what you would in do C#:
class Test
{
private:
std::string _label;
public:
Property<std::string> Label = Property<std::string>
(
[this]()->std::string
{
return this->_label;
},
[this](const std::string& value)
{
this->_label = value;
}
);
Property<unsigned int> LabelSize = Property<unsigned int>
(
[this]()->unsigned int
{
return this->_label.size();
}
);
};
Testing this code:
Test test;
test.Label = "std functional";
std::cout << "label = " << std::string(test.Label) << std::endl
<< "label size = " << int(test.LabelSize) << std::endl;
will output
label = std functional
label size = 14
I think this is as syntactic-sugar-coated as you can get it in c++ :)
Probably the best option currently is to use the microsoft's __declspec( property( get=get_func_name, put=put_func_name ) ) PropertyType PropertyName attribute.
it is also supported by clang,
it is converted into your getter/setter when compiled (won't add any new variables),
in use, it is the closest thing to a real property (can access property of a property...).
But if you're using other compilers, you could use macros:
#define PROPERTY_GEN(Class, Type, Name, GetMethod, SetMethod) \
class Property_##Name { \
public: \
Property_##Name(Class* parent) : _parent(parent) { } \
Type operator = (Type value) \
{ \
_parent->SetMethod(value); \
return _parent->GetMethod(); \
} \
operator Type() const \
{ \
return static_cast<const Class*>(_parent)->GetMethod(); \
} \
Property_##Name& operator =(const Property_##Name& other) \
{ \
operator=(other._parent->GetMethod()); return *this; \
}; \
Property_##Name(const Property_##Name& other) = delete; \
private: \
Class* _parent; \
} Name { this };
// PROPERTY - Declares a property with the default getter/setter method names.
#define PROPERTY(Class, Type, Name) \
PROPERTY_GEN(Class, Type, Name, get_##Name, set_##Name)
Then use them like:
class SomeClass
{
public:
PROPERTY(SomeClass, int, Value)
int get_Value() const { return _value; }
void set_Value(int value) { _value = value; }
private:
int _value = 0;
};
int main()
{
SomeClass s, c;
s.Value = 5;
c.Value = 3 * s.Value;
s.Value = c.Value;
}
You could also add other macro variants for read-only, write-only properties and read-only non-const getters. To be able to access sub-properties via ->, you could add operator-> overloads to the macro.
Compared to microsoft's __declspec(property(...)), getter and setter methods can be made private but this isn't a real advantage since client might need to take the address of a getter/setter sometimes.
There is also a disadvantage of having an additional _parent variable for every property, and you would need to explicitly define copy constructors for parent classes if they are used.
I realize it's this question is probably too old to add another answer but to expand on Moo-Juice's answer, I've come up with a pretty neat and simple solution:
/// Utility for functions get, set & ptr.
template<typename TVal>
using GetFn = std::function<const TVal& (void)>;
template<typename TVal>
using SetFn = std::function<void(const TVal&)>;
template<typename TVal>
using PtrFn = std::function<TVal* (void)>;
/// The property class and each specialization utility.
template<typename TVal, bool Delegate, bool ReadOnly>
class Property;
template<typename TVal>
using PropertyGetSet = Property<TVal, false, false>;
template<typename TVal>
using PropertyDelGetSet = Property<TVal, true, false>;
template<typename TVal>
using PropertyGet = Property<TVal, false, true>;
template<typename TVal>
using PropertyDelGet = Property<TVal, true, true>;
/// <summary>
/// Property get-set.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, false, false>
{
public:
typedef TVal Value;
Property(const TVal& val)
: m_value(val)
{}
inline const TVal& Get() const { return m_value; }
inline void Set(const TVal& val) { m_value = val; }
inline TVal* Ptr() { return &m_value; }
private:
TVal m_value;
};
/// <summary>
/// Property delegate get-set.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, true, false>
{
public:
typedef TVal Value;
Property(GetFn<TVal> getFn, SetFn<TVal> setFn, PtrFn<TVal> ptrFn)
: m_getFn(getFn)
, m_setFn(setFn)
, m_ptrFn(ptrFn)
{}
inline const TVal& Get() const { return m_getFn(); }
inline void Set(const TVal& val) { m_setFn(val); }
inline TVal* Ptr() { return m_ptrFn(); }
private:
GetFn<TVal> m_getFn;
SetFn<TVal> m_setFn;
PtrFn<TVal> m_ptrFn;
};
/// <summary>
/// Property get.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, false, true>
{
public:
typedef TVal Value;
Property(const TVal& val)
: m_value(val)
{}
inline const TVal& Get() const { return m_value; }
inline TVal* Ptr() { return &m_value; }
private:
TVal m_value;
};
/// <summary>
/// Property delegate get.
