operator[][] C++
I'd like to overload operator[][]
to give internal access to a 2D array of char in C++.
Right now I'm开发者_开发百科 only overloading operator[]
, which goes something like
class Object
{
char ** charMap ;
char* operator[]( int row )
{
return charMap[row] ;
}
} ;
It works ok.. Is it possible to override operator[][]
though?
Don’t try to do that – as others have said, overloading operator []
the way you do actually provides the [][]
syntax for free. But that’s not a good thing.
On the contrary – it destroys the encapsulation and information hiding of your class by turning an implementation detail – the char*
pointer – to the outside. In general, this is not advisable.
A better method would be to implement an operator [,]
which takes more than one argument, or indeed an operator [][]
. But neither exists in C++.
So the usual way of doing this is to ditch operator []
altogether for more than one dimension. The clean alternative is to use operator ()
instead because that operator can have more than one argument:
class Object
{
char ** charMap ;
char& operator ()(int row, int column)
{
return charMap[row][column];
}
};
For more information, see the article in the C++ FAQ Lite.
There is no operator [][]
: that's two []
operations in a row. You could:
- Have
Object::operator[]
return an object of a second class representing a row, which has its ownoperator[]
method that takes a column number; - Write a
get(int row, int column)
method and use that instead of operator overloading. I'd recommend this unless your object absolutely has to behave like an array.
There is no operator[][]
. Evaluating a[x][y]
first calls operator[]
on a
, and then operator[]
again on the result of that.
So the operator[]
of your object has to return another object with its own operator[]
, which then will access the requested value.
As far as I know there is no such thing as operator[][]
. What you can do is you could return from your operator[]
method something that has overloaded operator[]
.
Actually you are doing it now, because you return char*
which can be indexed using []
again.
There is no [][]
operator. What actually happens is that the second []
operates on the variable returned by the first []
. Because there is already that functionality, it would create ambiguity were there to exist a [][]
operator.
For example: let's say you have a variable x
of some type T
.
T x = new T();
If we use the []
operator, let's say a variable of other type Q
is returned:
Q y = x[0];
And then using the []
operator on a variable of type Q
might return a variable of type R
:
R z = y[0];
Therefore x[][]
returns a variable of t ype R.
Let's say we actually were able to overload [][]
for type T such that it returned a type S:
S a = x[0][0];
The compiler would have no way of knowing if it should use the [][]
operator on x
to return a type S
variable, or use the []
operator twice in a row to return a type R
variable. This is the ambiguity I mentioned above.
Your best bet if you're stuck on using square brackets is to have operator[]
return a variable which also has []
overloaded (or perhaps a variable of the same type, with a flag set), and have that initially returned variable deal with the second []
.
But the best solution here (as mentioned already in another answer) is to use a different operator such as ()
.
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