CODEWITHSUNDEEP
c++arrays
tttapa
tttapa

Reputation: 1417

Instantiate const array reference as constructor parameter using initializer list, allowed or not?

I have a constructor that takes a reference to an array as a parameter. Can I use a brace-enclosed initializer list to call that constructor?
Is the lifetime of the temporary array the lifetime of the constructor, or is that not guaranteed?

My main question:
Is the following example program correct? 

class Test {
public:
  template <size_t N>
  Test(const int (&numbers)[N]) {
    for (const int &number : numbers)
      sum += number;
  }

  int getSum() const {
    return sum;
  }

private:
    int sum = 0;
};

int main() {
  Test test({1, 2, 3});
  assert(test.getSum() == 6);
}

The code above works fine, however, the reason why I'm asking this is that I'm using the same approach in a larger project, where the elements of the array are not correctly initialized.

E.g. if {16} is used as the argument to the constructor, sometimes the value of numbers[0] is 0b00000000 00000000 00000000 00010000, which is correct, and sometimes it is a different number, e.g. 0b11111111 11111111 11111111 00010000 or 0b00000000 00000000 00000011 00010000.
The size of an int is 4 bytes on the microcontroller platform I'm working on, and when numbers[0] is wrong, I always see the same pattern: the least significant byte is correct, but in the 3 most significant bytes, there is always a block of all ones. This leads me to believe that it was not initialized correctly.

I haven't been able to isolate the problem or reproduce it using a small example.

Upvotes: 2

Views: 66

Answers (1)

Jarod42
Jarod42

Reputation: 217398

In Test test({1, 2, 3});, lifetime of the temporary array ends at ; (so after the constructor call finished).

So your usage in constructor is correct.

But if you kept reference to that array as member, that reference would become dangling.

Upvotes: 2

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