在 C++ 类中初始化二维数组

Question

I am trying to initialize nums 2D array in a Class Construct . I am using the default constructor to initialize it but since it has already been created I am unable to. I cannot initialize it in the class either. I have tried initializing each element manually and it works but I want to just initialize nums in one line.

#include <iostream> 
using namespace std; 

class Construct { 
public: 
    int nums[3][3]; 

    // Default Constructor 
    construct() 
    { 
        int nums[3][3] = {{4,5,42,34,5,23,3,5,2}}
    } 
}; 

int main() 
{ 

    Construct c; 
    cout << "a: " << c.nums[1][0] << endl 
        << "b: " << c.nums[0][1]; 
    return 1; 
} 

I have tried nums[1][0] = 5 ... but that is not very efficient. Any feedback would be great.

Answer 1

Use initializer list

Construct(): nums {{4,5,42},{34,5,23},{3,5,2}}
{ }

Answer 2

In case you want to be able to inititalize your class with custom values, you will need to provide a constructor that takes some kind of a container as an argument and uses it to initialize your array: ie Construct(const atd::array<int, 6>&) .

But to avoid the overhead of rewriting all the values of an already initialized array, you will need to use a Pack expansion.

Here is a customized class template that provides such a constructor:

#include <utility>
#include <array>

// typename of an underlying array, sz0 and sz1 - array dimensions
template <typename T, size_t sz0, size_t sz1,
    class = decltype(std::make_index_sequence<sz0 + sz1>())>
    class Construct_;

// expanding a Pack of indexes to access std::array's variables
template <typename T, size_t sz0, size_t sz1,
    size_t ... indx>
    class Construct_<T, sz0, sz1, 
        std::index_sequence<indx...>>
{
// make it private or use a struct instead of a class
    T nums_[sz0][sz1];

public:

    // nums_ is initialized with arr's values. No additional copies were made
    Construct_(const std::array<T, sz0 + sz1> &arr)
        : nums_{ arr[indx] ... }
    {}

};

using Construct33int = Construct_<int, 3, 3>;

int main()
{
    std::array<int, 6> arr{ 4, 8, 15, 16, 23, 42 };
    Construct33int c{ arr };

}