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How to: Convert with Constructors

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);
}
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);
}
13 12 500 2000

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