Introduce `private(this)` Access Modifier

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On Friday, 4 July 2025 at 09:26:30 UTC, Peter wrote:
> DIP: xxxx
> Title: Introduce `private(this)` Access Modifier
> Author: [Your Name]
> Status: Draft
> Created: 2025-07-04
>
> ## Abstract
>
> This DIP proposes adding a new access modifier, 
> `private(this)`, that applies only to class and struct members.
> Outside the defining module it behaves identically to `private`.
> Within the module, however, it restricts access to the scope of 
> the declaring type, preventing any other code in the same 
> module from touching it.
>
> ## Rationale
>
> Currently, D’s `private` grants access to all code in the same 
> module.
> While that can simplify internal communication, it undermines 
> encapsulation by allowing unrelated types
> and functions in the same module to directly manipulate class 
> or struct internals.
> Enforcing true type-level privacy strengthens abstraction 
> boundaries and helps maintain invariants.
>
> ## Specification
>
> - Applicable only to class and struct members.
>
> - Outside the defining module, `private(this)` == `private`.
>
> - Inside the defining module:
>   - Only code lexically inside the body of the same class or 
> struct may access the member.
>   - No free functions, mixins, nested types, or other types in 
> the module may access it.
>
> - All existing access rules (transitive private, inheritance, 
> package scopes) remain unchanged.
>
> ## Example
>
> module example;
>
> struct S
> {
>     private(this) int x;    // Only S’s code can touch x
>     int getX() { return x; }
> }
>
> void helper()
> {
>     S s;
>     // s.x = 5;             // Error: x not accessible here
> }
>
> Pros and Cons
> -------------
>
> Pros of not having private(this)
>
>     Simpler access model with just private at module level.
>
>     No new keyword to learn or confuse beginners.
>
>     Existing codebases remain fully functional without change.
>
> Cons of not having private(this)
>
>     Invariants can be broken by unrelated code in the same 
> module.
>
>     Difficult to enforce strict encapsulation; relies on 
> documentation and discipline.
>
>     Module-sized privacy may encourage large modules with weak 
> boundaries.
>
>
> Pros of having private(this)
>
>     Strong, compiler-enforced type-level encapsulation.
>
>     Clearer abstraction boundaries and safer internal 
> invariants.
>
>     Encourages smaller, focused modules and cleaner 
> object-oriented design.
>
> Cons of having private(this)
>
>     Introduces another access modifier and slightly raises 
> language complexity.
>
>     Potential confusion between private and private(this) for 
> new users.
>
>     May require refactoring of existing module-heavy code to 
> satisfy the stricter rules.

Upsides of Mutual Access to Private Members in D
D's ability to allow mutual access to private members within the 
same module offers several significant
advantages. Here's a breakdown of the key benefits:
1. Tight Coupling of Related Types
- Seamless Collaboration: D encourages tight coupling between 
components that are closely related.
Classes and functions within the same module can interact with 
each other’s internals, making it
easier to implement complex logic that involves multiple types.
- Easier Integration: With mutual access, it's simpler to 
integrate various parts of the code without
having to expose unnecessary getter/setter methods or interfaces, 
resulting in a more natural flow
of data and behavior.
2. Reduced Boilerplate Code
- Fewer Getter/Setter Methods: By allowing classes and functions 
within the same module to access
each other’s private members, you avoid the need for redundant 
getter/setter methods. This
reduces boilerplate code and keeps the codebase cleaner and 
easier to maintain.
- Cleaner Design: Without excessive public interfaces, the 
internal design remains more streamlined.
This allows for easier readability and understanding of how 
components interact internally.
3. Simplified Refactoring Within a Module
- Easier Refactoring: When classes and functions within the same 
module can access each other’s
private members, it becomes easier to refactor code without 
needing to expose or rewrite public
methods. You can change internal implementations more freely 
without breaking the code.
- Less Overhead: Refactoring a class doesn’t require adjusting 
numerous public methods or creating
additional interfaces to accommodate other parts of the module. 
It’s easier to modify internal logic
without affecting the module’s public API.
4. Better Encapsulation Within the Module
- Internal Access Control: While the private members are 
accessible within the module, they are still
protected from external access. This maintains encapsulation by 
keeping internal implementation
details hidden from other modules, while still allowing internal 
components to interact freely.
