C++ array member without initialization

Question

I tried to write an "inline-vector" class for storing a number of elements on the stack conveniently:

template<typename T, size_t size_ = 256>
struct InlineVector{
    T content[size_];
    size_t num;

    T() : num(0) {}

    ~T() { for(size_t s = 0; s < num; s++){ content[s]->~T(); } }

    template<typename _Up, typename... _Args>
    void emplace_back(_Args&&... __args) { new (&content[num++]) T(__args); }

    T& get(size_t i) { assert(i < num); return content[i]; }


}

For efficiency reasons, I want that content is not initialized in X's constructor, even if T has only a non-trivial constructor. As you see, the content is intialized later with placement new when it is actually inserted. However, C++ seems to enforce that all elements of content must be initialized in the initialization of X. For example, this does not compile:

struct Y{ Y(int){} }

X<Y> foo; // compile error, no constructor for Y is called in the construction of X

So, how is it possible to have an array member that is not initialized, even if the array element type needs a constructor?

Lightness Races in Orbit · Accepted Answer · 2014-11-10 12:33:12Z

6

By making the array an array of chars and later placement new-ing into it. char is the only type you're allowed to do this with.

Whilst you're at it, then, you might as well write your own allocator instead (with the array as one of its members), and plug it into std::vector rather than re-inventing the whole shebang.

std::vector<int, YourAllocatorType<int, 256>> v;

answered Nov 10, 2014 at 12:33

Lightness Races in Orbit

387k77 gold badges670 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

gexicide Over a year ago

Interesting idea with the allocator. And thanks for the actual answer of using a char array, as the allocator would have the same problem as the vector. Seems there is no way around the char array, no matter whether writing an allocator or a vector.

Lightness Races in Orbit Over a year ago

@gexicide: Yep not really :)

Tony Delroy · Accepted Answer · 2014-11-10 12:35:39Z

1

So, how is it possible to have an array member that is not initialized, even if the array element type needs a constructor?

It's not for T x[n]... but you can point any old T* at properly aligned uninitialised memory, then manage your own ad-hoc construction and destruction. You could also explore creating a union with your type and say a char, and have an array of the unions....

answered Nov 10, 2014 at 12:35

Tony Delroy

107k16 gold badges188 silver badges265 bronze badges

2 Comments

Jan Hudec Over a year ago

I don't think union is permitted with members with non-trivial constructors. But it's not needed; a char array and cast is fine and as long as one of the types is char, well defined.

Tony Delroy Over a year ago

@JanHudec: there are restrictions around the type, but I believe they were relaxed substantially for C++11... certainly something to keep in mind. Don't have time to dig for exact details myself right now....

Collectives™ on Stack Overflow

C++ array member without initialization

2 Answers 2

2 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related