Implicit Boxing
The Visual C++ compiler now boxes value types to Object. This is possible because of a compiler-defined conversion to convert value types to Object.
Remarks
Boxing and unboxing enable value types to be treated as objects. Value types, including both struct types and built-in types such as int, can be converted to and from the type Object.
For more information, see:
Example
The following sample shows how explicit boxing was done with Managed Extensions for C++.
// vcmcppv2_explicit_boxing.cpp
// compile with: /clr:oldSyntax
using namespace System;
__gc class A {
public:
void func(System::Object * o){Console::WriteLine("in A");}
};
__value class V {};
__gc __interface IFace {
void func();
};
__value class V1 : public IFace {
public:
void func() {
Console::WriteLine("Interface function");
}
};
int main() {
// example 1 explicit boxing
Int32 __box* bi = __box(1);
Console::WriteLine(bi);
// example 2 calling a member with explicit boxing
Int32 n = 10;
Console::WriteLine(S"xx = {0}", __box(n)->ToString());
// example 3 explicit boxing for function calls
A * a = new A;
a->func(__box(n));
// example 4 explicit boxing for WriteLine function call
V v;
Console::WriteLine(S"Class {0} passed using implicit boxing", __box(v)->ToString());
// example 5 casting to a base with explicit boxing
V1 v1;
IFace * iface = __box(v1);
iface->func();
}
1
xx = 10
in A
Class V passed using implicit boxing
Interface function
The following sample shows how implicit boxing works.
// vcmcppv2_explicit_boxing2.cpp
// compile with: /clr
using namespace System;
ref class A {
public:
void func(System::Object^ o){Console::WriteLine("in A");}
};
value class V {};
interface struct IFace {
void func();
};
value class V1 : public IFace {
public:
virtual void func() {
Console::WriteLine("Interface function");
}
};
value struct V2 {
// conversion operator to System::Object
static operator System::Object^(V2 v2) {
Console::WriteLine("operator System::Object^");
return (V2^)v2;
}
};
void func1(System::Object^){Console::WriteLine("in void func1(System::Object^)");}
void func1(V2^){Console::WriteLine("in func1(V2^)");}
void func2(System::ValueType^){Console::WriteLine("in func2(System::ValueType^)");}
void func2(System::Object^){Console::WriteLine("in func2(System::Object^)");}
int main() {
// example 1 simple implicit boxing
Int32^ bi = 1;
Console::WriteLine(bi);
// example 2 calling a member with implicit boxing
Int32 n = 10;
Console::WriteLine("xx = {0}", n.ToString());
// example 3 implicit boxing for function calls
A^ a = gcnew A;
a->func(n);
// example 4 implicit boxing for WriteLine function call
V v;
Console::WriteLine("Class {0} passed using implicit boxing", v);
Console::WriteLine("Class {0} passed with forced boxing", (V^)(v)); // force boxing
// example 5 casting to a base with implicit boxing
V1 v1;
IFace ^ iface = v1;
iface->func();
// example 6 user-defined conversion preferred over implicit boxing for function-call parameter matching
V2 v2;
func1(v2); // user defined conversion from V2 to System::Object preferred over implicit boxing
// Will call void func1(System::Object^);
func2(v2); // OK: Calls "static V2::operator System::Object^(V2 v2)"
func2((V2^)v2); // Using explicit boxing: calls func2(System::ValueType^)
}
1
xx = 10
in A
Class V passed using implicit boxing
Class V passed with forced boxing
Interface function
in func1(V2^)
in func2(System::ValueType^)
in func2(System::ValueType^)
Requirements
Compiler option: /clr