C++ custom assignment

Question

I have the following code:

template<size_t rows, size_t cols>
class Matrix{
    std::array<int, rows * cols>  data;
public:
    int  operator()(size_t i, size_t j){
        return data[i * cols + j];
    }
//  ...code...
}

What is the best way to achieve this:

Matrix<1,1> a;
a(0, 0) = 0;

avoiding the lvalue required as left operand of assignment error?

What is that line of code supposed to do exactly? Oh right, you have edited your question. But why not `std::array, rows> data`? I mean, if you _want_ a 2D array, then why don't you make yourself one? — Paul Sanders, Apr 26 '22 at 17:37

score 3 · Answer 1 · answered Apr 26 '22 at 17:32

3

You need to return the reference of the element from data like this:

// can change
int &operator()(size_t i, size_t j)
{
    return data[i * cols + j];
}

And every STL container includes a const function, for the cases like const Matrix &

// cannot change 
int operator()(size_t i, size_t j) const
{
    return data[i * cols + j];
}

answered Apr 26 '22 at 17:32

Darth-CodeX

1,851
4
19

3

Reason you need to return the reference: returning `int` returns a new, very short-lived temporary variable that contains a copy of the value at the requested index in the matrix. Changing this temporary copy is pointless because it will have no effect on the matrix. Returning `int &` returns a reference to the `int` at the requested index, not just it's value. Through the reference you can affect the `int` in the matrix. – user4581301 Apr 26 '22 at 17:44

wohlstad · Accepted Answer · 2022-04-27T03:50:24.247

You can change the following line:

int  operator()(size_t i, size_t j){

To:

int & operator()(size_t i, size_t j){

Returning a refernce (L value reference to be precise) to the element in the Matrix will allow you to assign to it.

Update: Some notes to complete my answer:

As @user4581301 commented: you can see more info about C++ value categories here: What are rvalues, lvalues, xvalues, glvalues, and prvalues?
As @Darth-CodeX and @HolyBlackCat mentioned, it is advisable to add a const overload for operator():

int const & operator()(int i, int j) const { /* same implementation */ }

You can use it with a const Matrix for reading elements values: if you have e.g. a Matrix const & m, you can use int val = m(0,0) to read an element value (of cource you will not be able to use it for assignment due to constness).

Terminology helper: [What are rvalues, lvalues, xvalues, glvalues, and prvalues?](https://stackoverflow.com/questions/3601602/what-are-rvalues-lvalues-xvalues-glvalues-and-prvalues) — user4581301, Apr 26 '22 at 17:48

C++ custom assignment

2 Answers2