Question

using namespace std;

vector<vector<T> > vec_collection(3);

vec_collection[0]=vector<T>(12);
vec_collection[1]=vector<T>(3);
vec_collection[2]=vector<T>(14);

Suppose I have an empty collection of 3 vectors and after initialization, I want the first, second, and third vector to be of specific size 12, 3, and 14 say. Is the above code snippet the right way to declare their sizes?

Answer 1

Your code in line of principle creates separated temporary vector instances that replace the existing zero-size ones, which in theory means that for every of those lines you have a constructor call (create the temporary), a copy-constructor call (copy it over the existing vector) and a destructor call (destroy the temporary).

Although I think that some optimizations could make it actually better than it sounds, it’s way easier and almost surely more efficient to simply do:

using namespace std;

vector< vector<T> > vec_collection(3);

vec_collection[0].resize(12);
vec_collection[1].resize(3);
vec_collection[2].resize(14);

On the other hand, if you need such vectors to be always of that fixed size, you should replace them with fixed-size data structures (e.g. a struct containing three C-style arrays or std::array), as in @sehe’s answer.