Square and round brackets operator, how to choose overloads? - c++

Question

I want to access to some class data using operator[] but depending on the index type into the square brackets return one kind of data or other. As a simplified example:
struct S
{
int &operator []( int index ) { std::cout << "[i]"; return i_buffer[index]; }
short &operator [](short index) { std::cout << "[s]"; return s_buffer[index]; }
private:
int i_buffer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_buffer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
There's no way to write a short literal, so the only way to choose the short overload is by casting:
S s;
std::cout << s[9] << '\n'; // prints [i]9
std::cout << s[(short)9] << '\n'; // prints [s]999
But I don't like it and I was wondering if there's different options.
What I've tried?
Tagged parameter.
First I've tried to use "tags":
struct S
{
enum class i_type : std::int32_t {};
enum class s_type : std::int32_t {};
int &operator [](i_type index)
{ std::cout << "[i]"; return i_buffer[static_cast<int>(index)]; }
short &operator [](s_type index)
{ std::cout << "[s]"; return s_buffer[static_cast<int>(index)]; }
private:
int i_buffer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_buffer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
That works but is still a little verbose:
S s;
std::cout << s[9] << '\n'; // error, no possible overload to be taken
std::cout << s[S::i_type{9}] << '\n'; // prints [i]9
std::cout << s[S::s_type{9}] << '\n'; // prints [s]999
Template.
As a crazy workaround I wanted to try to template the operator:
struct S
{
template <typename T>
T &operator [](T) { std::cout << "???"; return 0; }
private:
int i_buffer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_buffer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
template <>
int &S::operator [](int index) { std::cout << "[i]"; return i_buffer[index]; }
template <>
short &S::operator [](short index) { std::cout << "[s]"; return s_buffer[index]; }
The template version behaves as the original code, but there's no easy way to specify a type parameter along with the operator[]:
S s;
std::cout << s[9] << '\n'; // prints [i]9 like before
std::cout << s[(short)9] << '\n'; // prints [s]999 like before
std::cout << s<short>[9] << '\n'; // s is not template
std::cout << s[9]<short> << '\n'; // nonsense
// Correct but utterly verbose and hard to write and read
std::cout << s.operator[]<short>(9) << '\n';
Question.
All the issues described also happens with operator(), I want to know if there's more alternatives that I'm not aware of?

Answer 1

I think that using a named method is a much better idea than using operator[] in your situation, as it would be easier to understand that two separate buffers are being accessed by reading the source code.
Regardless, if you want to use your operator[] approach, you could use strong typedefs and user defined literals to have type-safety with minimal syntactic overhead:
BOOST_STRONG_TYPEDEF(std::size_t, int_index)
BOOST_STRONG_TYPEDEF(std::size_t, short_index)
struct S
{
auto& operator[](int_index i) { /* ... */ }
auto& operator[](short_index i) { /* ... */ }
};
auto operator "" _ii(unsigned long long int x) { return int_index{x}; }
auto operator "" _si(unsigned long long int x) { return short_index{x}; }
You can then call your methods as follows:
S s;
auto& some_int = s[15_ii];
auto& some_short = s[4_si];
wandbox example

Answer 2

I think I'd use std::tie from the <tuple> library and then write a little helper to find the correct reference type:
#include <tuple>
#include <iostream>
template<class As, class...Ts>
auto& as(std::tuple<const Ts&...>ts)
{
return std::get<As const&>(ts);
};
template<class As, class...Ts>
auto& as(std::tuple<Ts&...>ts)
{
return std::get<As &>(ts);
};
struct S
{
// both cost and mutable version provided for completeness.
auto operator[](std::size_t i) const {
return std::tie(i_buffer[i], s_buffer[i]);
}
auto operator[](std::size_t i) {
return std::tie(i_buffer[i], s_buffer[i]);
}
private:
int i_buffer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_buffer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
int main()
{
auto s = S();
const auto x = S();
std::cout << "short is : " << as<short>(s[5])<< '\n';
std::cout << "int is : " << as<int>(s[5])<< '\n';
std::cout << "short is : " << as<short>(x[6])<< '\n';
std::cout << "int is : " << as<int>(x[6])<< '\n';
}
This way, the code is explicit but still succinct.
expected output:
short is : 555
int is : 5
short is : 666
int is : 6
Having read the further comments, I might choose to store the matrix in (say) row-wise form and then provide a col-wise wrapper.
A barely functional example:
#include <tuple>
#include <iostream>
#include <array>
template<std::size_t Rows, std::size_t Cols>
struct RowWiseMatrix
{
auto& operator[](std::size_t i) { return data_[i]; }
std::array<std::array<double, Cols>, Rows> data_;
};
template<std::size_t Rows, std::size_t Cols>
struct ColumnProxy
{
ColumnProxy(std::array<std::array<double, Cols>, Rows>& data, std::size_t col)
: data_(data), col_(col)
{
}
auto& operator[](std::size_t i) { return data_[i][col_]; }
std::array<std::array<double, Cols>, Rows>& data_;
std::size_t col_;
};
template<std::size_t Rows, std::size_t Cols>
struct ColWiseProxy
{
ColWiseProxy(RowWiseMatrix<Rows, Cols>& mat) : underlying_(mat) {}
auto operator[](std::size_t i) { return ColumnProxy<Rows, Cols> { underlying_.data_, i }; }
RowWiseMatrix<Rows, Cols>& underlying_;
};
template<std::size_t Rows, std::size_t Cols>
auto& rowWise(RowWiseMatrix<Rows, Cols>& mat)
{
return mat;
};
template<std::size_t Rows, std::size_t Cols>
auto colWise(RowWiseMatrix<Rows, Cols>& mat)
{
return ColWiseProxy<Rows, Cols>(mat);
};
int main()
{
auto m = RowWiseMatrix<3, 3> {
std::array<double, 3>{ 1, 2, 3 },
std::array<double, 3>{ 4, 5, 6},
std::array<double, 3>{ 7, 8, 9}
};
std::cout << rowWise(m)[0][2] << '\n';
std::cout << colWise(m)[0][2] << '\n';
}
Expected output:
3
7

Answer 3

I agree with Vittorio Romeo that the best solution is a named method.
However here is a solution:
template <class T> struct S_proxy {
T* data;
T& operator[](std::size_t i) { return data[i]; }
};
struct S
{
auto i_type() { return S_proxy<int>{i_buffer}; };
auto s_type() { return S_proxy<short>{s_buffer}; };
private:
int i_buffer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_buffer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
and use:
S s;
return s.s_type()[2];

Answer 4

If i_type and s_type are supposed to have a meaning by themselves, it is possible to add semantics to operators []. Something like
#include <iostream>
struct Month {
explicit Month(int m)
: m(m)
{
}
int m;
};
struct Day {
explicit Day(short d)
: d(d)
{
}
short d;
};
struct S {
int& operator[](const Month& mes)
{
std::cout << "[i]";
return i_bufer[mes.m];
}
short& operator[](const Day& dis)
{
std::cout << "[s]";
return s_bufer[dis.d];
}
private:
int i_bufer[10]{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
short s_bufer[10]{ 0, 111, 222, 333, 444, 555, 666, 777, 888, 999 };
};
int main()
{
S s;
std::cout << s[Month(9)] << '\n'; // muestra [i]9
std::cout << s[Day(9)] << '\n'; // muestra [s]999
}