The type to cast from. The programmer must ensure it is legal to make this cast.
// Regular cast, which has been verified to be legal by the programmer: { long x; auto y = cast(int) x; } // However this will still compile if 'x' is changed to be a pointer: { long* x; auto y = cast(int) x; } // castFrom provides a more reliable alternative to casting: { long x; auto y = castFrom!long.to!int(x); } // Changing the type of 'x' will now issue a compiler error, // allowing bad casts to be caught before it's too late: { long* x; static assert( !__traits(compiles, castFrom!long.to!int(x)) ); // if cast is still needed, must be changed to: auto y = castFrom!(long*).to!int(x); }
A wrapper on top of the built-in cast operator that allows one to restrict casting of the original type of the value.
A common issue with using a raw cast is that it may silently continue to compile even if the value's type has changed during refactoring, which breaks the initial assumption about the cast.