C ++ 11，使用vs typedef，模板化

Question

Instead of

typedef struct
{
 double x,y;
} Point;

C++11 supports

using Point = struct {double x, y;};

Unfortunately, this approach does not work for the type T

template <typename T>
using Point = struct {T x, y;};

Is there any way of resolving the problem?

Answer 1

Is there a reason as to why you want to use a type alias in this case? If it's to avoid prepending the struct keyword, this is not the case in C++.

You would just define them directly: struct Point { double x, y; }; struct Point { double x, y; }; .

Same with the template case:

template <typename T>
struct Point { T x, y; };

Answer 2

using Point = ...; is a type alias (formally called just 'alias'). Ie, it's just a different syntax for typedef where the type involved is named in ... .

template<typename T> using Point = ...; is an alias template , where the type involved is again named in ... .

What both aliases and alias templates have in common is that both must refer to a type-id (C++11 [basic.scope.pdecl]p3). Type-ids must in turn name types . (Go figure.)

The problem is that template<typename T> struct {T x, y;} is not a type, but a class template, and as just established alias templates must refer to types. As to changing your code to resolve your problem, I've no idea as you haven't said what it is... ;-] Regarding that, please see What is the XY problem?

Answer 3

I somewhat disagree with the language lawyering in other answers, so I've decided to write my own.

In short, you cannot use template <typename T> using Point = struct {T x, y;}; for the same reason you cannot use template <typename T> struct {T x, y;}; .

Alias template is still a template. And although the standard doesn't state this explicitly, it implies that a template should have a name (see [temp]/4 - A template name has linkage ... ). Which you don't provide.

So no, you cannot "resolve the problem" without providing a name. But you also cannot just throw a name in an alias-declaration like this: template <typename T> using Point = struct P {T x, y;}; . The standard explicitly forbids it ([dcl.typedef]/2 - The defining-type-specifier-seq of the defining-type-id shall not define a class or enumeration ).

The only thing you can do will look something like this:

template <typename T>
struct Point {T x, y;};
template <typename T>
using PointAlias = Point<T>;

Which, as you can see, is pointless (unless, of course, your Point takes several template arguments; in this case, you can make a nifty alias template).

So stick with simple template <typename T> struct Point {T x, y;}; . It does exactly the same thing as your "proposed" syntax.