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Typedef

Started by TheShadow Mar 23, 2003 at 5:05 PM 5 replies 300+ views

Original Post

TheShadow

Mar 23, 2003 at 5:05 PM

I am reading a c++ tutorial and it uses the typedef word... I dont get what it is for or why to use it.

Neosmyle

Mar 23, 2003 at 5:11 PM

Basically just laziness and to save time.

All it does is create a "nickname" for a variable type, etc.

With that in mind you should be able to understand it from any c/c++ documentation.

TheShadow

Mar 23, 2003 at 5:17 PM

so if i have

  
struct house {
   char nameofpropietary;
};

typedef struct house TheMillersHouse;
typedef TheMillersHouse *MillersHouse;

What is that pointer useful for? And why use typedef?

JohnBolton

Mar 23, 2003 at 11:50 PM

I primarily use typedef for two reasons:

1. As an alias for long or complicated declarations.
2. An alias for template parameters.

  
template< typename C, typename T >
class izfstream : public std::basic_istream< C, T >
{
public:
    typedef C                          char_type;
    typedef T                          traits_type;
    typedef izfstream                  this_type;
    typedef std::basic_istream< C, T > base_type;
    typedef std::basic_ios< C, T >     ios_type;
    ...

Shannon Barber

Mar 24, 2003 at 12:24 AM

quote:
Original post by TheShadow
I am reading a c++ tutorial and it uses the typedef word...

I dont get what it is for or why to use it.

It let''s you apply a type re-direction in your code. Instead of using "int" as the error code, you can say
typedef int errorcode;
Then make functions that return an errorcode. Later you can replace the typedef with anohter typedef, or a class or structure, and change how it behaves.

With C code, you could write some algorithm and make a verion for both floats and doubles by changing a typedef. With C++ you would probably want to use a template, and then you use typedef''s so that the template class can remember it''s types.

Qw3r7yU10p!

Mar 24, 2003 at 2:09 AM

It's also useful for reducing coding and making code more readable and easier to change.

For example using the standard containers in the C++ library you can end up with long declarations:

  
std::map<Player*,IPAddress*> playerIPs;
std::map<Player*,IPAddress*>::iterator it = playerIPs.begin();
std::map<Player*,IPAddress*>::iterator itEnd = playerIPs.end();
//etc.

with a few typedefs you can make things a lot nicer

  
typedef std::map<Player*,IPAddress*> PlayerIPMap;
typedef PlayerIPMap::iterator PlayerIPMapIt;

PlayerIPMap playerIPs;
PlayerIPMapIt it= playerIPs.begin();
PlayerIPMapIt itEnd= playerIPs.end();
//etc.

The nice thing though is that if you have some class which you'd rather use than the standard map, one you've written yourself perhaps that will work with all the nice standard algorithms available in the standard library, you can just change the typedef and see how your program's performance changes.

eg

  
//typedef std::map<Player*,IPAddress*> PlayerIPMap;

typedef shadow::MyMap<Player*,IPAddress*> PlayerIPMap;
typedef PlayerIPMap::iterator PlayerIPMapIt;
//etc

[edited by - petewood on March 24, 2003 3:36:47 AM]

[edited by - petewood on March 24, 2003 3:37:48 AM]

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