Is there something like Java equals()? To compare if object is the same type ?
public boolean equals(Object obj) {
if (obj == null || !(obj instanceof ViewMode)) {
return false;
}
ViewMode dm = (ViewMode) obj;
return dm.width == w
&& dm.h == h
&& dm.b == b
&& dm.f == f;
}
public int hashCode() {
return w ^ h ^ f ^ b ;
}
For the idiomatic equivalent of your example, you would define operator== as follows:
friend bool operator==(const ViewMode &lhs, const ViewMode &rhs) {
return (lhs.w == rhs.w) && the rest;
}
friend bool operator!=(const ViewMode &lhs, const ViewMode &rhs) {
return !(lhs == rhs);
}
In C++ you don't normally write a function to allow ViewMode objects to be compared with something that has nothing at all to do with ViewMode. I suppose that if you really wanted that comparison to return false, rather than refusing to compile, then you could add a couple of template operators (as free functions, outside the class):
template <typename T>
bool operator==(const ViewMode &, const T&) {
return false;
}
template <typename T>
bool operator==(const T &, const ViewMode&) {
return false;
}
but I really don't recommend it. That Java idiom doesn't apply to C++, because in C++ you pretty much should never have an object, but have no idea of its type.
If you want your equals function to be virtual, then it's probably best to write an equals() virtual function, rather than using operator== for it. You'd write it to take a const ViewObject & as parameter, so no need for any equivalent to the instanceof check. Which is just as well, because C++ does not have any way to take an object of totally unknown type and test whether it is an instance of a specified type.
You rarely need a polymorphic equals function in C++, but if you were using it for example in std::unordered_map, then you'd specify the extra template parameters to the unordered_map. Give it an equality comparison function that takes two pointers and calls equals on one or the other, and a hash function that does something sensible.
No. C++ does not have a global type model like Java. There is no Object type from which all other types inherit, so there are no methods (like equals) that are defined for all classes.
C++ provides a framework for building a type model with a universal comparison operation: operator ==. It is up to you to build out all of the implementations of this in all of your classes so that they interact correctly. But there is no default implementation comparable to Object.equals.
You can define the operator== in your class.
You might be able to use the typeid operator for this.
Related
Say I have a class called book:
class Book {
int i;
public:
Book(int ii)
: i(ii) {
}
int ISBN() {
return i;
}
};
I want to overload the comparison operator for the Book class, so that I can create a bool function that will compare the member "i" when it comes across book1==book2.
bool is_same() {
return (book1==book2) ? true : false;
}
How would I go about this? This is the current operator overload function I have, it gives me an "invalid initialization of non-const reference of type 'Book&' from an rvalue of the type 'bool'" error. I currently have my overloaded function inside of the class Book as a public function.
Book& operator==(const Book& b) const {
return ISBN() == b.ISBN();
}
I'm relatively new to operator overloading, I have sifted through many answers but none of them resolve my issue. I understand how one could simply do book1==book2, but that would only return true if every single member was of the same value. In this case I have more than just one, but I only want to return true if "i" is the same for both objects.
You basically have 2 choices:
use a member operator with one argument:
class Book {
...
bool operator==( const Book &an ) const { return ISDN() == an.ISDN(); }
};
use a non-member operator (and possibly a friend statement) with 2 arguments:
bool operator==( const Book &b1, const Book &b2 )
{
return b1.ISBN() == b2.ISBN();
}
Note that ISDN() should be made const.
Either way, you need to return a bool, not a Book &, which is usually returned by the assignment operator =, not the comparison operator ==.
I am trying to overload the == operator in C++.
#include <string>
using namespace std;
namespace date_independent
{
class clock
{
public:
int clockPair[2] = {0,0};
operator string() const
{
string hourString;
string minString;
if(clockPair[0]<10)
{
hourString = "0"+to_string(clockPair[0]);
}
else
{
hourString = to_string(clockPair[0]);
}
if(clockPair[1]<10)
{
minString = "0"+to_string(clockPair[1]);
}
else
{
minString = to_string(clockPair[1]);
}
return hourString+":"+minString;
};
bool operator ==(const clock&clockOne, const clock&clockTwo) const
{
return string(clockOne)==string(clockTwo);
};
};
};
There is much more code than I have included, but this is the important part. I want it so that the == operator can compare two objects of class clock. E.g., object1==object2. Is there anybody that can help me?
