我已经更改了Adam D Ruppes模块notnull.d，只允许使用构造函数NotNull继承的类实例赋值给NotNull基类实例>
/** Assignment from $(D NotNull) Inherited Class $(D rhs) to $(D NotNull) Base
Class $(D this). */
typeof(this) opAssign(U)(NotNull!U rhs) @safe pure nothrow if (isAssignable!(T, U)) {
this._value = rhs._value;
return this;
}
这允许
unittest
{
class A {}
class B : A {}
NotNull!B b = assumeNotNull(new B);
NotNull!A a = assumeNotNull(new A);
a = b;
assert(a is b);
}
但是我还没弄明白如何允许
unittest
{
class A {}
class B : A {}
void f(NotNull!A a) {}
NotNull!B b = assumeNotNull(new B);
f(b);
}
编译，我认为应该自动生效。
alias this显然不足以完成这项工作。
我该如何实现？
模块notnull.d的完整来源如下：
#!/usr/bin/env rdmd-dev-module
module notnull;
import std.traits: isAssignable;
/** Note that NotNull!T is not NotNullable :) */
alias NotNullable(T) = isAssignable!(T, typeof(null));
/**
NotNull ensures a null value can never be stored.
* You must initialize it when declared
* You must never assign the null literal to it (this is a compile time error)
* If you assign a null value at runtime to it, it will immediately throw an Error
at the point of assignment.
NotNull!T can be substituted for T at any time, but T cannot become
NotNull without some attention: either declaring NotNull!T, or using
the convenience function, notNull.
Condition: T must be a reference type.
Instead of: __traits(compiles, { T t; assert(t is null); }.
TODO: Merge with http://arsdnet.net/dcode/notnullsimplified.d
Examples:
---
int myInt;
NotNull!(int *) not_null = &myInt;
// you can now use variable not_null anywhere you would
// have used a regular int*, but with the assurance that
// it never stored null.
---
*/
struct NotNull(T) if (NotNullable!T)
{
@disable this(); // Disallow default initialized (to null)
/** Assignment from $(D NotNull) Inherited Class $(D rhs) to $(D NotNull) Base
Class $(D this). */
typeof(this) opAssign(U)(NotNull!U rhs) @safe pure nothrow if (isAssignable!(T, U)) {
this._value = rhs._value;
return this;
}
NotNull!U opCast(U)() @safe pure nothrow if (isAssignable!(U, T)) {
return NotNull!_value;
}
// this could arguably break the static type check because
// you can assign it from a variable that is null.. but I
// think it is important that NotNull!Object = new Object();
// works, without having to say assumeNotNull(new Object())
// for convenience of using with local variables.
/// Constructs with a runtime not null check (via assert()).
this(T value) @safe pure nothrow
{
assert(value !is null);
_value = value;
}
/** Disable null construction. */
@disable this(typeof(null));
/** Disable null assignment. */
@disable typeof(this) opAssign(typeof(null));
private T _value;
@property inout(T) _valueHelper() inout
{
assert(_value !is null); // sanity check of invariant
return _value;
}
// Apparently a compiler bug - the invariant being uncommented breaks all kinds of stuff.
// invariant() { assert(_value !is null); }
alias _valueHelper this; /// this is substitutable for the regular (nullable) type
/* void toMsgpack (Packer) (ref Packer packer) const { packer.pack(_value); } */
/* void fromMsgpack(Unpacker)(auto ref Unpacker unpacker) { unpacker.unpack(_value); } */
}
/** A convenience function to construct a NotNull value from something $(D t)
you know isn't null.
*/
NotNull!T assumeNotNull(T)(T t) if (NotNullable!T)
{
return NotNull!T(t); // note the constructor asserts it is not null
}
/** A convenience function to check for null $(D t). If you pass null to $(D t),
it will throw an exception. Otherwise, return NotNull!T.
*/
NotNull!T enforceNotNull(T, string file = __FILE__, size_t line = __LINE__)(T t) if (NotNullable!T)
{
import std.exception: enforce;
enforce(t !is null, "t is null!", file, line);
return NotNull!T(t);
}
unittest
{
import core.exception;
import std.exception;
void NotNullCompiliationTest1()() // I'm making these templates to defer compiling them
{
NotNull!(int*) defaultInitiliation; // should fail because this would be null otherwise
}
assert(!__traits(compiles, NotNullCompiliationTest1!()()));
void NotNullCompiliationTest2()()
{
NotNull!(int*) defaultInitiliation = null; // should fail here too at compile time
}
assert(!__traits(compiles, NotNullCompiliationTest2!()()));
int dummy;
NotNull!(int*) foo = &dummy;
assert(!__traits(compiles, foo = null)); // again, literal null is caught at compile time
int* test;
test = &dummy;
foo = assumeNotNull(test); // should be fine
void bar(int* a) {}
// these should both compile, since NotNull!T is a subtype of T
bar(test);
bar(foo);
void takesNotNull(NotNull!(int*) a) { }
assert(!__traits(compiles, takesNotNull(test))); // should not work; plain int might be null
takesNotNull(foo); // should be fine
takesNotNull(assumeNotNull(test)); // this should work too
assert(!__traits(compiles, takesNotNull(assumeNotNull(null)))); // notNull(null) shouldn't compile
test = null; // reset our pointer
assertThrown!AssertError(takesNotNull(assumeNotNull(test))); // test is null now, so this should throw an assert failure
void takesConstNotNull(in NotNull!(int *) a) {}
test = &dummy; // make it valid again
takesConstNotNull(assumeNotNull(test)); // should Just Work
NotNull!(int*) foo2 = foo; // we should be able to assign NotNull to other NotNulls too
foo2 = foo; // including init and assignment
}
unittest
{
class A {}
class B : A {}
NotNull!B b = assumeNotNull(new B);
NotNull!A a = assumeNotNull(new A);
a = b;
assert(a is b);
}
答案 0 :(得分：0)
至于我，这是个坏主意，因为NotNull只是一个容器，只有一个值，而协变/逆变容器在运行时完整性方面并不安全。让我们想象一下这个想法是如何实现的，并考虑这个稍微修改过的例子：
unittest
{
class A {}
class B : A { void bb(){} }
void f(ref NotNull!A a) {
a = assumeNotNull(new A);
}
NotNull!B b = assumeNotNull(new B);
f(b);
b.bb();
}
b.bb（）调用一个对象，如果f（）驻留在另一个模块中，那么编译器就没有机会发现这个问题。
使其协变的唯一正确方法是删除所有消费方法（opAssign，fromMsgpack），使其实际上是不可变的。
补充阅读： Variance in C++， Cheat Codes