I would like the following to compile, but it does not:
template <typename T>
struct Odp
{
public:
operator*() const
{
return m_p;
}
T* operator->() const
{
return m_p;
}
T** operator&()
{
return &m_p;
}
private:
T* m_p;
};
struct Ftw : public Odp<int>
{
bool operator==(const Ftw& rhs)
{
return m_p == rhs.m_p; // C2248 cannot access private member
}
};
Is there any way to make this work? I can't modify Odp.
Odp overloads operator* to return m_p. You can invoke the operator on *this and rhs:
struct Ftw : public Odp<int>
{
bool operator==(const Ftw& rhs) const
{
return **this == *rhs;
}
};
The operator* overload is a bit unusual, however: it should probably return *m_p instead, since operator-> returns m_p (this would result in your class having consistent pointer-like semantics). If you did this, you would then have to do the following to do the comparison:
return &**this == &*rhs; // or explicitly as:
return &this->operator*() == &rhs.operator*();
This is getting a bit messy, and it won't necessarily work if the unary & is overloaded for T (but, you really, really shouldn't do that...). You can also obtain the pointer by explicitly calling operator->, which might be preferable:
return this->operator->() == rhs.operator->();
The real question is, "what is this Odp, why are you using it, and why can you not modify it?"
On an unrelated note, your operator== should either be implemented as a const member function or, preferably, as a friend function:
bool operator==(const Ftw& rhs) const { /* ... */ }
friend bool operator==(const Ftw& lhs, const Ftw& rhs) { /* ... */ }
On another unrelated note, overloading the unary & is almost certainly a bad idea.
The compiler is telling you that m_p is private. If you want to access m_p in the derived class you need to make it either protected or public.
Since Odp is giving the pointer out for free in its methods (even the address of it, OMG! it's like making door with many locks and then giving the keys to every thief around), you can just do
bool operator==(const Ftw& rhs)
{
return **this == *rhs;
}
Had Odp implemented its own comparison operator, you could use it like this:
bool operator==(const Ftw& rhs)
{
return Odp<int>::operator==(rhs) && ... other conditions ...;
}
If you can't modify Odp, you can call operator->() explicitly. It returns what you need and should get inlined.
Related
Hello I am writing code for school project.
In my current implementation I am using polymorphism in some cases. Like for example when I have a set or a vector:
std::vector<Parent *> vec;
vec.push_back(new Child1()); // adding instance of derived class
vec.push_back(new Child2()); // adding other instance of derived class
I can use this code to add objects of derived classes, but in case of using a set for example, I cannot prevent duplicated by using this method, because the set will compare the memory address to another, not the objects themselves, so I want to use wrappers. On another note, my teacher suggests to use smart pointers, like unique_ptr or shared_ptr so that memory is properly cleaned up. I find the second option to be easier to work with. In any case even if I have:
std::set<std::shared_ptr<Parent>> s;
s.insert(std::make_shared<Child1>(Child1());
// ...
s.insert(std::make_shared<ChildN>(ChildN());
It still works just with regular pointers and duplicates are allowed. So I want to write a wrapper class to prevent this from happening and I have something like this:
template<typename T>
class PolymorphicWrapper {
private:
std::shared_ptr<T> ptr;
public:
PolymorphicWrapper(const std::shared_ptr<T> &ptr) : ptr(ptr) {}
const std::shared_ptr<T> &getPtr() const {
return ptr;
}
void setPtr(const std::shared_ptr<T> &ptr) {
PolymorphicWrapper::ptr = ptr;
}
bool operator==(const PolymorphicWrapper &rhs) const {
return *ptr == *rhs.ptr;
}
bool operator!=(const PolymorphicWrapper &rhs) const {
return rhs != *this;
}
bool operator<(const PolymorphicWrapper &rhs) const {
return *ptr < *rhs.ptr;
}
bool operator>(const PolymorphicWrapper &rhs) const {
return rhs < *this;
}
bool operator<=(const PolymorphicWrapper &rhs) const {
return rhs >= *this;
}
bool operator>=(const PolymorphicWrapper &rhs) const {
return *this >= rhs;
}
};
But this approach doesn't work with derived classes at all!