A binary operator like == can be overloaded either as a member function with a single parameter (this being the left-hand operand, and the parameter being the right-hand one), or as a non-member function with two parameters for the two operands.
So either
move your operator declaration outside the class declaration (moving the definition to a source file, or declaring it inline if you keep the definition in the header); or
add friend to the definition, so that it declares a non-member in the surrounding namespace; or
remove the first argument from the member function, using this instead.
As a member, it would look like
bool operator==(const const & clockTwo) const {
return string(*this) == string(clockTwo);
}
You might also want to compare the two integer values directly to save the expense of making strings. You should also remove the rogue ; after the function and namespace definitions, although most modern compilers shouldn't object to their presence.
Your comparison function has been written to take two clock objects and compare them, so it should be a non-member function (after the class definition), without the const qualifier.
bool operator==(const clock& clockOne, const clock& clockTwo)
{
return string(clockOne) == string(clockTwo);
}
When you have an operator inside the class definition, the left-hand argument is implicitly provided for you (it's *this), so if you wanted to implement it there you'd need something like:
bool operator==(const clock& clockTwo) const
{
return string(*this) == string(clockTwo);
}
Still, that's not recommended for == as if you have say an implicit constructor from another type T, you won't be able to write code ala my_t == my_clock with the member version unless T provides a suitable comparison operator (for clock or string). A non-member operator gives more symmetric operation.
Overloading can be done inside or outside the class definition. If you want to do it inside, the function receives only one argument. You should compare this with that argument.
bool operator ==(const clock&clockTwo) const
{
return string(*this)==string(clockTwo);
}
Note the const after the argument, it means that you won't change this inside the function.
On the other hand, if you want to do it outside the class definition, it needs two arguments, and you should compare them.
bool operator ==(const clock&clockOne, const clock&clockTwo)
{
return string(clockOne)==string(clockTwo);
}
Also note that it'll be faster to compare the clockPair of the objects rather than making the string and comparing them.
Though your question is poorly worded I believe that you are asking why the operator you've defined is not working?
If you are defining the operator as a member of the class it only takes one parameter. For example:
class clock {
bool operator ==(const clock& rhsClock) const
{
// Note: this is the lhsClock
return string(*this) == string(otherClock);
}
};
When you define the operator as a free function (not as a part of the class) then you need to define both parameters:
class clock {
// ... class declaration ...
};
bool operator ==(const clock& lhsClock, const clock& rhsClock)
{
return string(lhsClock) == string(rhsClock)
}
Where the comparison would look like this:
if (lhsClock == rhsClock) // ... do something ...
This question already has answers here:
What are the basic rules and idioms for operator overloading?
(8 answers)
Closed 8 years ago.
i have a class which needs to implement the operator== and return type of it is bool. I'm not sure is below code the correct way to implement it.
//MyTest.hpp file
class Test {
public:
Test();
virtual ~Test();
int x,y;
bool operator==(const Test &v);
};
// MyTest.cpp file
bool Test::operator==(const Test &v) {
return (x== v.x) && (y==v.y);
}
even though the code compiles is this standard way to implement we need to use template.
Should i use template way of implementation like below code :
template <class T>
bool operator==(const T &v);
Don't use templates unless you need to. Does it make sense for Test to be comparable with any type? I doubt so.
Also there's no "Standard way" because the standard does not impose a strict way to implement operator== (I think it mentions something on the return type, but I doubt it actually enforces anything).
It's a good idea to make it return a bool and make it does what people expect it does (compare the two objects in a meaningful way).
And finally, you should mark your current one const:
bool operator==(const Test &v) const;
Unless it actually modifies the invoking object, which is definitely something you don't want it to do.
