User-Defined Conversions (C++/CLI)
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The latest version of this topic can be found at User-Defined Conversions (C++/CLI).
This section discusses user-defined conversions (UDC) when one of the types in the conversion is a reference or instance of a value type or reference type.
Implicit and explicit conversions
A user-defined conversion can either be implicit or explicit. A UDC should be implicit if the conversion does not result in a loss of information. Otherwise an explicit UDC should be defined.
A native class's constructor can be used to convert a reference or value type to a native class.
For more information about conversions, see Boxing and Standard Conversions.
// mcpp_User_Defined_Conversions.cpp
// compile with: /clr
#include "stdio.h"
ref class R;
class N;
value class V {
static operator V(R^) {
return V();
}
};
ref class R {
public:
static operator N(R^);
static operator V(R^) {
System::Console::WriteLine("in R::operator N");
return V();
}
};
class N {
public:
N(R^) {
printf("in N::N\n");
}
};
R::operator N(R^) {
System::Console::WriteLine("in R::operator N");
return N(nullptr);
}
int main() {
// Direct initialization:
R ^r2;
N n2(r2); // direct initialization, calls constructor
static_cast<N>(r2); // also direct initialization
R ^r3;
// ambiguous V::operator V(R^) and R::operator V(R^)
// static_cast<V>(r3);
}
Output
in N::N
in N::N
Convert-From Operators
Convert-from operators create an object of the class in which the operator is defined from an object of some other class.
Standard C++ does not support convert-from operators; standard C++ uses constructors for this purpose. However, when using CLR types, Visual C++ provide syntactic support for calling convert-from operators.
To interoperate well with other CLS-compliant languages, you may wish to wrap each user-defined unary constructor for a given class with a corresponding convert-from operator.
Convert-from operators:
Shall be defined as static functions.
Can either be implicit (for conversions that do not lose precision such as short-to-int) or explicit, when there may be a loss of precision.
Shall return an object of the containing class.
Shall have the "from" type as the sole parameter type.
The following sample shows an implicit and explicit "convert-from", user-defined conversion (UDC) operator.
// clr_udc_convert_from.cpp
// compile with: /clr
value struct MyDouble {
double d;
MyDouble(int i) {
d = static_cast<double>(i);
System::Console::WriteLine("in constructor");
}
// Wrap the constructor with a convert-from operator.
// implicit UDC because conversion cannot lose precision
static operator MyDouble (int i) {
System::Console::WriteLine("in operator");
// call the constructor
MyDouble d(i);
return d;
}
// an explicit user-defined conversion operator
static explicit operator signed short int (MyDouble) {
return 1;
}
};
int main() {
int i = 10;
MyDouble md = i;
System::Console::WriteLine(md.d);
// using explicit user-defined conversion operator requires a cast
unsigned short int j = static_cast<unsigned short int>(md);
System::Console::WriteLine(j);
}
Output
in operator
in constructor
10
1
Convert-to operators
Convert-to operators convert an object of the class in which the operator is defined to some other object. The following sample shows an implicit, convert-to, user-defined conversion operator:
// clr_udc_convert_to.cpp
// compile with: /clr
using namespace System;
value struct MyInt {
Int32 i;
// convert MyInt to String^
static operator String^ ( MyInt val ) {
return val.i.ToString();
}
MyInt(int _i) : i(_i) {}
};
int main() {
MyInt mi(10);
String ^s = mi;
Console::WriteLine(s);
}
Output
10
An explicit user-defined convert-to conversion operator is appropriate for conversions that potentially lose data in some way. To invoke an explicit convert-to operator, a cast must be used.
// clr_udc_convert_to_2.cpp
// compile with: /clr
value struct MyDouble {
double d;
// convert MyDouble to Int32
static explicit operator System::Int32 ( MyDouble val ) {
return (int)val.d;
}
};
int main() {
MyDouble d;
d.d = 10.3;
System::Console::WriteLine(d.d);
int i = 0;
i = static_cast<int>(d);
System::Console::WriteLine(i);
}
Output
10.3
10
To convert generic classes
You can convert a generic class to T.
// clr_udc_generics.cpp
// compile with: /clr
generic<class T>
public value struct V {
T mem;
static operator T(V v) {
return v.mem;
}
void f(T t) {
mem = t;
}
};
int main() {
V<int> v;
v.f(42);
int i = v;
i += v;
System::Console::WriteLine(i == (42 * 2) );
}
Output
True
A converting constructor takes a type and uses it to create an object. A converting constructor is called with direct initialization only; casts will not invoke converting constructors. By default, converting constructors are explicit for CLR types.
// clr_udc_converting_constructors.cpp
// compile with: /clr
public ref struct R {
int m;
char c;
R(int i) : m(i) { }
R(char j) : c(j) { }
};
public value struct V {
R^ ptr;
int m;
V(R^ r) : ptr(r) { }
V(int i) : m(i) { }
};
int main() {
R^ r = gcnew R(5);
System::Console::WriteLine( V(5).m);
System::Console::WriteLine( V(r).ptr);
}
Output
5
R
In this code sample, an implicit static conversion function does the same thing as an explicit conversion constructor.
public value struct V {
int m;
V(int i) : m(i) {}
static operator V(int i) {
V v(i*100);
return v;
}
};
public ref struct R {
int m;
R(int i) : m(i) {}
static operator R^(int i) {
return gcnew R(i*100);
}
};
int main() {
V v(13); // explicit
R^ r = gcnew R(12); // explicit
System::Console::WriteLine(v.m);
System::Console::WriteLine(r->m);
// explicit ctor can't be called here: not ambiguous
v = 5;
r = 20;
System::Console::WriteLine(v.m);
System::Console::WriteLine(r->m);
}
Output
13
12
500
2000