在D中迭代const集合

但它失败了.我有一个错误: Error: nested type List.List!int.List.Range should have the same or weaker constancy as enclosing type const(List!int)

怎么了？

解决方案是使您的范围存在于容器外部,但仍然引用它.如果范围在容器内,则它受传递const规则的约束,但如果它在外面,则可以在可变范围内保留const引用.如果在同一文件中定义它们,则范围仍然可以看到容器的私有成员.

注意:

class Container(T) {
        private T[] contents;

        this(T[] contents) {
                this.contents = contents;
        }

        RangeOver!(Container!T, T) getRange() {
                return RangeOver!(Container!T, T)(this);
        }
        RangeOver!(const(Container!T), const(T)) getRange() const {
                return RangeOver!(const(Container!T), const(T))(this);
        }
}

struct RangeOver(Container, T) {
        Container container;
        size_t iterationPosition;
        this(Container container) {
                this.container = container;
                this.iterationPosition = 0;
        }

        ref T front() {
                return container.contents[iterationPosition];
        }

        bool empty() {
                return iterationPosition == container.contents.length;
        }

        void popFront() {
                iterationPosition++;
        }
}


void main() {
        import std.stdio;

        // mutable iteration
        {
                writeln("about to mutate...");
                auto container = new Container!int([1,2,3]);
                foreach(ref item; container.getRange()) {
                        writeln(item);
                        item += 5;
                }
                writeln("mutation done");
                // changes seen
                foreach(item; container.getRange())
                        writeln(item);
        }

        // const iteration
        {
                writeln("consting it up y0");
                const container = new Container!int([1,2,3]);
                // allowed
                foreach(item; container.getRange())
                        writeln(item);
        }
}