Compiler Support for Type Traits (C++/CLI and C++/CX)

Article
08/03/2021

The Microsoft C++ compiler supports type traits for C++/CLI and C++/CX extensions, which indicate various characteristics of a type at compile time.

All Runtimes

Remarks

Type traits are especially useful to programmers who write libraries.

The following list contains the type traits that are supported by the compiler. All type traits return false if the condition specified by the name of the type trait is not met.

(In the following list, code examples are written only in C++/CLI. But the corresponding type trait is also supported in C++/CX unless stated otherwise. The term, "platform type" refers to either Windows Runtime types or common language runtime types.)

__has_assign( type )

Returns true if the platform or native type has a copy assignment operator.

ref struct R {
void operator=(R% r) {}
};

int main() {
System::Console::WriteLine(__has_assign(R));
}

__has_copy( type )

Returns true if the platform or native type has a copy constructor.

ref struct R {
R(R% r) {}
};

int main() {
System::Console::WriteLine(__has_copy(R));
}

__has_finalizer( type )

(Not supported in C++/CX.) Returns true if the CLR type has a finalizer. See Destructors and finalizers in How to: Define and consume classes and structs (C++/CLI) for more information.
```
using namespace System;
ref struct R {
~R() {}
protected:
!R() {}
};

int main() {
Console::WriteLine(__has_finalizer(R));
}
```

__has_nothrow_assign( type )

Returns true if a copy assignment operator has an empty exception specification.

#include <stdio.h>
struct S {
void operator=(S& r) throw() {}
};

int main() {
__has_nothrow_assign(S) == true ?
printf("true\n") : printf("false\n");
}

__has_nothrow_constructor( type )

Returns true if the default constructor has an empty exception specification.

#include <stdio.h>
struct S {
S() throw() {}
};

int main() {
__has_nothrow_constructor(S) == true ?
printf("true\n") : printf("false\n");
}

__has_nothrow_copy( type )

Returns true if the copy constructor has an empty exception specification.

#include <stdio.h>
struct S {
S(S& r) throw() {}
};

int main() {
__has_nothrow_copy(S) == true ?
printf("true\n") : printf("false\n");
}

__has_trivial_assign( type )

Returns true if the type has a trivial, compiler-generated assignment operator.

#include <stdio.h>
struct S {};

int main() {
__has_trivial_assign(S) == true ?
printf("true\n") : printf("false\n");
}

__has_trivial_constructor( type )

Returns true if the type has a trivial, compiler-generated constructor.

#include <stdio.h>
struct S {};

int main() {
__has_trivial_constructor(S) == true ?
printf("true\n") : printf("false\n");
}

__has_trivial_copy( type )

Returns true if the type has a trivial, compiler-generated copy constructor.

#include <stdio.h>
struct S {};

int main() {
__has_trivial_copy(S) == true ?
printf("true\n") : printf("false\n");
}

__has_trivial_destructor( type )

Returns true if the type has a trivial, compiler-generated destructor.

// has_trivial_destructor.cpp
#include <stdio.h>
struct S {};

int main() {
__has_trivial_destructor(S) == true ?
printf("true\n") : printf("false\n");
}

__has_user_destructor( type )

Returns true if the platform or native type has a user-declared destructor.

// has_user_destructor.cpp

using namespace System;
ref class R {
~R() {}
};

int main() {
Console::WriteLine(__has_user_destructor(R));
}

__has_virtual_destructor( type )

Returns true if the type has a virtual destructor.

__has_virtual_destructor also works on platform types, and any user-defined destructor in a platform type is a virtual destructor.
```
// has_virtual_destructor.cpp
#include <stdio.h>
struct S {
virtual ~S() {}
};

int main() {
__has_virtual_destructor(S) == true ?
printf("true\n") : printf("false\n");
}
```
__is_abstract( type )

Returns true if the type is an abstract type. For more information on native abstract types, see Abstract Classes.

__is_abstract also works for platform types. An interface with at least one member is an abstract type, as is a reference type with at least one abstract member. For more information on abstract platform types, see abstract.
```
// is_abstract.cpp
#include <stdio.h>
struct S {
virtual void Test() = 0;
};

int main() {
__is_abstract(S) == true ?
printf("true\n") : printf("false\n");
}
```
__is_base_of( base , derived )

Returns true if the first type is a base class of the second type, or if both types are the same.

__is_base_of also works on platform types. For example, it will return true if the first type is an interface class and the second type implements the interface.
```
// is_base_of.cpp
#include <stdio.h>
struct S {};
struct T : public S {};

int main() {
__is_base_of(S, T) == true ?
printf("true\n") : printf("false\n");

__is_base_of(S, S) == true ?
printf("true\n") : printf("false\n");
}
```

__is_class( type )

Returns true if the type is a native class or struct.