For example:
std::set<PolymorphicWrapper<Parent>> s;
s.insert(PolymorphicWrapper<Parent>(std::make_shared<ChildN>(ChildN()); // doesn't work
Is there a simple fix? I am not too good with programming and have trouble understating hard solutions. But it is an exam I have to pass to keep going with other subjects.
The std::set template lets you specify a key comparison function. Instead of messing with the pointer class, just use a comparison function for the pointers that compares what they point to.
See: https://en.cppreference.com/w/cpp/container/set
class Media {
public:
bool operator==(const Media& other) const {}
bool operator!=(const Media& other) const {}
};
class Book : public Media {
public:
bool operator==(const Book& other) const {} // commenting out this line solves this issue.
bool operator!=(const Book& other) const {}
};
class Game : public Media {
public:
bool operator==(const Game& other) const {}
bool operator!=(const Game& other) const {}
};
int main() {
Book book;
Game game;
bool res = book == game; // doesn't compile.
}
I have these 3 classes and they must have their own == and != operators defined. But then I also have to compare between two siblings using those operators.
I could've written a (pure) virtual function, say, virtual bool equals(const Media& other) const in the base class that subclasses override. And then call that function in the bodies of == and != opertor definition in base class Media. But that feature is gone when I add another bool operator==(const Book& other) const {} in the Book class (the same goes for the Game class too).
Now I want to compare between siblings using those operators and still have all 6 definition in those 3 classes. How do I make it work?
You mentioned in the comments that this form of comparison is an imposed restriction (to compare among siblings of a child type). If its an imposed restriction that you need to somehow perform this with inheritance, then one option is to fulfill the base signature and use dynamic_cast. Note that this is not a clean approach, but it might be the expected solution for this problem if this is some form of assignment.
dynamic_cast uses Runtime Type Information (RTTI) to determine whether an instance to a base class is actually an instance of the derived class. When you use it with a pointer argument, it returns nullptr on failure -- which is easily testable:
auto p = dynamic_cast<const Book*>(&other);
if (p == nullptr) { // other is not a book
return false;
}
// compare books
You can use this along with a virtual function to satisfy the hierarchy. However, to avoid possible ambiguities with c++20's generated symmetric operator==/operator!= functions, it's usually better to do this through a named virtual function rather than the operator== itself in order to prevent ambiguity:
class Media {
public:
virtual ~Media() = default;
bool operator==(const Media& other) const { return do_equals(other); }
private:
virtual bool do_equals(const Media& other) const = 0;
};
class Book : public Media {
...
private:
bool do_equals(const Media& other) const override {
auto* p = dynamic_cast<const Book*>(&other);
if (p == nullptr) { return false; }
return (... some comparison logic ...);
}
...
};
... Same with Game ...
Since we never define operator==(const Book&) or operator==(const Game&), we won't see this shadow the base-class' operator==; instead it always dispatches through the base's operator==(const Media&) -- which is non-virtual and prevents ambiguity.
This would allow a Book and a Game to be comparable, but to return false -- whereas two Book or two Game objects may be compared with the appropriate logic.
Live Example
That said...
This approach is not a good design, as far as software architecture goes. It requires the derived class to query what the type is -- and usually by the time you need to do this, that's an indication that the logic is funky. And when it comes to equality operators, it also leads to complications with symmetry -- where a different derived class may choose to compare things weirdly with different types (imagine a Media that may compare true with other different media; at which point, the order matters for the function call).
A better approach in general is to define each of the respective equality operators between any types that logically require equality comparison. If you are in C++20 this is simple with symmetric equality generation; but pre-C++20 is a bit of a pain.