- Module Boundaries: This design keeps the module's internal 
details encapsulated from other
modules, ensuring that the outside world can only interact with 
the module’s public API, while the
internal structure remains hidden and flexible.
5. Performance Considerations
- Lower Overhead: The lack of getter/setter methods and the 
reduced need for intermediary layers
means that code can be more efficient. Direct access to private 
members reduces the need for
additional indirection or abstraction layers, leading to 
potential performance gains.
- No Extra Abstraction: Since classes and functions within the 
module can directly access each
other’s private data, you avoid the overhead of additional layers 
of abstraction that might
otherwise be necessary to facilitate communication.
6. Greater Flexibility in Code Design
- More Freedom for Developers: D allows greater flexibility in 
how internal components can interact
with one another. Developers can design more flexible solutions 
without being forced to overabstract or expose unnecessary 
details to the outside world.
- Easier to Express Complex Logic: By reducing the need for 
strict encapsulation between related
components, you can implement more complex logic directly within 
the module. The components
can work together without unnecessary restrictions, resulting in 
easier and more powerful
designs.
7. Facilitates Testing and Debugging
- Simpler Unit Testing: Since functions and classes can access 
each other’s private members, testing
individual components becomes easier. For example, you can 
directly test internal states of a class
without requiring elaborate mocking or stubbing.
- Enhanced Debugging: During debugging, it's easier to inspect 
and manipulate the private state of
classes within the same module. This can help you quickly 
identify issues and fix bugs without
needing to write extra code to expose the internals.
8. Better Maintainability in Small to Medium-Sized Modules
- Easier Collaboration: For small to medium-sized modules, mutual 
access promotes collaboration
between related classes and functions. This makes it easier for 
multiple developers to work on
different parts of the module without stepping on each other's 
toes or having to work through
rigid public APIs.
- Lower Overhead: For less complex modules, the simplicity of 
allowing private members to be
accessed across components reduces the overhead of defining 
complex access control mechanisms
and unnecessary abstraction.
9. Encourages Modular and Cohesive Design
- Internal Module Cohesion: D encourages a design where 
components within the same module can
interact easily, leading to modular, cohesive designs. This is 
beneficial when building systems that
have clear module boundaries but need tight collaboration between 
related components.
- Less Fragmentation: When related classes/functions can directly 
interact with each other’s private
state, it helps avoid the fragmentation of internal logic across 
multiple interfaces. This reduces the
risk of creating fragmented or overly-complicated code.
Conclusion
The ability to allow mutual access to private members within a 
module in D brings several advantages:
1. Tight coupling between related types, promoting seamless 
integration and interaction.
2. Reduced boilerplate code, making the codebase cleaner and more 
efficient.
3. Easier refactoring without the need for changing public 
interfaces.
4. Better encapsulation within the module, while still allowing 
internal access.
5. Improved performance due to fewer abstraction layers.
6. Greater flexibility for expressing complex logic and designs.
7. Simplified testing and debugging by directly accessing private 
state.
8. Better maintainability in smaller modules due to simplified 
access.
9. Modular and cohesive design, reducing unnecessary 
fragmentation.
When used appropriately, this feature in D can significantly 
streamline development and make the
codebase more manageable and flexible.
==============================================
Downsides of Mutual Access to Private Members in D
While D's approach to mutual access of private members within a 
module offers significant flexibility,
there are several potential downsides and trade-offs:
1. Lack of Strict Encapsulation
- Less Fine-Grained Control: By allowing multiple types (such as 
classes and functions) within the
same module to access each other’s private members, D reduces the 
boundaries of encapsulation.
While this is fine for tightly-coupled internal code, it might 
lead to accidental misuse or unintended
dependencies between classes that could otherwise remain isolated.
- Hidden Dependencies: Internal classes and functions become more 
coupled, making it harder to
maintain isolation between components. A change in one class can 
unexpectedly affect others,
leading to tight interdependencies that are harder to track.
2. Can Lead to Over-Exposure of Implementation Details
- Increased Risk of Misuse: When private members are accessible 
to other components within the
same module, it can encourage over-exposure of implementation 
details, which might lead to
misuse by other classes/functions within the module.
