具有函数运算符的C ++数组类，用于赋值

Question

I want to write my own 2d array class. I want the class to be able to assign a value to an element like this.

a(0,0) = 1

I know this must be possible because I can do it with the matrix classes from blaze and eigen . My first idea was to overload the function operator () , which already works great to access the elements. When I try to assign a value with a(0,2) = 0; I get a compiler error.

lvalue required as left operand of assingment

What operator do I have to overload that the assignment also works?

Answer 1

You need to build a function with this prototype:

T& operator()(std::size_t, std::size_t);

Where T is your element type. This function needs to return a reference to an element in the array. This allows you to modify the value of the element via that reference in the caller.

As well as the above, you ought to provide the function

const T& operator()(std::size_t, std::size_t) const;

for read-only element access. I've rather dogmatically insisted on std::size_t for your indexing. In reality, you might want a typedef of your own to stand in for the index type.

Answer 2

You need to provide two overloads - one const and another one not, returning a reference. For example:

template <typename TVal> class val {
  std::map<int, TVal> values;

public:
  // the distinction is between a const version ...
  TVal operator()(int key) const {
    auto found = values.find(key);
    if (found != values.end())
      return found->second;

    return TVal();
  }

  // ... and a non-const, returning a reference
  TVal& operator()(int key) {
   return values[key];
  }
};

and then you can use it as follows:

val<int> v;
v(1) = 41;
v(1)++;
std::cout << v(1) << std::endl; // -> 42
std::cout << v(2) << std::endl; // -> 0

Answer 3

Please post the compile-error, for others to see and precisely answer it.

Having said that, I am pretty sure that it is because you are not returning a reference from your operator.