If a Book is meant to be comparable to a Game, then define operator==(const Game&) or operator==(const Book&, const Game&). Yes, this may mean you have a large number of operator==s to define for each of them; but its much more coherent, and can get better symmetry (especially with C++20's symmetric equality):
bool operator==(const Game&, const Book&);
bool operator==(const Book&, const Game&); // Generated in C++20
bool operator==(const Game&, const Game&);
bool operator==(const Book&, const Book&);
In an organization like this, Media may not even be logical as a 'Base class'. It may be more reasonable to consider some form of static polymorphism instead, such as using std::variant -- which is touched on in #Jarod42's answer. This would allow the types to be homogeneously stored and compared, but without requiring casting from the base to the derived type:
// no inheritance:
class Book { ... };
class Game { ... };
struct EqualityVisitor {
// Compare media of the same type
template <typename T>
bool operator()(const T& lhs, const T& rhs) const { return lhs == rhs; }
// Don't compare different media
template <typename T, typename U>
bool operator()(const T&, const U&) const { return false; }
};
class Media
{
public:
...
bool operator==(const Media& other) const {
return std::visit(EqualityVisitor{}, m_media, other.m_media);
}
private:
std::variant<Book, Game> m_media;
};
Live Example
This would be my recommended approach, provided the forms of media are meant to be fixed and not extended.
You might do double dispatch thanks to std::visit/std::variant (C++17):
class Media;
class Book;
class Game;
using MediaPtrVariant = std::variant<const Media*, const Book*, const Game*>;
class Media {
public:
virtual ~Media () = default;
virtual MediaPtrVariant asVariant() const { return this; }
};
class Book : public Media {
public:
MediaPtrVariant asVariant() const override { return this; }
};
class Game : public Media {
public:
MediaPtrVariant asVariant() const override { return this; }
};
struct EqualVisitor
{
template <typename T>
bool operator()(const T*, const T*) const { return true; }
template <typename T, typename U>
bool operator()(const T*, const U*) const { return false; }
};
bool operator ==(const Media& lhs, const Media& rhs)
{
return std::visit(EqualVisitor(), lhs.AsVariant(), rhs.AsVariant());
}
bool operator !=(const Media& lhs, const Media& rhs)
{
return !(lhs == rhs);
}
int main()
{
Book book;
Game game;
bool res = book == game;
}
Demo
So in the interest of creating a Minimal. Complete, Verifiable Example I have created a toy iterator here (I know it's not perfect, it's just for the purposes of asking a question):
class foo : public iterator<input_iterator_tag, string> {
string _foo;
static const size_t _size = 13;
public:
const string& operator*() { return _foo; }
const foo& operator++() {
_foo += '*';
return *this;
}
const foo operator++(int) {
auto result = *this;
_foo += '*';
return result;
}
bool operator==(const foo& rhs) { return _foo.empty() != rhs._foo.empty() && _foo.size() % _size == rhs._foo.size() % _size; }
bool operator!=(const foo& rhs) { return !operator==(rhs); }
};
I read that an InputIterator needs to have defined the Member Selection Operator. The Indirection Operator makes sense, but a Member Selection Operator is confusing to me here. How would an Member Selection Operator be implemented for foo?
const string* operator->() const { return &_foo; }
Example usage:
foo i;
++i;
assert(i->length() == 1);
The way this works is that the compiler will generate repeated calls to operator-> until the return type is a raw pointer (so in this case just one call to foo::operator->), then do the regular member selection operation on that pointer.
The operator->() should return a pointer type of the type the container holds that the iterator is used on. So if you have a container that holds a std::string then the iterator::operator-> should return a std::sting*. In your case since you derive from std::iterator you can use the pointer typedef for the return type.
I'm trying to overload the assignment operator and would like to clear a few things up if that's ok.
I have a non member function, bool operator==( const MyClass& obj1, const myClass& obj2 ) defined oustide of my class.
I can't get at any of my private members for obvious reasons.
So what I think I need to do is to overload the assignment operator. And make assignments in the non member function.
With that said, I think I need to do the following:
use my functions and copy information using strcpy or strdup. I used strcpy.
go to the assignment operator, bool MyClass::operator=( const MyClass& obj1 );
Now we go to the function overloading (==) and assign obj2 to obj1.
