Initializing std::array private member through con

问题:

I have for example the following code:

#include <iostream>
#include <array>

class Base {
public:
  Base() : mA(std::array<int,2>()) {}
  Base(std::array<int,2> arr) : mA(arr) {}
  Base(/* what to write here ??? */);
private:
  std::array<int,2> mA;
};

int main() 
{
    std::array<int,2> a = {423, 12}; // Works fine
    Base b(a); // Works fine
    Base c({10, 20}); // This is what I need. 

    return 0;
}

How should I define constructor to allow initialization with as shown in the 3rd line inside "main" above? In general, I need a configurable (in length in compile / run time) structure that will allow initialization with list of numbers, like {1, 2, 3} or (1, 2, 3) or something similar without need to element-by-element copying. I chose std::array for simplicity, but I'm afraid it might not work with this kind of initialization. What container would your recommend?

Thanks, Kostya

回答1:

You could add a constructor that takes an std::initializer_list<int> and copy the contents into the array:

#include <initializer_list>
#include <algorithm>

....

Base(std::initializer_list<int> a) {
   // check size first
   std::copy(a.begin(), a.end(), mA.begin()); }
}

Note: If you wanted to hold a number of elements defined at runtime, then you should use a an std::vector<int> This has a constructor from initializer_list<int> so the code is simpler:

class Foo {
public:
  Foo() {}
  Foo(const std::vector<int>& arr) : mA(arr) {}
  Foo(std::initializer_list<int> a) : mA(a) {}
private:
  std::vector<int> mA;
};

You can initialize it like this:

Foo f1({1,2,3,4,5});

or

Foo f2{1,2,3,4,5};