Question

Let’s I have

struct Vector {

    float i,j,k;
}

I want to zero all elements of vec declared below (i,j,k=0)

std::vector <Vector> vec;
vec.resize(num,0);

I don’t want to use reserve() and then push_back() zeroes one by one.
Another thing is, after succesfully initializing vec, I want to set all members of vec to zero again after it is manipulated. Is there something like memset for vectors?

EDIT:
I compared all of the methods in Mike Seymour’s and Xeo’s answers and as a result
size_t size = vec.size();
vec.clear();
vec.resize(size); is the fastest if they are repeated frequently in a loop.

Answer 1

That’s very simple:

vec.resize(num);

or initialise it with the required size:

std::vector<Vector> vec(num);

Both the constructor and resize will fill new elements with value-initialised objects. A value-initialised object of a type with no default constructor (such as your Vector) will have all numeric members initialised to zero.

To reset everything to zero, either

size_t size = vec.size();
vec.clear();
vec.resize(size);

or:

std::fill(vec.begin(), vec.end(), Vector());

or, less efficiently but with a strong exception guarantee:

std::vector<Vector>(vec.size()).swap(vec);