In a C++ class, do you have to initialize member variables that are arrays?

Question

For example:

class Cls
{
private:
    int x;
    int y;
    int arr[10];

public:
    Cls();
};

Cls::Cls() : x(0), y(0) {}

My understanding is that an uninitialized array will have garbage values. As long as I don't write code that expects the array to have a specific value before filling it, is there any issue with not initializing the array? Or do I need something like this as the constructor:

Cls::Cls() : x(0), y(0), arr() {}

@n.m. That's not what I asked, is it? I'm aware of the disadvantages of C-style arrays, but in this particular case, I'm obligated to use them. Also, even if I weren't gonna use them, it would be nice to know how they worked. — ubadub, Oct 31 '18 at 04:29
This is a comment. Comments are not answers, they are exactly for things that were *not* asked. — n. m. could be an AI, Oct 31 '18 at 06:07
`arr()` is not legal. You can say `arr{}` in C++11 and up if you want to. Don't have to though, as the answer says. Just like any other array or non-array variable in C++ btw. Class members are not special. — n. m. could be an AI, Oct 31 '18 at 06:19
@n.m. how is that possible? I just compiled and ran a file that does exactly that: `arr()` as part of an initializer list — ubadub, Oct 31 '18 at 15:22

score 3 · Accepted Answer · answered Oct 31 '18 at 04:18

3

The constructor of Cls is not required to initialize the array member. Failing to initialize it will leave the elements with indeterminate values. Contrary to popular belief, objects with indeterminate values aren't merely objects whose values are unspecified; as a matter of fact, attempting to read them, before first writing definite values to them, causes undefined behaviour. For this reason, it's generally advisable to set the values as soon as possible.

answered Oct 31 '18 at 04:18

Brian Bi

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Right, but if I'm not planning on reading them, only writing to them, it doesn't really matter? – ubadub Oct 31 '18 at 04:18
2

@ubadub It's fine to leave them uninitialized if you're sure that you'll write to them before reading. (If you want to only write and *never* read, what's the point?) – Brian Bi Oct 31 '18 at 04:19
Yea, that's what I meant- the class I'm writing stores a `size` referring to the number of elements written to the array and never reads past index `size - 1`. – ubadub Oct 31 '18 at 04:20
See here for more detail on indeterminate values and UB [Has C++ standard changed with respect to the use of indeterminate values and undefined behavior in C++14?](https://stackoverflow.com/q/23415661/1708801) – Shafik Yaghmour Oct 31 '18 at 04:36

score 1 · Answer 2 · answered Oct 31 '18 at 04:38

If your logic is that you definitely won't write code that accesses them before setting them, then no you don't need to initialise them. If for instance you straight up did not use the array at all, you could simply leave them unitialised.

However it is simply best to initialise them since there's no reason not to, even if you initialise them to bad values. What might happen is although you always write before reading now, that may change in the future. 6 months down the track someone may refactor something, and now a read happens before a write. Because that's undefined behaviour, maybe your program will seem to function correctly, or it might work on their machine but crash on another. If you initialise them, you just won't get these problems.

Perhaps your concern is that 0 is a valid value and you don't want your program to accidentally get that value, you'd rather make it clear it's not properly initialised, but with undefined behaviour you could get 0 or some other valid value or a crash, and it may be inconsistent. You will probably thank yourself if you just initialise everything properly.

The C++ police or a compiler won't stop you from doing so, and your program can still be well formed, but you may as well guarantee it by just initialising them.

In a C++ class, do you have to initialize member variables that are arrays?

2 Answers2