CODEWITHSUNDEEP
c++vector
smilingbuddha
smilingbuddha

Reputation: 14660

declaring sizes of vectors in a collection. c++

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?

Upvotes: 1

Views: 2919

Answers (4)

sehe
sehe

Reputation: 393467

Yes. Allthough it doesn't make the vectors fixed-size (vectors can be resized explicitely, or implicitely - e.g. using push_back or insert).

Finally, vectors can simply be assigned over.

If you need strongtype, fixed-size, jagged nested vectors (urrfff that's a mouth-full):

#include <tuple>
#include <array>

using std::array;
using std::tuple;

typedef tuple<
       array<T, 12>,
       array<T, 3>,
       array<T, 14>
    > vec_collection;

That would be a bit more awkward to work with, but if speed and container size guarantees were of the essence, go for it.

Note that in pre-TR1 world (say, c++98 with a c++03 library), this would be pretty close to equivalent (but even less comfortable to work with):

struct vec_collection
{
     T a[12];
     T b[3];
     T c[14];
};

Upvotes: 3

Matteo Italia
Matteo Italia

Reputation: 126867

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.

Upvotes: 7

TotoroTotoro
TotoroTotoro

Reputation: 17622

vector< vector<T>* > vec_collection(3); // i changed it to a vector of vector<T>*


 vec_collection[0]=new vector<MyType>(12);
 //etc

You probably want to replace with in the code. E.g. 

vector<int>(12);

Moreover, if your vector are fixed size, you can just use arrays.

Upvotes: -1

Seth Carnegie
Seth Carnegie

Reputation: 75150

If you have C++11, you can do it with an initialiser list, which is more efficient:

vector<vector<T>> vec_collection(3) { vector<T>(12), vector<T>(3), vector<T>(14) };

Otherwise, you can do it the way you're doing which is fine, or just call resize on the elements like in Matteo's answer.

Upvotes: 2

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