/// </summary>
/// <typeparam name="TVal">Value type.</typeparam>
template<typename TVal>
class Property<TVal, true, true>
{
public:
typedef TVal Value;
Property(GetFn<TVal> getFn, PtrFn<TVal> ptrFn)
: m_getFn(getFn)
, m_ptrFn(ptrFn)
{}
inline const TVal& Get() const { return m_getFn(); }
inline TVal* Ptr() { return m_ptrFn(); }
private:
GetFn<TVal> m_getFn;
PtrFn<TVal> m_ptrFn;
};
And then to use it:
PropertyGetSet<std::string> strGetSet = PropertyGetSet<std::string>("GetSet");
std::string m_strGetSet = "DelGetSet";
PropertyDelGetSet<std::string> strDelGetSet =
PropertyDelGetSet<std::string>(
[&]() -> const std::string& { return m_strGetSet; },
[&](const std::string& val) { m_strGetSet = val; },
[&]() { return &m_strGetSet; /* throw? */ });
// The get (read-only) version is the same but without the set function
Some caveats:
The get function returns a const& so you are not able to use it for changing the value, this is by design as it would allow people to use the reference to set the value instead of the explicit Set which gives the advantage of knowing when the value is set.
There is no syntactic sugar for the get-set-ptr functions, personally, I didn't like using operators cause it made the underlying system more obtuse, so using explicit functions lets the user know that it's a property and not something else. But if you may, you could sprinkle some operator overloads.
All specializations have a Ptr function which will be the pointer of the data. However, when using the delegate version, you can choose to throw so anyone trying to use it will have to work around it. The reason it's there is that in the worst-case scenario you may try to use the pointer for a very particular situation, I would highly advise not to use this tho, so feel free to remove it or make an extra specialization for it.
Lastly, it's a bit verbose, you could wrap the usage in a macro to make the syntax a bit shorter, but personally, I like it the way it is as it's more explicit that way.
EDIT:
You may run into the same issue I had with this design, have a look at the following link for the issue and the solution I've come up with: https://stackoverflow.com/a/68563492/3339838
Another try to enhance Moo-Juice's answer, by defining a Getter class (which the client can only get) and a Getter-Setter class which is also assignable:
template <typename T>
class Getter {
protected:
T &_value;
public:
Getter(T &value) : _value(value) {}
operator const T() const {
return _value;
}
};
template <typename T>
class GetterSetter : public Getter<T> {
using Getter<T>::_value;
using Getter<T>::Getter;
public:
GetterSetter<T> & operator=(const T val) {
_value = val;
return *(this);
}
};
This gives you the option to decide which properties could be changed from outside the class, and which will only change internally.
No, there is not. You would just create getter and setter functions:
someThing.setText("blah");
std::string blah = someThing.getText();

What's the best way to implement AST using visitor pattern with return value?

