Finding the type of an object in C

Question

I have a class A and another class that inherits from it  B  I am overriding a function that accepts an object of type A as a parameter  so I have to accept an A  However  I later call functions that only B has  so I want to return false and not proceed if the object passed is not of type B   What is the best way to find out which type the object passed to my function is

User · Answer

You are looking for dynamic cast lt B  gt  pointer

User · Answer

If you can access boost library  maybe type id with cvr   function is what you need  which can provide data type without removing const  volatile   amp  and  amp  amp  modifiers  Here is an simple example in C  11    include  lt iostream gt   include  lt boost type index hpp gt   int a  int amp  ff          return a     int main         ff     10      using boost  typeindex  type id with cvr      std  cout  lt  lt  type id with cvr lt int amp  gt    pretty name    lt  lt  std  endl      std  cout  lt  lt  type id with cvr lt decltype ff    gt    pretty name    lt  lt  std  endl      std  cout  lt  lt  typeid ff    name    lt  lt  std  endl      Hope this is useful

User · Answer

dynamic cast should do the trick TYPE amp  dynamic cast lt TYPE amp  gt   object   TYPE  dynamic cast lt TYPE  gt   object    The dynamic cast keyword casts a datum from one pointer or reference type to another  performing a runtime check to ensure the validity of the cast  If you attempt to cast to pointer to a type that is not a type of actual object  the result of the cast will be NULL  If you attempt to cast to reference to a type that is not a type of actual object  the cast will throw a bad cast exception  Make sure there is at least one virtual function in Base class to make dynamic cast work  Wikipedia topic Run-time type information  RTTI is available only for classes that are polymorphic  which means they have at least one virtual method  In practice  this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer

User · Answer

Probably embed into your objects an ID  tag  and use it to distinguish between objects of class A and objects of class B   This however shows a flaw in the design  Ideally those methods in B which A doesn t have  should be part of A but left empty  and B overwrites them  This does away with the class-specific code and is more in the spirit of OOP

User · Answer

This is called RTTI  but you almost surely want to reconsider your design here  because finding the type and doing special things based on it makes your code more brittle

User · Answer

Dynamic cast is the best for your description of problem   but I just want to add that you can find the class type with    include  lt typeinfo gt       string s   typeid YourClass  name

User · Answer

Probably embed into your objects an ID  tag  and use it to distinguish between objects of class A and objects of class B   This however shows a flaw in the design  Ideally those methods in B which A doesn t have  should be part of A but left empty  and B overwrites them  This does away with the class-specific code and is more in the spirit of OOP

User · Answer

If you can access boost library  maybe type id with cvr   function is what you need  which can provide data type without removing const  volatile   amp  and  amp  amp  modifiers  Here is an simple example in C  11    include  lt iostream gt   include  lt boost type index hpp gt   int a  int amp  ff          return a     int main         ff     10      using boost  typeindex  type id with cvr      std  cout  lt  lt  type id with cvr lt int amp  gt    pretty name    lt  lt  std  endl      std  cout  lt  lt  type id with cvr lt decltype ff    gt    pretty name    lt  lt  std  endl      std  cout  lt  lt  typeid ff    name    lt  lt  std  endl      Hope this is useful

User · Answer

Use overloaded functions  Does not require dynamic cast or even RTTI support   class A     class B   public A      class Foo   public      void Bar A amp  a               do something           void Bar B amp  b            Bar static cast lt A amp  gt  b               do B specific stuff

User · Answer

Use overloaded functions  Does not require dynamic cast or even RTTI support   class A     class B   public A      class Foo   public      void Bar A amp  a               do something           void Bar B amp  b            Bar static cast lt A amp  gt  b               do B specific stuff

User · Answer

As others indicated you can use dynamic cast  But generally using dynamic cast for finding out the type of the derived class you are working upon indicates the bad design  If you are overriding a function that takes pointer of A as the parameter then it should be able to work with the methods data of class A itself and should not depend on the the data of class B  In your case instead of overriding if you are sure that the method you are writing will work with only class B  then you should write a new method in class B

User · Answer

Your description is a little confusing    Generally speaking  though some C   implementations have mechanisms for it  you re not supposed to ask about the type  Instead  you are supposed to do a dynamic cast on the pointer to A  What this will do is that at runtime  the actual contents of the pointer to A will be checked  If you have a B  you ll get your pointer to B  Otherwise  you ll get an exception or null