- Hidden Dependencies: Classes might start depending on private 
details of other classes, which
makes internal refactoring difficult. Changes to one class could 
inadvertently break other parts of
the module that rely on those private details.
3. Harder to Maintain Large Modules
- Unclear Boundaries: With mutual access, it becomes unclear what 
should be the public API of the
module versus what is internal. Over time, this can lead to 
bloated code with tangled
interdependencies, making maintenance more difficult.
- Fragile Code: A small change in one part of the module could 
propagate to unexpected areas,
making the system more fragile and increasing the risk of bugs 
when modifying or extending the
module.
4. Violates the Principle of Least Privilege
- Excessive Access: The principle of least privilege is violated 
when classes and functions are granted
more access than they need. This might lead to accidental 
modification of internal state, which
could cause unintended side effects.
- Unnecessary Dependencies: Functions or classes that don’t need 
access to private members might
accidentally misuse them, creating dependencies that should not 
exist.
5. Can Encourage Bad Design Practices
- Lack of Proper Interfaces: Developers might skip creating 
proper public interfaces and instead
access private members directly. This leads to poor abstraction 
and makes the code harder to
refactor in the future.
- Tight Coupling: The design could encourage tight coupling 
between classes, which makes the code
harder to extend or modify without affecting other components in 
the module.
6. Makes Code Less Reusable
- Interdependencies: With private members exposed, code becomes 
less reusable. If classes or
functions are tightly coupled within the module, reusing 
components in different contexts becomes
difficult. The code becomes less modular, which reduces its 
flexibility.
- Reduced Modularity: If components within a module depend on 
each other's internals, it’s harder
to decouple them for reuse in other projects or modules.
7. Potential for Increased Technical Debt
- Accumulation of Implicit Dependencies: As more components 
access each other's private
members, the dependency graph becomes harder to track, and 
changes in one part of the module
can have unintended effects elsewhere, increasing the technical 
debt over time.
- Difficulty in Testing: When private members are freely accessed 
across the module, it complicates
unit testing. You might need to write integration tests instead 
of unit tests, making it harder to
ensure correctness.
Conclusion
While D’s ability to allow mutual access between private members 
of classes and functions within the
same module provides flexibility and simplicity, it comes with 
several potential downsides:
1. Weakened encapsulation, leading to accidental misuse or 
unwanted interdependencies.
2. Increased fragility of the code due to tight coupling and 
interdependencies.
3. Potential violation of design principles like least privilege 
and good abstraction.
4. Increased technical debt and difficulty in maintaining or 
testing large codebases.
It’s important for developers to use this feature judiciously and 
ensure that the internal access is
intentional and not abused for convenience. Like all powerful 
language features, it can be a double-edged
sword: used well, it can streamline development; used poorly, it 
can lead to tightly coupled, fragile code
that’s hard to maintain and extend.
============================
What if D has another accessibitly modifier, called 
private(this), so that class members can use that to restrict
visibilty to the class scope, and not allow other code in the 
same module to access private(this) members of a class.
Introducing an access modifier like private(this) in D would be 
an interesting way to further restrict visibility
and potentially enhance encapsulation in the language, adding a 
more fine-grained control over how
private members are accessed. Here's a breakdown of what this 
would imply:
Concept of private(this) in D
If D were to introduce an access modifier like private(this), it 
would have the following characteristics:
- Class-Level Visibility: The private(this) modifier would 
restrict access to only the class in which the
member is defined. This would mean that even functions and 
classes within the same module
would not be able to access the member, except for the class 
itself.
- Module-Level Access: In contrast, the current private modifier 
in D allows access from any function
or type within the same module, making it more permissive.
Implications of private(this) in D
Let’s explore the pros and cons, and how this could fit into D’s 
existing design.
Upsides of private(this) in D
1. Stronger Encapsulation:
- By restricting access to just the class itself, private(this) 
would enforce stricter encapsulation.
This is particularly useful when you want to ensure that no other 
code (even within the
same module) can interact with certain private members. This 
could prevent unintended
changes or misuse of internal state.
2. Better Design for Internal Complexity:
- Some classes may have complex internal logic that should be 
hidden even from other
components in the same module. Using private(this) would allow 
the class to maintain total
control over its internal state and interactions, without other 
parts of the module
potentially messing with it.
