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digitalmars.D.learn - what is the different bettwen typeid and .classinfo?

reply lilijreey <lilijreey 126.com> writes:
Hi:
    I write code like this
```D
class Base
{
int base;
}

class CC :Base
{
  int cc;
}

auto cc = new CC;

//I want check cc object type is CC
if (typeid(cc) == typeid(CC)) // ok ==

if (cc.classinfo == AstAssignNode) //error

if (cc.classinfo == AstAssignNode.classinfo) // ok ==
```
Mar 30 2020
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 3/30/20 10:38 AM, lilijreey wrote:

 Hi:
     I write code like this
 ```D
 class Base
 {
 int base;
 }
 
 class CC :Base
 {
   int cc;
 }
 
 auto cc = new CC;
 
 //I want check cc object type is CC
 if (typeid(cc) == typeid(CC)) // ok ==
 
 if (cc.classinfo == AstAssignNode) //error
 
 if (cc.classinfo == AstAssignNode.classinfo) // ok ==
 ```


They are both the same. Historically they were different (ClassInfo was 
its own type different from TypeInfo_Class). I would recommend typeid 
for future-proof code (it's possible, however unlikely, that .classinfo 
at some point goes away).

-Steve
Mar 30 2020
parent reply Pham <home home.com> writes:
On Monday, 30 March 2020 at 15:15:08 UTC, Steven Schveighoffer 
wrote:

 On 3/30/20 10:38 AM, lilijreey wrote:

 Hi:
     I write code like this
 ```D
 class Base
 {
 int base;
 }
 
 class CC :Base
 {
   int cc;
 }
 
 auto cc = new CC;
 
 //I want check cc object type is CC
 if (typeid(cc) == typeid(CC)) // ok ==
 
 if (cc.classinfo == AstAssignNode) //error
 
 if (cc.classinfo == AstAssignNode.classinfo) // ok ==
 ```


 They are both the same. Historically they were different 
 (ClassInfo was its own type different from TypeInfo_Class). I 
 would recommend typeid for future-proof code (it's possible, 
 however unlikely, that .classinfo at some point goes away).

 -Steve


Will it be the same if using "is", the reason is for function 
that use "nothrow" attribute?
if (cc.classinfo is typeid(CC))
Mar 30 2020
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 3/30/20 1:06 PM, Pham wrote:

 On Monday, 30 March 2020 at 15:15:08 UTC, Steven Schveighoffer wrote:

 They are both the same. Historically they were different (ClassInfo 
 was its own type different from TypeInfo_Class). I would recommend 
 typeid for future-proof code (it's possible, however unlikely, that 
 .classinfo at some point goes away).


 
 Will it be the same if using "is", the reason is for function that use 
 "nothrow" attribute?
 if (cc.classinfo is typeid(CC))


It should be identical.

For fun I did an AST dump for this code:

void main()
{
     Object o;
     auto ti = typeid(o);
     auto ci = o.classinfo;
}

And this is what I get:

void main()
{
	Object o = null;
	TypeInfo_Class ti = typeid(o);
	TypeInfo_Class ci = **o;
	return 0;
}

So it looks like typeid(o) is not lowered to something, but o.classinfo 
is lowered to **o (the first field in an object is the TypeInfo).

I would say that this is going to reduce to the same thing in both 
cases, but I find it interesting that the AST is different.

I don't really understand the question about "nothrow".

-Steve
Mar 30 2020
parent reply Pham <home home.com> writes:
On Monday, 30 March 2020 at 18:20:00 UTC, Steven Schveighoffer 
wrote:

 On 3/30/20 1:06 PM, Pham wrote:



 
 Will it be the same if using "is", the reason is for function 
 that use "nothrow" attribute?
 if (cc.classinfo is typeid(CC))



 I don't really understand the question about "nothrow".

 -Steve


void main()
{
	static class Foo { int i; }
	
	static bool checkFoo() nothrow
	{
		// error when compile
		//return typeid(Foo) == Foo.classinfo;
		
		// ok when compile
		return typeid(Foo) is Foo.classinfo;
	}
		
	checkFoo();
}
Mar 30 2020
parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 3/30/20 5:17 PM, Pham wrote:

 On Monday, 30 March 2020 at 18:20:00 UTC, Steven Schveighoffer wrote:

 I don't really understand the question about "nothrow".


 
 void main()
 {
      static class Foo { int i; }
 
      static bool checkFoo() nothrow
      {
          // error when compile
          //return typeid(Foo) == Foo.classinfo;
 
          // ok when compile
          return typeid(Foo) is Foo.classinfo;
      }
 
      checkFoo();
 }


In this case, the difference is == vs. `is`, not classinfo vs typeid:

typeid(Foo) == typeid(Foo); // throws

Foo.classinfo == Foo.classinfo; // throws

typeid(Foo) is typeid(Foo); // nothrow

Foo.classinfo is Foo.classinfo; // nothrow

`is` between class instances means "is this the same instance". It is a 
simple comparison of two pointers basically.

== between classes means calling object.opEquals [1], which is not 
nothrow (or nogc, or pure).

In fact, they should be the same instance, because typeid and classinfo 
do the same thing.

-Steve

[1] https://dlang.org/phobos/object.html#.opEquals
Mar 30 2020