why use c++ Typedef?

Question

My understanding of typedef is to give a declaration an alias. Such the declaration for int will now be referred to as Integer. But why? Why would someone use typedef? What's the more advantageous reason?

typedef int Integer;

Integer blamo = 50;

cout << blamo << endl;

Answer 1

that isn't a good example because Integer and int don't have any different meaning. But imagine something like,

typedef int BlamoType;

Now, in the future when 'blamo' becomes a 64-bit integer, you can just change this typedef, and all other code stays the same.

Another reason for typedef is to shorten type names. For example instead of

boost::shared_ptr<std::map<std::wstring, std::vector<int> > > blamo1;
boost::shared_ptr<std::map<std::wstring, std::vector<int> > > blamo2;
std::vector<boost::shared_ptr<std::map<std::wstring, std::vector<int> > > > blamoList;

you can do:

typedef boost::shared_ptr<std::map<std::wstring, std::vector<int> > > BlamoType;
BlamoType blamo1;
BlamoType blamo2;
std::vector<BlamoType> blamoList;

Answer 2

A reason that no-one mentioned here: to write generic code. Consider an implementation of some container like std::vector. It should have a type called std::vector<T>::value_type so anyone that writes generic code could use it:

template<class Container> void f(const Container &c)
{
    typename Container::value_type v = c.back();
    // ...
}

How can you introduce such a type into your class? The only way is a typedef:

// !!!EXPOSITION ONLY!!!
template<class T> class vector
{
public:
    typedef T value_type;
    // ...
};