Copy constructor of vector c++

Question

So i have this class that represtents matrix in my header file. My question is how do i initialize my org_matrix by already created matrix for example:

Matrix_t m{
        {INF, 10, 8,   19, 12},
        {10, INF, 20,  6,  3},
        {8,   20, INF, 4,  2},
        {19,  6,  4, INF,  7},
        {12,  3,  2,   7, INF}
};

Can i define a copy constructor of that object?

using Matrix_t = std::vector<std::vector<double>>;

class Matrix{
public:
    Matrix(const Matrix_t& m);
private:
    Matrix_t org_matrix;
};

`Matrix(const Matrix&) = default;` does the job. (you should then also add `operator=(const MAtrix&)` and move version). — Jarod42, Nov 05 '19 at 09:24

score 2 · Answer 1 · answered Nov 05 '19 at 09:08

2

Matrix(const Matrix_t& m); is not a copy constructor, just a constructor that takes Matrix_t.

Thanks to std::vector copy constructor, its code is very simple:

Matrix(const Matrix_t& m) : org_matrix(m)
{}

However, using std::vector<std::vector<double>> for a matrix is not a good idea. Use single std::vector<double> and then calculate an index in this "flattened" array as row * cols() + col or col * rows() + row.

answered Nov 05 '19 at 09:08

Evg

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Thx for the edit. But I disagree on a vector of vectors not being a good idea. It is not most efficient, but very easy to use. So it all depends on the purpose. – JHBonarius Nov 05 '19 at 09:21
@JHBonarius, raw pointers are also very easy to use, but we still should prefer smart ones. Not always. People should not pick bad habits from the very beginning. In some simple cases `vector>` can be used to represent a matrix, but in general it is a bad idea that should be warned about. This question might be found later by someone else. – Evg Nov 05 '19 at 09:38
then maybe elaborate on /why/ it is a bad idea ;) p.s. a non-owning raw pointer to the data object of a smart pointer is still common use... or `std::ref_wrapper>` or so, which imho is effectively the same... – JHBonarius Nov 05 '19 at 09:56
1

@JHBonarius, a raw pointer is a perfect tool, when used appropriately. I do not suggest to replace every raw pointer by a smart one. But many beginners definitely use raw pointers more often than they should. As for nested vectors, I guess it's a separate question. Good answers can be found [here](https://scicomp.stackexchange.com/questions/3159/is-it-a-good-idea-to-use-vectorvectordouble-to-form-a-matrix-class-for-high/3162) and [here](https://stackoverflow.com/questions/45747848/performance-impact-of-nested-vectors-vs-contiguous-arrays). – Evg Nov 05 '19 at 10:10

score 1 · Answer 2 · edited Nov 05 '19 at 09:18

1

Of course! Why wouldn't you? But Matrix_t is the type of an internal data stage object. A copy constructor should copy from an object of the same class. I.e.

Matrix(const Matrix& m);

... actually Matrix_t should be an internal, private type. You are probably looking for an initializer list constructor.

Matrix(initializer_list<initializer_list<double>>);

edited Nov 05 '19 at 09:18

Evg

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answered Nov 05 '19 at 09:14

JHBonarius

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Copy constructor of vector c++

2 Answers2