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17.9.3 Conversion operators

王朝other·作者佚名  2006-05-21
窄屏简体版  字體: |||超大  

A conversion operator declaration introduces a user-defined conversion (§13.

4), which augments the predefined

implicit and explicit conversions.

A conversion operator declaration that includes the implicit keyword

introduces a user-defined implicit

conversion. Implicit conversions can occur in a variety of situations,

including function member invocations, cast

expressions, and assignments. This is described further in §13.1.

Chapter 17 Classes

257

A conversion operator declaration that includes the explicit keyword

introduces a user-defined explicit

conversion. Explicit conversions can occur in cast expressions, and are

described further in §13.2.

A conversion operator converts from a source type, indicated by the

parameter type of the conversion operator, to

a target type, indicated by the return type of the conversion operator. A

class or struct is permitted to declare a

conversion from a source type S to a target type T provided all of the

following are true:

? S and T are different types.

? Either S or T is the class or struct type in which the operator

declaration takes place.

? Neither S nor T is object or an interface-type.

? T is not a base class of S, and S is not a base class of T.

From the second rule it follows that a conversion operator must convert

either to or from the class or struct type

in which the operator is declared. [Example: For example, it is possible

for a class or struct type C to define a

conversion from C to int and from int to C, but not from int to bool. end

example]

It is not possible to redefine a pre-defined conversion. Thus, conversion

operators are not allowed to convert from

or to object because implicit and explicit conversions already exist

between object and all other types.

Likewise, neither the source nor the target types of a conversion can be a

base type of the other, since a

conversion would then already exist.

User-defined conversions are not allowed to convert from or to

interface-types. In particular, this restriction

ensures that no user-defined transformations occur when converting to an

interface-type, and that a conversion to

an interface-type succeeds only if the object being converted actually

implements the specified interface-type.

The signature of a conversion operator consists of the source type and the

target type. (This is the only form of

member for which the return type participates in the signature.) The

implicit or explicit classification of a

conversion operator is not part of the operator?s signature. Thus, a class

or struct cannot declare both an

implicit and an explicit conversion operator with the same source and

target types.

[Note: In general, user-defined implicit conversions should be designed to

never throw exceptions and never lose

information. If a user-defined conversion can give rise to exceptions (for

example, because the source argument

is out of range) or loss of information (such as discarding high-order

bits), then that conversion should be defined

as an explicit conversion. end note]

[Example: In the example

using System;

public struct Digit

{

byte value;

public Digit(byte value) {

if (value < 0 || value > 9) throw new ArgumentException();

this.value = value;

}

public static implicit operator byte(Digit d) {

return d.value;

}

public static explicit operator Digit(byte b) {

return new Digit(b);

}

}

the conversion from Digit to byte is implicit because it never throws

exceptions or loses information, but the

conversion from byte to Digit is explicit since Digit can only represent a

subset of the possible values of a

byte. end example]

 
 
 
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