I don't have a copy constructor, so I'm stuck with these:
class Class
{
private:
m_1;
m_2;
public:
..
};
void Class::Func1(char buff[]) const
{
strcpy( buff, m_1 );
return;
}
void Class::Func2(char buff[]) const
{
strcpy( buff, m_2 );
return;
}
bool Class& Class::operator=(const Class& obj)
{
if ( this != &obj ) // check for self assignment.
{
strcpy( m_1, obj.m_1 );
// do this for all other private members.
}
return *this;
}
bool operator== (const Class& obj1, const Class& obj2)
{
Class MyClass1, MyClass2;
MyClass1 = obj1;
MyClass2 = obj2;
MyClass2 = MyClass1;
// did this change anything?
// Microsofts debugger can not get this far.
return true;
}
So as you can probably tell, I'm completely lost in this overloading. Any tips? I do have a completed version overloading the same operator, only with ::, so my private members won't lose scope. I return my assignments as true and it works in main. Which is the example that I have in my book.
Will overloading the assignment operator and then preforming conversions in the operator== non member function work? Will I then be able to assign objects to each other in main after having completed that step?
You have a couple of obvious mistakes here and there is some confusion about what you are actually trying to achieve. Firstly, the assignment operator operator = is meant to copy the value from one instance to another. The return value of the assignment operator is almost always a non constant reference to the target of the copy, so that you can chain assignments:
Class & operator=(const Class &rhs)
{
// copy the members
return *this;
}
The comparison operator operator == is meant to perform a comparison of two instances. It returns a boolean true if they are equal:
boolean operator==(const Class &rhs) const
{
// check if they are equal
return something;
}
The confusion is why are you trying to copy values around, or maybe assign to the instances in the comparison operator?
Op== isn't the assignment operator. T& Op= (const T&) is.
bool operator==(const T& lhs, const T& rhs) is the operation to compare two Ts. It returns true if lhs is equal to rhs, for whatever definition of "equal" you want to code.
I am guessing that you want to compare the two objects. In that case, you can just overload the operator == in class "Class". You don't need assignment operator.
class Class
{
public:
Class(int i) : m_i(i){}
bool operator==( const Class& rhs)
{
return m_i == rhs.m_i;
}
private:
int m_i;
};
int main()
{
Class t1(10), t2(10);
bool b = (t1 == t2);
}
I am not sure whether I understood the question correctly. But if you trying to check the equality using a non-member function and can't do this only because you can't access the private members of the class, then you can declare the non-member function as a friend function and use it like this:
class Test
{
public:
Test(int i) : m_i(i){}
private:
int m_i;
friend bool operator==(Test& first, Test& second);
};
bool operator==(Test& first, Test& second)
{
return first.m_i == second.m_i;
}
int main()
{
Test t1(10), t2(10);
bool b = (t1 == t2);
}
In my classes I often write a quick operator!= by returning !(*this == rhs), e.g.:
class Foo
{
private:
int n_;
std::string str_;
public:
...
bool operator==(const Foo& rhs) const
{
return n_ == rhs.n_ && str_ == rhs.str_;
}
bool operator!=(const Foo& rhs) const
{
return !(*this == rhs);
}
};
I can't see any obvious problems with doing this but thought I'd ask if anyone knows of any.
I believe that's the preferred method of implementing operator!= so that you don't repeat yourself, and you have a guaranteed correct relationship with operator==.
Defining operator!= as !operator== is just fine
For getting these trivial equivalent operators easily defined, I always use Boost.Operators.
The case with only operator== and operator!= (i.e. using equality_comparable<>) doesn't gain very much.
But when you need less and greater than too, or some combination of operator+, operator* etc. this becomes very convenient.
An example for your case would read
class Foo : private boost::equality_comparable< Foo >
{
private:
int n_;
std::string str_;
public:
...
bool operator==(const Foo& rhs) const
{
return n_ == rhs.n_ && str_ == rhs.str_;
}
};
No, that's absolutely fine - I do exactly the same.