I'm trying to implement a simple abstract syntax tree (AST) in C++ using the visitor pattern. Usually a visitor pattern does not handle return value. But in my AST there are expressions nodes which care about the return type and value of its children node. For example, I have a Node structure like this:
class AstNode
{
public:
virtual void accept(AstNodeVisitor&) = 0;
void addChild(AstNode* child);
AstNode* left() { return m_left; }
AstNode* right() { return m_right; }
...
private:
AstNode* m_left;
AstNode* m_right;
};
class CompareNode : public AstNode
{
public:
virtual void accept(AstNodeVisitor& v)
{
v->visitCompareNode(this);
}
bool eval(bool lhs, bool rhs) const
{
return lhs && rhs;
}
};
class SumNode : public AstNode
{
public:
virtual void accept(AstNodeVisitor& v)
{
v->visitSumNode(this);
}
int eval(int lhs, int rhs) const
{
return lhs + rhs;
}
};
class AstNodeVisitor
{
public:
...
bool visitCompareNode(CompareNode& node)
{
// won't work, because accept return void!
bool lhs = node.left()->accept(*this);
bool rhs = node.right()->accept(*this);
return node.eval(lhs, rhs);
}
int visitSumNode(Node& node)
{
// won't work, because accept return void!
int lhs = node.left()->accept(*this);
int rhs = node.right()->accept(*this);
return node.eval(lhs, rhs);
}
};
In this case both CompareNode and SumNode are binary operators but they rely on the return type of their children's visit.
As far as I can see to make it work, there are only 2 options:
accept can still return void, save the return value in a context object which is passed to each accept and visit function, and use them in the visit function, where I know what type to use. This should work but feels like a hack.
make AstNode a template, and accept function a none virtual, but return type depends on template parameter T.But if I do this, I no longer have a common AstNode* class and can't save any AstNode* in the children list.
for example:
template <typename T`>
class AstNode
{
public:
T accept(AstNodeVisitor&);
...
};
So is there a more elegant way to do this? This should be a fairly common problem for people implementing AST walking so I'd like to know what's the best practice.
Thanks.
The Visitor can have member that it can use to store result, something like:
class AstNodeVisitor
{
public:
void visitCompareNode(CompareNode& node)
{
node.left()->accept(*this); // modify b
bool lhs = b;
node.right()->accept(*this); // modify b
bool rhs = b;
b = node.eval(lhs, rhs);
}
void visitSumNode(Node& node)
{
node.left()->accept(*this); // modify n
int lhs = n;
node.right()->accept(*this); // modify n
int rhs = n;
n = node.eval(lhs, rhs);
}
private:
bool b;
int n;
};
You may also want to save the type of last result or use something like boost::variant.
template<class T> struct tag { using type=T; };
template<class...Ts> struct types { using type=types; }
template<class T>
struct AstVisitable {
virtual boost::optional<T> accept( tag<T>, AstNodeVisitor&v ) = 0;
virtual ~AstVisitable() {};
};
template<>
struct AstVisitable<void> {
virtual void accept( tag<void>, AstNodeVisitor&v ) = 0;
virtual ~AstVisitable() {};
};
template<class Types>
struct AstVisitables;
template<>
struct AstVisibables<types<>> {
virtual ~AstVisitables() {};
};
template<class T0, class...Ts>
struct AstVisitables<types<T0, Ts...>>:
virtual AstVisitable<T0>,
AstVisitables<types<Ts...>>
{
using AstVisitable<T0>::accept;
using AstVisitables<types<Ts...>>::accept;
};
using supported_ast_return_types = types<int, bool, std::string, void>;
class AstNode:public AstVisitables<supported_ast_return_types> {
public:
void addChild(AstNode* child);
AstNode* left() { return m_left.get(); }
AstNode* right() { return m_right.get(); }
private:
std::unique_ptr<AstNode> m_left;
std::unique_ptr<AstNode> m_right;
};
template<class types>
struct AstVisiablesFailAll;
template<>
struct AstVisiablesFailAll<> {
virtual ~AstVisiablesFailAll() {};
};
template<class T>
struct AstVisitableFailure : virtual AstVisitable<T> {
boost::optional<T> accept( tag<T>, AstNodeVisitor& ) override {
return {};
}
};
template<>
struct AstVisitableFailure<void> : virtual AstVisitable<void> {
void accept( tag<void>, AstNodeVisitor& ) override {
return;
}
};
template<class T0, class...Ts>
struct AstVisitablesFailAll<types<T0, Ts...>>:
AstVisitableFailure<T0>,
AstVisitableFailAll<types<Ts...>>
{
using AstVisitableFailure<T0>::accept;
using AstVisitableFailAll<types<Ts...>>::accept;
};
So now you can boost::optional<bool> lhs = node.left()->accept( tag<bool>, *this );, and from the state of lhs know if the left node can be evaluated in a bool context.