User · Answer

Probably embed into your objects an ID  tag  and use it to distinguish between objects of class A and objects of class B   This however shows a flaw in the design  Ideally those methods in B which A doesn t have  should be part of A but left empty  and B overwrites them  This does away with the class-specific code and is more in the spirit of OOP

User · Answer

dynamic cast should do the trick TYPE amp  dynamic cast lt TYPE amp  gt   object   TYPE  dynamic cast lt TYPE  gt   object    The dynamic cast keyword casts a datum from one pointer or reference type to another  performing a runtime check to ensure the validity of the cast  If you attempt to cast to pointer to a type that is not a type of actual object  the result of the cast will be NULL  If you attempt to cast to reference to a type that is not a type of actual object  the cast will throw a bad cast exception  Make sure there is at least one virtual function in Base class to make dynamic cast work  Wikipedia topic Run-time type information  RTTI is available only for classes that are polymorphic  which means they have at least one virtual method  In practice  this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer

User · Answer

Just to be complete  I ll build build off of Robocide and point out that typeid can be used alone without using name      include  lt typeinfo gt   include  lt iostream gt   using namespace std   class A   public      virtual  A     default     We re not polymorphic unless we                                have a virtual function     class B   public A       class C   public A        int main int argc  char  argv          B b      A amp  a   b       cout  lt  lt   a is B     lt  lt  boolalpha  lt  lt   typeid a     typeid B    lt  lt  endl      cout  lt  lt   a is C     lt  lt  boolalpha  lt  lt   typeid a     typeid C    lt  lt  endl      cout  lt  lt   b is B     lt  lt  boolalpha  lt  lt   typeid b     typeid B    lt  lt  endl      cout  lt  lt   b is A     lt  lt  boolalpha  lt  lt   typeid b     typeid A    lt  lt  endl      cout  lt  lt   b is C     lt  lt  boolalpha  lt  lt   typeid b     typeid C    lt  lt  endl      Output   a is B  true a is C  false b is B  true b is A  false b is C  false

User · Answer

Use overloaded functions  Does not require dynamic cast or even RTTI support   class A     class B   public A      class Foo   public      void Bar A amp  a               do something           void Bar B amp  b            Bar static cast lt A amp  gt  b               do B specific stuff

User · Answer

This is called RTTI  but you almost surely want to reconsider your design here  because finding the type and doing special things based on it makes your code more brittle

User · Answer

Your description is a little confusing    Generally speaking  though some C   implementations have mechanisms for it  you re not supposed to ask about the type  Instead  you are supposed to do a dynamic cast on the pointer to A  What this will do is that at runtime  the actual contents of the pointer to A will be checked  If you have a B  you ll get your pointer to B  Otherwise  you ll get an exception or null

User · Answer

You are looking for dynamic cast lt B  gt  pointer

User · Answer

As others indicated you can use dynamic cast  But generally using dynamic cast for finding out the type of the derived class you are working upon indicates the bad design  If you are overriding a function that takes pointer of A as the parameter then it should be able to work with the methods data of class A itself and should not depend on the the data of class B  In your case instead of overriding if you are sure that the method you are writing will work with only class B  then you should write a new method in class B

User · Answer

Because your class is not polymorphic  Try   struct BaseClas   int base  virtual  BaseClas        class Derived1   public BaseClas   int derived1       Now BaseClas is polymorphic  I changed class to struct because the members of a struct are public by default

User · Answer

You are looking for dynamic cast lt B  gt  pointer

User · Answer

Probably embed into your objects an ID  tag  and use it to distinguish between objects of class A and objects of class B   This however shows a flaw in the design  Ideally those methods in B which A doesn t have  should be part of A but left empty  and B overwrites them  This does away with the class-specific code and is more in the spirit of OOP

User · Answer

This is called RTTI  but you almost surely want to reconsider your design here  because finding the type and doing special things based on it makes your code more brittle

User · Answer

As others indicated you can use dynamic cast  But generally using dynamic cast for finding out the type of the derived class you are working upon indicates the bad design  If you are overriding a function that takes pointer of A as the parameter then it should be able to work with the methods data of class A itself and should not depend on the the data of class B  In your case instead of overriding if you are sure that the method you are writing will work with only class B  then you should write a new method in class B

