Vector Initialization

Question

I am working with C++ vectors that I faced with the following snippets code :

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    using MyVector = vector<int>;

    MyVector vectorA(1);
    cout << vectorA.size() << " " << vectorA[0] << endl;

    MyVector vectorB(1, 10);
    cout << vectorB.size() << " " << vectorB[0] << endl;

    MyVector vectorC{ 1, 10 , 100, 1000 };
    cout << vectorC.size() << " " << vectorC[3] << endl;
    return 0;
}

Why in this code vector object defined with using keyword? I can't understand why vector used in this code with this approach.

Almost a duplicate of [What is the difference between 'typedef' and 'using' in C++11?](http://stackoverflow.com/questions/10747810/what-is-the-difference-between-typedef-and-using-in-c11), but first you need to know what a typedef is. Hopefully the link helps you out. — user4581301, Dec 17 '15 at 22:27
As for why, the writer didn't want to have to keep typing `vector` over and over and seems to prefer typing three less characters and sewing chaos. — user4581301, Dec 17 '15 at 22:29
Oh thanks user4581301. I get it why using keyword used in this code. — Adonaim, Dec 17 '15 at 22:33

score 4 · Accepted Answer · answered Dec 17 '15 at 22:30

4

With using keyword you create alias MyVector for type vector<int>. You can read more about it here: http://en.cppreference.com/w/cpp/language/type_alias The effect is the same as with typedef vector<int> MyVector.

answered Dec 17 '15 at 22:30

Ardavel

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score 2 · Answer 2 · answered Dec 17 '15 at 22:30

2

It can be annoying to continuously type out template arguments especially when they get really long. The using syntax is handy to keep things short and consistent without needing to type vector<...> everytime.

answered Dec 17 '15 at 22:30

Justin Meiners

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score 1 · Answer 3 · answered Dec 17 '15 at 23:03

In this case, using A = B;, using allows you to substitute data type name A for data type name B in a more controlled manner than a macro definition and similar, if not identical to, a typedef.

The intent is to replace some arcane bit of wizardry with a term easier to convey and use. Here you're typically looking to improve one or both of clarity and brevity. You are trying to make something easier to read and easier to type.

The example of using MyVector = vector<int>; is almost meaningless. vector<int> is short and clear. You know exactly what vector<int> is at a glance, so no clarity benefit is gained.

Brevity is questionable. A whole three characters are saved. Over time this will add up, but I'm not sure it will match the time wasted by others looking up what a MyVector is. Fortunately modern IDEs are good at this for you so you don't waste time and hair looking for MyVector.h.

However for something like using CubeDataIterator = std::list<core::utils::DataRecord<UEI::Cube>>::iterator, is a kindness to the eyes. It is an iterator for Cube data, whatever the heck that is. But if you already know what a Cube is, you are rocking.

score 0 · Answer 4 · answered Dec 17 '15 at 22:41

C++ keywords: using

Usage

using-directives for namespaces and using-declarations for namespace members
using-declarations for class members
type alias and alias template declaration (since C++11)

In your sample

using MyVector = vector<int>;

is identical to:

typedef  vector<int> MyVector;

Vector Initialization

4 Answers4