SumNode looks like this:
class SumNode :
public AstNode,
AstVisiablesFailAll<supported_ast_return_types>
{
public:
void accept(tag<void>, AstNodeVisitor& v) override
{
accept(tag<int>, v );
}
boost::optional<int> accept(tag<int>, AstNodeVisitor& v) override
{
return v->visitSumNode(this);
}
int eval(int lhs, int rhs) const {
return lhs + rhs;
}
};
and visitSumNode:
boost::optional<int> visitSumNode(Node& node) {
// won't work, because accept return void!
boost::optional<int> lhs = node.left()->accept(tag<int>, *this);
boost::optional<int> rhs = node.right()->accept(tag<int>, *this);
if (!lhs || !rhs) return {};
return node.eval(*lhs, *rhs);
}
The above assumes that visiting a+b in a void context is acceptable (like in C/C++). If it isn't, then you need a means for void visit to "fail to produce a void".
In short, accepting requires context, which also determines what type you expect. Failure is an empty optional.
The above uses boost::optional -- std::experimental::optional would also work, or you can roll your own, or you can define a poor man's optional:
template<class T>
struct poor_optional {
bool empty = true;
T t;
explicit operator bool() const { return !empty; }
bool operator!() const { return !*this; }
T& operator*() { return t; }
T const& operator*() const { return t; }
// 9 default special member functions:
poor_optional() = default;
poor_optional(poor_optional const&)=default;
poor_optional(poor_optional const&&)=default;
poor_optional(poor_optional &&)=default;
poor_optional(poor_optional &)=default;
poor_optional& operator=(poor_optional const&)=default;
poor_optional& operator=(poor_optional const&&)=default;
poor_optional& operator=(poor_optional &&)=default;
poor_optional& operator=(poor_optional &)=default;
template<class...Ts>
void emplace(Ts&&...ts) {
t = {std::forward<Ts>(ts)...};
empty = false;
}
template<class...Ts>
poor_optional( Ts&&... ts ):empty(false), t(std::forward<Ts>(ts)...) {}
};
which sucks, because it constructs a T even if not needed, but it should sort of work.
For completion sake I post the template version that is mentioned by the OP
#include <string>
#include <iostream>
namespace bodhi
{
template<typename T> class Beignet;
template<typename T> class Cruller;
template<typename T> class IPastryVisitor
{
public:
virtual T visitBeignet(Beignet<T>& beignet) = 0;
virtual T visitCruller(Cruller<T>& cruller) = 0;
};
template<typename T> class Pastry
{
public:
virtual T accept(IPastryVisitor<T>& visitor) = 0;
};
template<typename T> class Beignet : public Pastry<T>
{
public:
T accept(IPastryVisitor<T>& visitor)
{
return visitor.visitBeignet(*this);
}
std::string name = "Beignet";
};
template<typename T> class Cruller : public Pastry<T>
{
public:
T accept(IPastryVisitor<T>& visitor)
{
return visitor.visitCruller(*this);
}
std::string name = "Cruller";
};
class Confectioner : public IPastryVisitor<std::string>
{
public:
virtual std::string visitBeignet(Beignet<std::string>& beignet) override
{
return "I just visited: " + beignet.name;
}
virtual std::string visitCruller(Cruller<std::string>& cruller) override
{
return "I just visited: " + cruller.name;
}
};
}
int main()
{
bodhi::Confectioner pastryChef;
bodhi::Beignet<std::string> beignet;
std::cout << beignet.accept(pastryChef) << "\n";
bodhi::Cruller<std::string> cruller;
std::cout << cruller.accept(pastryChef) << "\n";
return 0;
}
Every pastry is a node and every visitor can implement its accepted return type. Having multiple visitor could visit the same pastry.