User · Answer

This is called RTTI  but you almost surely want to reconsider your design here  because finding the type and doing special things based on it makes your code more brittle

User · Answer

As others indicated you can use dynamic cast  But generally using dynamic cast for finding out the type of the derived class you are working upon indicates the bad design  If you are overriding a function that takes pointer of A as the parameter then it should be able to work with the methods data of class A itself and should not depend on the the data of class B  In your case instead of overriding if you are sure that the method you are writing will work with only class B  then you should write a new method in class B

User · Answer

dynamic cast should do the trick TYPE amp  dynamic cast lt TYPE amp  gt   object   TYPE  dynamic cast lt TYPE  gt   object    The dynamic cast keyword casts a datum from one pointer or reference type to another  performing a runtime check to ensure the validity of the cast  If you attempt to cast to pointer to a type that is not a type of actual object  the result of the cast will be NULL  If you attempt to cast to reference to a type that is not a type of actual object  the cast will throw a bad cast exception  Make sure there is at least one virtual function in Base class to make dynamic cast work  Wikipedia topic Run-time type information  RTTI is available only for classes that are polymorphic  which means they have at least one virtual method  In practice  this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer

User · Answer

Just to be complete  I ll build build off of Robocide and point out that typeid can be used alone without using name      include  lt typeinfo gt   include  lt iostream gt   using namespace std   class A   public      virtual  A     default     We re not polymorphic unless we                                have a virtual function     class B   public A       class C   public A        int main int argc  char  argv          B b      A amp  a   b       cout  lt  lt   a is B     lt  lt  boolalpha  lt  lt   typeid a     typeid B    lt  lt  endl      cout  lt  lt   a is C     lt  lt  boolalpha  lt  lt   typeid a     typeid C    lt  lt  endl      cout  lt  lt   b is B     lt  lt  boolalpha  lt  lt   typeid b     typeid B    lt  lt  endl      cout  lt  lt   b is A     lt  lt  boolalpha  lt  lt   typeid b     typeid A    lt  lt  endl      cout  lt  lt   b is C     lt  lt  boolalpha  lt  lt   typeid b     typeid C    lt  lt  endl      Output   a is B  true a is C  false b is B  true b is A  false b is C  false

User · Answer

You are looking for dynamic cast lt B  gt  pointer

User · Answer

Your description is a little confusing    Generally speaking  though some C   implementations have mechanisms for it  you re not supposed to ask about the type  Instead  you are supposed to do a dynamic cast on the pointer to A  What this will do is that at runtime  the actual contents of the pointer to A will be checked  If you have a B  you ll get your pointer to B  Otherwise  you ll get an exception or null

User · Answer

dynamic cast should do the trick TYPE amp  dynamic cast lt TYPE amp  gt   object   TYPE  dynamic cast lt TYPE  gt   object    The dynamic cast keyword casts a datum from one pointer or reference type to another  performing a runtime check to ensure the validity of the cast  If you attempt to cast to pointer to a type that is not a type of actual object  the result of the cast will be NULL  If you attempt to cast to reference to a type that is not a type of actual object  the cast will throw a bad cast exception  Make sure there is at least one virtual function in Base class to make dynamic cast work  Wikipedia topic Run-time type information  RTTI is available only for classes that are polymorphic  which means they have at least one virtual method  In practice  this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer

User · Answer

Dynamic cast is the best for your description of problem   but I just want to add that you can find the class type with    include  lt typeinfo gt       string s   typeid YourClass  name

User · Answer

Your description is a little confusing    Generally speaking  though some C   implementations have mechanisms for it  you re not supposed to ask about the type  Instead  you are supposed to do a dynamic cast on the pointer to A  What this will do is that at runtime  the actual contents of the pointer to A will be checked  If you have a B  you ll get your pointer to B  Otherwise  you ll get an exception or null

User · Answer

Use overloaded functions  Does not require dynamic cast or even RTTI support   class A     class B   public A      class Foo   public      void Bar A amp  a               do something           void Bar B amp  b            Bar static cast lt A amp  gt  b               do B specific stuff

User · Answer

Because your class is not polymorphic  Try   struct BaseClas   int base  virtual  BaseClas        class Derived1   public BaseClas   int derived1       Now BaseClas is polymorphic  I changed class to struct because the members of a struct are public by default

[c++] Finding the type of an object in C++

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