Overloading + operator on generic class in C++
I'm trying to overload the + operator in a forest class, a forest being a collection of trees, and the + operator is supposed to combine two forests into one. I have the following code as my class definition:
template<typename NODETYPE>
class Forest
{
public:
friend Forest& operator+<>(Forest&, Forest&);
friend ostream& operator<<<>(ostream&, const Forest&);
friend istream& operator>><>(istream&, Forest&);
Forest();
Forest( const Forest& otherForest);
~Forest();
void nodes(int&) const;
private:
ForestNode<NODETYPE> *开发者_如何学编程root;
ForestNode<NODETYPE> *getNewNode( const NODETYPE &);
};
The following is my implementation of operator+:
template<typename NODETYPE>
Forest& operator+<>(Forest& f1, Forest& f2)
{
f3 = new Forest();
f3.root = *f1.*root;
f3.root.sibling = *f2.*root;
*f1.root = 0;
*f2.root = 0;
return f3;
}
I get the following error on compile:
|28|error: expected constructor, destructor, or type conversion before '&' token|
line 28 refers to the signature of my operator+ implementation.
I think to correct it i am supposed to add to the return type, giving:
template<typename NODETYPE>
Forest<NODETYPE>& operator+<>(Forest& f1, Forest& f2)
{
f3 = new Forest();
f3.root = *f1.*root;
f3.root.sibling = *f2.*root;
*f1.root = 0;
*f2.root = 0;
return f3;
}
But that gives me the following errors:
|28|error: declaration of 'operator+' as non-function| |28|error: missing template arguments before '&' token| |28|error: 'f1' was not declared in this scope| |28|error: missing template arguments before '&' token| |28|error: 'f2' was not declared in this scope|
Can anyone help me with this? I'd be very very thankful.
The key to writing operator+ is don't write operator+. Instead, write a copy ctor and operator+=:
template<class NodeType>
struct Forest {
//...
Forest(Forest const &other);
//...
Forest& operator+=(Forest const &other) {
// code here
return *this;
}
//...
};
Now we add operator+:
template<class NodeType>
struct Forest {
//...
friend Forest operator+(Forest a, Forest const &b) {
a += b;
return a;
}
//...
};
And that's it! Copying is usually straight-forward (sometimes by being disallowed) and it may be simpler to think in terms of += than + (you have two objects and mutate one, rather than create a third object out of two). This pattern for op+ works with any similar type, and even for similar operators such as -, *, and /.
Operator overloading can be a good or a bad thing. Good when it leads to simpler looking code. Bad when it leads to writers either overloading with incorrect semantics (yet a solution that compiles) or where the intuitive way to use the operator leads to highly inefficient code.
Note the latter statement can apply to std::string too, which could potentially make large numbers of copies, and which is why the C++03 standard states that a string does not have to be stored internally in a contiguous buffer (in the old days they used copy-on-write references to and could store such references to both strings being concatenated until required. Subsequently it was found to be non-threadsafe and making it so was more costly than simply copying the buffer so now they copy every time and are inefficient again).
(Note that the C++11 standard which recognises threading and atomic issues ensures that the underlying does need to be contiguous and null-terminated to make read operations safe).
The correct signature of operator+ (in the case all are the same type) is as follows:
T operator+( const T&, const T& );
As a member function it would be:
class T
{
// make public if necessary
T operator+( const T& rhs ) const;
};
You can implement operator+ automatically as a template whenever operator += is available with
template<typename T, typename R>
T operator+( const T& lhs, const R& rhs )
{
T copy(lhs);
return copy += rhs;
}
If you want to declare an overloaded operator of your template as a friend, this is the correct way to do it. I will show it with operator<<
// first some forward declarations, assume ostream already declared with #include <iosfwd> minimum
template< typename T > class Forest;
template< typename T > std::ostream & operator<<( std::ostream & os, const Forest<T> & for );
template< typename T> class Forest
{
friend std::ostream& operator<< <>( std::ostream&, const Forest<T> & );
//rest of class Forest
};
template< typename T >
std::ostream & operator<<( std::ostream& os, const Forest<T> & forest )
{
// implement
return os;
}
You would apply a similar technique to any other external function you wish to declare as a friend to your class, i.e.
- Forwardly declare your class as a template
- Forwardly declare the method as a template function
- Make the function a friend using <> before the opening parentheses denoting the parameters
- Implement the function after your class.
You have to provide a template arguments for all Forest
parameters.
template<typename NODETYPE>
Forest<NODETYPE> operator+(Forest<NODETYPE>& f1, Forest<NODETYPE>& f2)
Also, consider making the arguments const references to make sure you do not manipulate them.
There are several questions on stackoverflow regarding friend function templates. The C++ FAQ also has a page on them that explains some basics.
You can define an operator+
template as follows:
template< class NodeType >
Forest<NodeType> operator+( Forest<NodeType> const& f1, Forest<NodeType> const& f2)
{
// Implementation.
}
Cheers & hth.,
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