What are the differences between typedef and using?

What are the differ开发者_开发百科ences between using

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

or

using Some::Nested::Namespace::TypeName;

to provide the shorthand TypeName in the local scope?

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typedef gives an alias name for the type.

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

Once you do that, You can refer Some::Nested::Namespace::TypeName just by saying TypeName in the local namespace.

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using declaration makes the type visible in the current namespace.

using Some::Nested::Namespace::TypeName;

Imports the type in the current namespace.

In this case, using the either of the above you can refer Some::Nested::Namespace::TypeName by just using TypeName in the local namespace.

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Using just brings declaration into the local scope, while typedef introduces a typedef-name. One difference between them is elaborated type specifiers, e.g.:

namespace n
{
  class foo
  {
  };
}

typedef n::foo n_foo;

using n::foo;

int main()
{
  class foo f1; // ok, declares f1 variable of type n::foo.
  class n_foo f2; // error, typedef-name can't be used in elaborated-type-specifier.
}

---

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 <typename T>
using equiv_map = std::map<T,T>;

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:

<name> = <expression>;

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.