CODEWITHSUNDEEP
c++vector
Chameleon
Chameleon

Reputation: 2137

std::vector elements initializing

std::vector<int> v1(1000);
std::vector<std::vector<int>> v2(1000);
std::vector<std::vector<int>::const_iterator> v3(1000);

How elements of these 3 vectors initialized?

About int, I test it and I saw that all elements become 0. Is this standard? I believed that primitives remain undefined. I create a vector with 300000000 elements, give non-zero values, delete it and recreate it, to avoid OS memory clear for data safety. Elements of recreated vector were 0 too.

What about iterator? Is there a initial value (0) for default constructor or initial value remains undefined? When I check this, iterators point to 0, but this can be OS

When I create a special object to track constructors, I saw that for first object, vector run the default constructor and for all others it run the copy constructor. Is this standard?

Is there a way to completely avoid initialization of elements? Or I must create my own vector? (Oh my God, I always say NOT ANOTHER VECTOR IMPLEMENTATION) I ask because I use ultra huge sparse matrices with parallel processing, so I cannot use push_back() and of course I don't want useless initialization, when later I will change the value.

Upvotes: 8

Views: 17784

Answers (4)

pmr
pmr

Reputation: 59811

In C++11 the specification for the constructor vector::vector(size_type n) says that n elements are default-inserted. This is being defined as an element initialized by the expression allocator_traits<Allocator>::construct(m, p) (where m is of the allocator type and p a pointer to the type stored in the container). For the default allocator this expression is ::new (static_cast<void*>(p)) T() (see 20.6.8.2). This value-initializes each element.

Upvotes: 5

Attila
Attila

Reputation: 28762

You are using this constructor (for std::vector<>):

explicit vector (size_type n, const T& value= T(), const Allocator& = Allocator());

Which has the following documentation:

Repetitive sequence constructor: Initializes the vector with its content set to a repetition, n times, of copies of value.

Since you do not specify the value it takes the default-value of the parameter, T(), which is int in your case, so all elements will be 0

Upvotes: 13

Ed Swangren
Ed Swangren

Reputation: 124632

They are default initialized.

About int, I test it and I saw that all elements become 0. Is this standard? I believed that primitives remain undefined.

No, an uninitialized int has an indeterminate value. These are default initialized, i.e.,

int i;          // uninitialized, indeterminate value
int k = int();  // default initialized, value == 0

Upvotes: 5

Mark Ransom
Mark Ransom

Reputation: 308121

The elements of a vector are default initialized, which in the case of POD types means zero initialized. There's no way to avoid it with a standard vector.

Upvotes: 3

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