d - 在构造函数之外初始化 const 对象

Question

在下面的代码中:

class A
{
    void aMethod() { }
    void aConstMethod() const { }
}

class B
{
    const A a; // Not initialized in the constructor, but at a latter time

    void initA()
    {
        a = new A(); // Error: can only initialize const member 'a' inside constructor
    }

    void doStuff()
    {
        //a.aMethod(); shouldn't be allowed to call this here, B can only read from A.
        a.aConstMethod();
    }
}

我希望类 B 只能从 A 调用 const 或 immutable 方法。但是，B 只能在构造完成后才能创建 A 的实例，因此无法在构造函数中初始化 A。我可以在不从 var a 中删除 const 的情况下修复上面的代码吗？

Answer 1

使用std.typecons.Rebindable :

class A
{
    void aMethod() { }
    void aConstMethod() const { }
}

class B
{
    import std.typecons: Rebindable;

    Rebindable!(const A) a; // Not initialized in the constructor, but at a latter time

    void initA()
    {
        a = new A(); // Error: can only initialize const member 'a' inside constructor
    }

    void doStuff()
    {
        static assert(!__traits(compiles, a.aMethod())); // shouldn't be allowed to call this here, B can only read from A.
        a.aConstMethod();
    }
}