As others have mentioned, using a template without any reason to do so is neither standard nor recommended. However assuming you want operator== to be more like one of the default comparison operators you should either mark it const (otherwise you cannot compare two const Test) like so
class Test {
public:
Test();
virtual ~Test();
int x,y;
bool operator==(const Test &v) const;
};
// MyTest.cpp file
bool Test::operator==(const Test &v) const {
return (x== v.x) && (y==v.y);
}
, or make it a free function (in the same namespace for ADL) taking two const Test & (preferred) like so
class Test {
public:
Test();
virtual ~Test();
int x,y;
};
bool operator==(const Test &a, const Test &b);
// MyTest.cpp file
bool operator==(const Test &a, const Test &b) {
return (a.x==b.x) && (a.y==b.y);
}
If any access to private members is required the free function can always be marked friend. In simple cases such as the one in this example it can be advantageous to furthermore define it in the header and marking it inline.
Please do not use templates matching anything:
template <class T>
bool operator==(const T &, const T &);
That one will put you in all kinds of trouble !
I would like to expose C++ code with a
std::vector<A>
to python. My
class A{};
does not have a comparison operator implemented. When I try
BOOST_PYTHON_MODULE(libmyvec)
{
using namespace boost::python;
class_<A>("A");
class_<std::vector<A> >("Avec")
.def(boost::python::vector_indexing_suite<std::vector<A> >());
}
I get an error about comparison operators. If I change the definition of A to
class A {
public:
bool operator==(const A& other) {return false;}
bool operator!=(const A& other) {return true;}
};
It works like a charm.
Why do I need to implement these comparison operators? Is there any way to use the vector_indexing_suite without them?
vector_indexing_suite implements a __contains__ member function, which requires the presence of an equality operator. As a consequence, your type must provide such an operator.
The sandbox version of Boost.Python solve this issue by using traits to determine what kind of operations are available on containers. For instance, find will only be provided if the values are equality comparable.
By default, Boost.Python consider all values to be equality comparable and less-than comparable. Since your type does not meet these requirements, you need to specialize the traits to specify what operations it supports:
namespace indexing {
template<>
struct value_traits<A> : public value_traits<int>
{
static bool const equality_comparable = false;
static bool const lessthan_comparable = false;
};
}
This is documented here.
I'm making some stack, in which I need to uses this kind of comparison in some function. But I got stuck since I don't know how the prototype for this should look like.
I have the following line in a function.
template <class T>
void function1(T i)
{
if(i == 'a')
//do something
}
I wonder know how the overload prototype should look like for it?
EDIT
Dunno if it's worth to mention, anyway this is what I have tried so far
template
bool Stack<T>::operator==(char c) const
{
cout << c << endl; // just some test
}
No need to comment how this function works, as I have not finished it yet. This part will compile, however at the part where I call this function for the first time is in the Stack::push(T i). The compiler will complain that there are no matching function for this.
error: no match for 'operator==' in 'i == '#''
For overloading operators, the name of the function is operator followed by the actual operator, so operator==. It returns bool. I don't know what your arguments should be based on your code. Probably Stack<T>&, and you need two of them to compare if it's a free function, and one to compare to this if it's a member function.
If you have ways to convert to a Stack<T>, then prefer a free function so that you can convert the left-hand-side.
I'm not sure I understand your question. In order for an instantiation of template function function1 to be well-formed, you'll have to provide a operator== which compares a T and (I'll suppose) a char.
Now, you have two options here :
Provide a bool operator==(char) const member function in your type, for example :
struct A {
bool operator==(char) const { /* ... */ }
};
function1(A()); // OK : comparison uses A::operator==(char)
Provide bool operator==(const T &, char) as a free function, for example :
struct A { /* ... */ };
bool operator==(const A &, char) { /* ... */ }
function1(A()); // OK : comparison uses operator==(const A &, char)
So every T in your function1(t) has to implement operator ==;
For example, as a member function
class A
{
public:
bool operator == (char) const;
};
or a non-member operator:
class A
{
public:
friend bool operator == (const A&, char);
};