Thursday, October 31, 2013

auto variable in C++ 11

C++ 11 standard made lot many improvements to C++. New features like rvalue reference, lambda functions, auto variable and many more added to C++. There was auto variable in previous C++ also, but that has completely new meaning now. C++ 11 standard committee members, which also includes big organizations like, Microsoft, Apple, Google, IBM, decided C++98 auto variable is pretty useless, and new meaning can be given to auto keyword with little impact.

In this article we will discuss about auto variables.

Let’s start simple first,

       int i = 10;
       auto a = 10;

       cout<<typeid(i).name() << " "<<typeid(a).name()<<endl;// int int

We have explicitly mentioned ‘i’ as integer. For variable ‘a’ we left to compiler to decide its type.  As 10 is integer, type of a would be int. So cout will print ‘int int’ as output.
Let’s take bit complex example,

       vector<int> v;

       v.push_back(1);
       v.push_back(2);
       v.push_back(3);
       v.push_back(4);
       v.push_back(5);

       //print all elements of vector -- old C++ way
       for(vector<int>::iterator it = v.begin();it!= v.end(); it++)
       {
              cout<<*it;
       }

       //print all elements of vector -- used C++ 11 auto
       for(auto it = v.begin(); it != v.end(); it++)
       {
              cout<<*it;
       }

You can see how clumsy looking iterator variable definition can be easily replaced by auto keyword. You need not to explicitly specify which container you are using, and no need to remember what it contains, just v.begin() tells compiler its type.

At very least it makes code look cleaner.

One thing I wanted to bring to notice that C++ is statically typed language, and compiler has to know data type of object at compile time. So, data type of auto variables will also be decided at compile time only.
There are more concrete reasons to use auto variable with lambda functions that I will cover in article for lambda functions.


If you have any queries or suggestions, please put comments below. 

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