C++ constructor with static array

Question

Trying to learn C++ and hitting a wall with a few things. Would appreciate some pointers from the C++ experts on explaining on what is happen ing under the hood.

My BigNum class is below. My questions are

BigNum b1 = BigNum(2, {2, 8}); does not work Error: No matching constructor for initialization of 'BigNum'
- But int arr[] = {2, 8}; BigNum b1 = BigNum(2, arr); works
I also see some compiler warnings for all constructors below. E.g:Candidate constructor not viable: requires 0 arguments, but 2 were provided for the default constructor below
Is there a one-line way to initialize the private members in the constructor. E.g:
- BigNum(int numDigits, int digits[]): _n(numDigits), _digits(digits) {};
- Or even better BigNum(int numDigits = 0, int digits[] = {}): _n(numDigits), _digits(digits) {};


class BigNum {
    int _n = 0;
  int _digits[MAX_DIGITS] = {};
public:
  BigNum() {};
  BigNum(int numDigits, int digits[]) {
    if (numDigits >= MAX_DIGITS || numDigits < 0) {
      return;
    }
    _n = numDigits;
    memcpy(_digits, digits, _n * sizeof(_digits[0]));
  };
  BigNum(const BigNum &bigNum) {
    _n = bigNum._n;
    memcpy(_digits, bigNum._digits, _n * sizeof(_digits[0]));
  };
};

1. You likely want a constructor taking std::initializer_list. With such a constructor, you could do BigNum({2, 8}) 2. Those are not separate warnings, but further explanations for the original error. The compiler tells you what constructors it tried, and why each one was found unsuitable. — Igor Tandetnik
– Igor Tandetnik, Commented Aug 31, 2020 at 0:20
3. There would be if you use something better than a plain C array for storing your digits - e.g. std::array or std::vector. These can be initialized in the constructor initializer list, while plain array cannot. — Igor Tandetnik
– Igor Tandetnik, Commented Aug 31, 2020 at 0:24
Thanks does the std::initializer_list work if data member is an int array. Seems to only work for std::vector — Arvind Swami
– Arvind Swami, Commented Aug 31, 2020 at 0:28
Just saw your comment for 3. Thanks. yes using std::initializer_list for the param and using for loop to initialize _digits works. Can you make this the answer, so that I can accept? — Arvind Swami
– Arvind Swami, Commented Aug 31, 2020 at 0:33

catnip · Accepted Answer · 2020-08-31 00:37:36Z

3

So, to echo what Igor said in the comments, std::vector has a convenient constructor from an initializer_list, so you could do:

#include <vector>
#include <initializer_list>

class BigNum {
    std::vector <int> digits;
public:
    BigNum (int max_digits) : digits (max_digits) {}
    BigNum (std::initializer_list <int> initial_digits) : digits (initial_digits) {}
};

int main()
{
    BigNum bn ( { 1, 2, 3, 4 } );
    BigNum bn2 (5);
}

Note that we now don't need a copy constructor - the default one generated by the compiler will work just fine.

edited Aug 31, 2020 at 0:37

answered Aug 31, 2020 at 0:25

catnip

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