C++ overload constructor with const array and initializer_list

问题

I have a class that takes a "list" of pins as argument in the constructor. Right now, this "list" is just a const array (const unsigned int (&pins)[N]). I'd like to overload the constructor in such a way that you can use an initializer_list as well as a (const) array.
I can get it working with separate classes, but I'm unable to combine them.

The basic structure of my test class:

#include <iostream>
#include <cstring>
using namespace std;

class X
{
  public:
    X(... pins);
    ~X();
    void print() {
        for (unsigned int i = 0; i < length; i++)
            cout << pins[i] << ' ';
    }

  private:
    (const) unsigned int *pins;
    const size_t length;
};

Class A (using std::initializer_list)

  public:   
    A(initializer_list<unsigned int> pins)
        : length(pins.size()) {
        this->pins = (unsigned int *)malloc(sizeof(unsigned int) * pins.size());
        memcpy(this->pins, pins.begin(), sizeof(unsigned int) * pins.size());
    }
    ~A() {
        free(pins);
    }
  private:
    unsigned int *pins;

Class B (using an array)

  public:
    template <size_t N>
    B(unsigned int (&pins)[N])
        : length(N) {
        this->pins = pins;
    }
  private:
    unsigned int *pins;

Class C (using a const array)

  public:
    template <size_t N>
    C(const unsigned int (&pins)[N])
        : length(N) {
        this->pins = pins;
    }
  private:
    const unsigned int *pins;

This allows me to do things like:

A a({1, 2, 3});

unsigned int pb[] = {4, 5, 6};
B b(pb);

const unsigned int pc[] = {7, 8, 9};
C c(pc);

a.print();
b.print();
c.print();

And this prints 1 2 3 4 5 6 7 8 9 as expected.

How can I combine these different constructors in one class? I want to be able to initialize an object in either of the three ways shown above.

If I make pins a const, I can't do things like memcpy(pins, ...), and if I don't make it a const, I can't use a const array as argument for the constructor.

(The target platform is Arduino, so most fancy and memory-intensive C++11 tricks are not supported.)

回答1:

Something along these lines, perhaps:

class X
{
  public:
    X(std::initializer_list<unsigned int> aPins)
        : storage(aPins), length(aPins.size()) {
      pins = length ? &storage[0] : nullptr;
    }

    template <size_t N>
    X(unsigned int (&aPins)[N])
        : length(N) {
        pins = aPins;
    }

    template <size_t N>
    X(const unsigned int (&aPins)[N])
        : length(N) {
        pins = aPins;
    }

    void print() {
        for (unsigned int i = 0; i < length; i++)
            std::cout << pins[i] << ' ';
    }

  private:
    const unsigned int* pins;
    const size_t length;
    std::vector<unsigned int> storage;
};

Demo

来源：https://stackoverflow.com/questions/45772095/c-overload-constructor-with-const-array-and-initializer-list

标签

c++

arrays

c++11

initializer-list