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Thinking in C++
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Aliasing names with typedef

This keyword promises more than it delivers: typedef suggests “type definition” when “alias” would probably have been a more accurate description, since that’s what it really does. The syntax is:

typedef existing-type-description alias-name

People often use typedef when data types get slightly complicated, just to prevent extra keystrokes. Here is a commonly-used typedef:

typedef unsigned long ulong; 

Now if you say ulong the compiler knows that you mean unsigned long. You might think that this could as easily be accomplished using preprocessor substitution, but there are key situations in which the compiler must be aware that you’re treating a name as if it were a type, so typedef is essential.

One place where typedef comes in handy is for pointer types. As previously mentioned, if you say:

int* x, y;

This actually produces an int* which is x and an int (not an int*) which is y. That is, the ‘*’ binds to the right, not the left. However, if you use a typedef:

typedef int* IntPtr;
IntPtr x, y;

Then both x and y are of type int*.

You can argue that it’s more explicit and therefore more readable to avoid typedefs for primitive types, and indeed programs rapidly become difficult to read when many typedefs are used. However, typedefs become especially important in C when used with struct.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire