封装的目的是为了代码安全,调试容易,当然还有方便理解.
可在构/类级别,或在模块级别指定封装关系.

词 意思
public 本程序任意访问
private 限制只能由定义他的模块访问.此外子类不能覆盖基类私成员.
package 包级访问,属于某包都可以访问.包属性仅包含最里边的包.vertebrate下的所有包都可以访问在animal.vertebrate.cat里定义的包定义函数(有包),子类不能覆盖包成员函数
protected 继承类可访问
export 外部程序都可访问,最大 
//java式
private int foo;

private void bar() {
    // ...
}
//c++式
private:
    // ...
    // ... 私
    // ...

protected:
    // ...
    // ... 保护
    // ...

//块式
private {
    // ...
    // ... 私
    // ...
}

导入模块默认为私.可以加个公开

module school.school;

public import school.student;//可间接访问
//从而
import school.school;

void main() {
    auto student = Student("Tim", Gender.male);

    // ...
}

封装是面向对象,最强大之处.示例:

module school.student;

import std.string;
import std.conv;

enum Gender { female, male }

struct Student {
    package string name;
    package Gender gender;

    string toString() const {
        return format("%s 是%s 学生.",
                      name, to!string(gender));
    }
}
module school.school;

public import school.student;                  // 1

import std.string;

class School {
private:                                       // 2

    Student[] students;
    size_t femaleCount;
    size_t maleCount;

public:                                        // 3

    void add(in Student student) {
        students ~= student;

        final switch (student.gender) {        // 4a

        case Gender.female:
            ++femaleCount;
            break;

        case Gender.male:
            ++maleCount;
            break;
        }
    }

    override string toString() const {//盖,继承自`object`
        string result = format(
            "%s 女, %s 男; 总共 %s 学生",
            femaleCount, maleCount, students.length);

        foreach (i, student; students) {
            result ~= (i == 0) ? ": " : ", ";
            result ~= student.name;            // 4b
        }

        return result;
    }
}
import std.stdio;
import school.school;

void main() {
    auto student = Student("Tim", Gender.male);
    writeln(student);

    auto school = new School;

    school.add(Student("Lindsey", Gender.female));
    school.add(Student("Mark", Gender.male));
    school.add(Student("Nancy", Gender.female));

    writeln(school);
}