How to assign two dimensional initializer list in c++?

Question

I inherited my class from vector and I would like to be able to assign list to my class like to vector.

My code is as follows:

#include <vector> using namespace std; template<typename T> class Matrix : public vector<vector<T>> { public: Matrix( vector<vector<T>> && m ) : vector<vector<T>>( m ) {} // Tried this approach, but it doesn't work // Matrix(std::initializer_list<std::initializer_list<T>> l){ // } } int main() { Matrix<int> m({{0, 1}, {2, 3}}); // it works // Matrix<int> m = {{0, 1}, {2, 3}}; // error: no instance of constructor "Matrix<T>::Matrix [with T=int]" matches the argument list -- argument types are: ({...}, {...}) }

The "error" line is not assignment, it's constructor syntax that looks like assignment. — Eljay
– Eljay, Commented May 9, 2022 at 13:59
Try this: Matrix<int> m = {{{0, 1}, {2, 3}}}; For the gory details, obligatory The Nightmare of Initialization in C++ by Nicolai Josuttis. An hour long presentation on the initialization syntax pain point in C++. — Eljay
– Eljay, Commented May 9, 2022 at 14:03
To expand on the comment by @Eljay, the first {} pair is for the Matrix<int> class, the next {} pair is for the outer vector. And the innermost {} pairs is for the inner vectors. — Some programmer dude
– Some programmer dude, Commented May 9, 2022 at 14:05

Marek R · Accepted Answer · 2022-05-09 14:02:37Z

5

Just bring std::vector constructor to your class scope:

template <typename T> class Matrix : public vector<vector<T>> { public: using vector<vector<T>>::vector; };

https://godbolt.org/z/b3bdx53d8

Offtopic: inheritance is bad solution for your case.

answered May 9, 2022 at 14:02

Marek R

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4 Comments

Andrey Over a year ago

could you please advice the better approach? I need a matrix with dimensions unknown at compile time, and I want to multiply them (by overloading * operator). And I can not use other libraries (like eigen)

ChrisMM Over a year ago

@Andrey, instead of inheriting from the vector class, have a vector as a member of your Matrix.

Andrey Over a year ago

@ChrisMM Thanks, but in that case I have to use . to access matrix: like myObject.mat instead of just myObject. The last option looks better for me

ChrisMM Over a year ago

@Andrey, you should read this

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