Using the subscript operator to access elements in a collection

Accessing elements of an array is a basic feature not just in C++ but also in any programming language that supports arrays. The syntax is also the same across many programming languages. In C++, the subscript operator used for this purpose, [], can be overloaded to provide access to data in a class. Typically, this is the case for classes that model containers. In this recipe, we’ll see how to leverage this operator and what changes C++23 brings.

How to do it…

To provide random access to elements in a container, overload the subscript operator as follows:

• For one-dimensional containers, you can overload the subscript operator with one argument, regardless of the version of the standard:

  template <typename T>
  struct some_buffer
  {
     some_buffer(size_t const size):data(size)
     {}
     size_t size() const { return data.size(); }
     T const& operator[](size_t const index) const
     {
        if(index >= data.size())
           std::runtime_error("invalid index");
        return data[index];
     }
     T & operator[](size_t const index)
     {
        if (index >= data.size())
           std::runtime_error("invalid index");
        return data[index];
     }
  private:
     std::vector<T> data;
  };
  

• For multidimensional containers, in C++23, you can overload the subscript operator with multiple arguments:

  template <typename T, size_t ROWS, size_t COLS>
  struct matrix
  {
     T& operator[](size_t const row, size_t const col)
     {
        if(row >= ROWS || col >= COLS)
           throw std::runtime_error("invalid index");
        return data[row * COLS + col];
     }
     T const & operator[](size_t const row,                         size_t const col) const
     {
        if (row >= ROWS || col >= COLS)
           throw std::runtime_error("invalid index");
        return data[row * COLS + col];
     }
  private:
     std::array<T, ROWS* COLS> data;
  };
  

How it works…

The subscript operator is used to access elements in an array. However, it is possible to overload it as a member function in classes typically modeling containers (or collections in general) to access its elements. Standard containers such as std::vector, std::set, and std::map provide overloads for the subscript operator for this purpose. Therefore, you can write code as follows:

std::vector<int> v {1, 2, 3};
v[2] = v[1] + v[0];

In the previous section, we saw how the subscript operator can be overloaded. There are typically two overloads, one that is constant and one that is mutable. The const-qualified overload returns a reference to a constant object, while the mutable overload returns a reference.

The major problem with the subscript operator was that, prior to C++23, it could only have one parameter. Therefore, it could not be used to provide access to elements of a multidimensional container. As a result, developers usually resorted to using the call operator for this purpose. An example is shown in the following snippet:

template <typename T, size_t ROWS, size_t COLS>
struct matrix
{
   T& operator()(size_t const row, size_t const col)
   {
      if(row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
   T const & operator()(size_t const row, size_t const col) const
   {
      if (row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
private:
   std::array<T, ROWS* COLS> data;
};
matrix<int, 2, 3> m;
m(0, 0) = 1;

To help with this, and allow a more consistent approach, C++11 made it possible to use the subscript operator with the syntax [{expr1, expr2, …}]. A modified implementation of the matrix class that leverages this syntax is shown next:

template <typename T, size_t ROWS, size_t COLS>
struct matrix
{
   T& operator[](std::initializer_list<size_t> index)
   {
      size_t row = *index.begin();
      size_t col = *(index.begin() + 1);
      if (row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
   T const & operator[](std::initializer_list<size_t> index) const
   {
      size_t row = *index.begin();
      size_t col = *(index.begin() + 1);
      if (row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
private:
   std::array<T, ROWS* COLS> data;
};
matrix<int, 2, 3> m;
m[{0, 0}] = 1;

However, the syntax is rather cumbersome and was probably rarely used in practice. For this reason, the C++23 standard makes it possible to overload the subscript operator using multiple parameters. A modified matrix class is shown here:

template <typename T, size_t ROWS, size_t COLS>
struct matrix
{
   T& operator[](size_t const row, size_t const col)
   {
      if(row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
   T const & operator[](size_t const row, size_t const col) const
   {
      if (row >= ROWS || col >= COLS)
         throw std::runtime_error("invalid index");
      return data[row * COLS + col];
   }
private:
   std::array<T, ROWS* COLS> data;
};
matrix<int, 2, 3> m;
m[0, 0] = 1;

This makes the calling syntax consistent with accessing one-dimensional containers. This is used by std::mdspan to provide element access. This is a new C++23 class that represents a non-owning view into a contiguous sequence (such as an array), but it reinterprets the sequence as a multidimensional array.

The matrix class shown previously can actually be replaced with an mdspan view over an array, as shown in the following snippet:

int data[2*3] = {};
auto m = std::mdspan<int, std::extents<2, 3>> (data);
m[0, 0] = 1;

See also

• Chapter 5, Writing your own random-access iterator, to see how you can write an iterator for accessing the elements of a container
• Chapter 6, Using std::mdspan for multidimensional views of sequences of objects, to learn more about the std::mdspan class

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