using namespace used as type C++

Question

I found a piece of code that uses the following statement:

using flags = std::ios_base::fmtflags;
flags fl = std::ios_base::dec;

This is the first time that I encountered it. How can you use a namespace as a type.

using namespace and using other_stuff are different. You can read them as plain English to understand the meaning. — ForceBru
– ForceBru, Commented May 12, 2017 at 20:13

François Andrieux · Accepted Answer · 2017-05-12 20:26:44Z

5

You seem to be confusing type aliasing with using declerations. Type aliasing behaves like a typedef, it defines another name for an existing type. Your example is equivalent to the following :

typedef std::ios_base::fmtflags flags;
flags fl = std::ios_base::dec;

edited May 12, 2017 at 20:26

answered May 12, 2017 at 20:12

François Andrieux

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2 Comments

François Andrieux Over a year ago

@SergeyA using can be templated.

SergeyA Over a year ago

Than it becomes alias template, not type alias.

SergeyA · Accepted Answer · 2017-05-12 20:09:46Z

3

Nobody uses a namespace as a type in this snippet. std::ios_base::fmtflags is a type, not a namespace, and this using flags = std::ios_base::fmtflags; line just introduces an alias to it.

answered May 12, 2017 at 20:09

SergeyA

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