#include <stdio.h>
struct S {};

int main() {
__is_class(S) == true ?
printf("true\n") : printf("false\n");
}

__is_convertible_to( from , to )

Returns true if the first type can be converted to the second type.

#include <stdio.h>
struct S {};
struct T : public S {};

int main() {
S * s = new S;
T * t = new T;
s = t;
__is_convertible_to(T, S) == true ?
printf("true\n") : printf("false\n");
}

__is_delegate( type )

Returns true if type is a delegate. For more information, see delegate (C++/CLI and C++/CX).
```
delegate void MyDel();
int main() {
System::Console::WriteLine(__is_delegate(MyDel));
}
```

__is_empty( type )

Returns true if the type has no instance data members.

#include <stdio.h>
struct S {
int Test() {}
static int i;
};
int main() {
__is_empty(S) == true ?
printf("true\n") : printf("false\n");
}

__is_enum( type )

Returns true if the type is a native enum.

// is_enum.cpp
#include <stdio.h>
enum E { a, b };

struct S {
enum E2 { c, d };
};

int main() {
__is_enum(E) == true ?
printf("true\n") : printf("false\n");

__is_enum(S::E2) == true ?
printf("true\n") : printf("false\n");
}

__is_interface_class( type )

Returns true if passed a platform interface. For more information, see interface class.

// is_interface_class.cpp

using namespace System;
interface class I {};
int main() {
Console::WriteLine(__is_interface_class(I));
}

__is_pod( type )

Returns true if the type is a class or union with no constructor or private or protected non-static members, no base classes, and no virtual functions. See the C++ standard, sections 8.5.1/1, 9/4, and 3.9/10 for more information on PODs.

__is_pod will return false on fundamental types.
```
#include <stdio.h>
struct S {};

int main() {
__is_pod(S) == true ?
printf("true\n") : printf("false\n");
}
```

__is_polymorphic( type )

Returns true if a native type has virtual functions.

#include <stdio.h>
struct S {
virtual void Test(){}
};

int main() {
__is_polymorphic(S) == true ?
printf("true\n") : printf("false\n");
}

__is_ref_array( type )

Returns true if passed a platform array. For more information, see Arrays.

using namespace System;
int main() {
array<int>^ x = gcnew array<int>(10);
Console::WriteLine(__is_ref_array(array<int>));
}

__is_ref_class( type )

Returns true if passed a reference class. For more information on user-defined reference types, see Classes and Structs.

using namespace System;
ref class R {};
int main() {
Console::WriteLine(__is_ref_class(Buffer));
Console::WriteLine(__is_ref_class(R));
}

__is_sealed( type )

Returns true if passed a platform or native type marked sealed. For more information, see sealed.
```
ref class R sealed{};
int main() {
System::Console::WriteLine(__is_sealed(R));
}
```

__is_simple_value_class( type )

Returns true if passed a value type that contains no references to the garbage-collected heap. For more information on user-defined value types, see Classes and Structs.

using namespace System;
ref class R {};
value struct V {};
value struct V2 {
R ^ r;   // not a simple value type
};

int main() {
Console::WriteLine(__is_simple_value_class(V));
Console::WriteLine(__is_simple_value_class(V2));
}

__is_union( type )

Returns true if a type is a union.

#include <stdio.h>
union A {
int i;
float f;
};

int main() {
__is_union(A) == true ?
printf("true\n") : printf("false\n");
}

__is_value_class( type )

Returns true if passed a value type. For more information on user-defined value types, see Classes and Structs.
```
value struct V {};

int main() {
System::Console::WriteLine(__is_value_class(V));
}
```

Windows Runtime

Remarks

The __has_finalizer(type) type trait is not supported because this platform does not support finalizers.

Requirements

Compiler option: /ZW

Common Language Runtime

Remarks

(There are no platform-specific remarks for this feature.)

Requirements

Compiler option: /clr

Examples

Example

The following code example shows how to use a class template to expose a compiler type trait for a /clr compilation. For more information, see Windows Runtime and Managed Templates.

// compiler_type_traits.cpp
// compile with: /clr
using namespace System;

template <class T>
ref struct is_class {
   literal bool value = __is_ref_class(T);
};

ref class R {};

int main () {
   if (is_class<R>::value)
      Console::WriteLine("R is a ref class");
   else
      Console::WriteLine("R is not a ref class");
}

R is a ref class

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Compiler Support for Type Traits (C++/CLI and C++/CX)

All Runtimes

Remarks

Windows Runtime

Remarks

Requirements

Common Language Runtime

Remarks

Requirements

Examples

See also

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