What are the differences between typedef and using?

Question

What are the differences between using

typedef Some::Nested::Namespace::TypeName TypeName;

or

using Some::Nested::Namespace         


        

Answer 1

They have different origins and different uses.

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typedef comes from C: recall that the C way to declare a struct is:

typedef struct _MyStruct { .... } MyStruct;

It allows you to introduce an alias for a type only. It can be used for the type of a function, with an awkward syntax...

typedef void (*Func)(Foo, Bar);

Where Func is now a pointer to a function taking two arguments by copy (of types Foo and Bar respectively) and returning nothing.

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using has, originally, a different meaning. It is meant to inject a name into a scope. Any name (nearly) can be injected: types, functions, variables (but not enum values...)

With C++11, the syntax has been enhanced to allow template aliasing:

template 
using equiv_map = std::map;

This powered-up using means that aliasing (see below) is now possible, on top of the previous functionalities.

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This C++11 change is a clear direction toward syntax harmonization. Note how the definition of an alias is now similar to the definition of a variable:

 = ;

Unfortunately it seems the Standard reserved this aliasing to template situations, so for now both typedef and using coexist, each with its own hunting ground.