3. Clearer Boundaries:
- With private(this), it becomes very clear which members are 
truly private to the class, and
which ones are shared within the module. This could make the code 
easier to understand
and maintain, as developers would be forced to respect the 
class’s internal boundaries.
4. More Control Over Class Internal Changes:
- Since only the class itself can modify its private(this) 
members, any refactoring or internal
changes within the class can be done with confidence that no 
other code in the module will
break the logic by depending on these private members.
5. Improved Testing:
- Since private(this) members wouldn’t be accessible outside the 
class, it could encourage
better unit testing practices. Classes would be forced to expose 
only the necessary public
methods for interaction, and internal testing could focus on 
ensuring that the public
interface behaves correctly without relying on direct access to 
private state.
Downsides of private(this) in D
1. Reduced Flexibility:
- The major downside of private(this) is the reduced flexibility 
it would impose on a module.
Currently, D allows more flexible interaction within a module, 
which can make it easier to
share logic across components without needing public methods or 
interfaces. private(this)
would restrict that, potentially making some tasks more 
cumbersome or requiring more
boilerplate.
2. Increased Boilerplate:
- With private(this), helper functions or internal methods that 
need to access class members
might need to be declared public or internal within the class, 
leading to additional code.
This could increase boilerplate code and reduce the overall 
conciseness of the module.
3. Harder Refactoring for Cross-Class Dependencies:
- If classes within the same module need to interact closely but 
the members they use are
private(this), refactoring those interactions might become more 
difficult. You’d either have to
create public methods or internal interfaces, which could 
increase coupling between
classes and make the module less cohesive.
4. Performance Overhead:
- If classes need to go through additional public or internal 
methods to access members
previously marked as private(this), this could add an extra layer 
of indirection. While this
overhead may not be significant in many cases, in 
performance-critical applications, it could
be a factor to consider.
5. Lack of Flexibility for Testing Across Classes:
- If you're trying to test the interaction between two closely 
related classes within the same
module, and their members are private(this), testing could become 
harder. You might need to
expose more members to make testing easier, or rely on more 
complex test setups.
Comparison with Current private in D
The existing private modifier in D allows other functions and 
types within the same module to access
members marked as private. If private(this) were introduced, the 
class would retain full control over its
internal members, preventing any external code within the same 
module from accessing them.
- private: Accessible to any function or type within the same 
module.
- private(this): Accessible only within the class that defines 
the member.
Let's look at a practical example to see how private(this) might 
be used in D:
module mymodule;
class MyClass
{
private(this) int secret;
this(int value){ secret = value; }
// Can access secret because it's in the same class
void printSecret() { writeln("Secret: ", secret);
}
}
void main()
{
MyClass obj = new MyClass(42);
obj.printSecret(); // Works fine
// obj.secret = 100; // Error: 'secret' is private(this) and 
can't be
accessed outside MyClass
}
In this example, the secret member is only accessible within 
MyClass. Any other function or class in the same module
(including the main function) cannot access or modify secret 
directly, even though they are in the same module. The
class is forced to expose only the necessary public methods 
(e.g., printSecret()), maintaining strong encapsulation.
Would private(this) Be a Useful Addition?
- Yes, for Strict Encapsulation: If you want to make certain 
class members completely hidden from
everything except the class itself, private(this) would be a 
great tool. It would encourage better
encapsulation and reduce the risk of accidentally exposing 
internal state that should remain
private.
- Maybe Not Always Ideal: In scenarios where classes are closely 
interrelated and need to share
internal state in a more flexible way, private(this) might add 
unnecessary rigidity to the design. The
existing private modifier gives more freedom for interacting 
between related classes, which is often
useful when developing modular code in the same module.
Conclusion
Introducing a private(this) modifier in D would offer several 
benefits, such as:
- Stronger encapsulation.
- Clearer separation between internal and external logic.
- More control over class state and internal changes.
However, it would also come with trade-offs, including:
- Reduced flexibility within modules.
- Increased boilerplate and potential for harder refactoring.
- More complexity for testing and interacting with closely 
related classes.
Whether this feature would be beneficial depends on the specific 
use case and design philosophy of the
project. For cases where strong internal isolation is crucial, 
private(this) would certainly be a valuable
addition. For more modular, flexible designs, however, the 
current private modifier may still be preferable.