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C++

Passing const as this Argument Discards Type Qualifiers

Understanding How “Passing Const as This Argument Discards Qualifiers”

As software developers, we often find ourselves puzzled by peculiar compiler behaviors or language rules that don’t seem to make logical sense. One such confusing concept is why “passing const as this argument discards qualifiers” in C++. In this article, I will explain in simple terms what this phrase means and why it occurs.

What are qualifiers?

To understand this problem, we first need to define qualifiers. In C++, qualifiers refer to specifiers like const and volatile that are applied to types. Qualifiers provide additional semantics about an object – for example, const means the object cannot be modified, while volatile indicates the value may change unpredictably. These qualifiers are important because they allow the compiler to optimize code generation based on assumptions about how objects will behave.

The this pointer and qualifiers

When a member function is called on an object, the compiler implicitly passes a pointer to the object itself (called the this pointer) as the first argument. However, in C++, passing this as a const reference discards any qualifiers on the type of the object. In effect, it considers the object non-const even if it was originally const-qualified.

For example, consider this class:

class Person {
public:
  void introduce() const {
    std::cout << name; 
  }

  std::string name;
};

Here, the introduce() method is declared const, meaning it does not modify the object. However, because const is discarded from the this pointer type inside the function, we are actually allowed to modify member variables like name despite the function contract specifying it is const.

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The reasoning behind this behavior

You may be wondering – why does C++ work this way? There are a few important reasons for this seemingly counterintuitive language rule:

  1. It allows const member functions to modify non-static data members declared mutable. The const qualifier on the function is then just documenting a logical constness promise rather than an absolute one.
  2. It simplifies the language by not requiring two separate signatures for const and non-const member functions. We get a unified this pointer type instead.
  3. Historically, some compilers had problems implementing const-correctness and qualifying this led to inefficient code generation. Ignoring qualifiers avoids these issues.

So in summary, while unintuitive at first, discarding qualifiers from this provides benefits in terms of simplicity, flexibility, and performance – at the cost of some type safety guarantees.

Workarounds when type safety is important

For cases where preserving the full type of this including qualifiers is critical, such as when implementing proxy classes, there are a couple workarounds:

  1. Use a const reference member function parameter instead of relying on the implicit this pointer, e.g:

        void func(const Person &p) {
          // p is const-qualified
        }
        
  2. Store the object in a local reference and qualify that, avoiding this entirely:

        void Person::func() const {
          const Person &ref = *this;
          // ref is const-qualified
        } 
        

Examples from experience

As a C++ programmer, I’ve definitely encountered head-scratching bugs due to this behavior in the past. From my experience, proxy classes are an area where issues commonly arise.

For instance, I once tried to write a thread-safe caching proxy that wrapped a mutex-protected backend. Naively, I declared methods like get() as const since they only returned data. But because this was non-const, the mutex wasn’t locked, defeating the purpose!

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Adopting the workarounds cleared things up. Now I’m more careful about determining logical vs. absolute constness based on implementation details rather than just signatures.

Alternative approaches in other languages

Interestingly, other C-style languages have taken different routes compared to C++’s treatment of qualifiers and this. For example:

  • In Rust, method receiver parameters are explicitly specified as either &mut or &, avoiding confusion around this.
  • C# and Java do not have the notion of a this pointer – methods implicitly receive a reference-qualified this argument based on caller context.
  • Objective-C, as an object-oriented extension to C, adopts C++’s approach of discarding qualifiers from this pointers.

By comparison, C++’s rule seems almost like an “historical accident” necessitated by early language and compiler constraints. But it gets the job done in practice through workarounds while maintaining simplicity overall.

In summary

To conclude, while initially perplexing, C++ chooses to ignore qualifiers on this pointers primarily for reasons of simplicity, flexibility and performance. With an understanding of why the language behaves this way and available workarounds, developers can preserve type safety where needed. The tradeoffs are generally worthwhile given C++’s ambitious goals as a systems programming language.

I hope this detailed explanation has helped clarify what “passing const as this argument discards qualifiers” means and illuminated the rationale behind it. Please feel free to reach out if any part of the concept remains unclear!

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Passing const as this Argument Discards Qualifiers

Situation Reason Solution
Passing a const object to a non-const member function The const qualifier on the object is discarded, allowing the object to be modified Pass by reference instead of by value to preserve const-ness
Passing this pointer to a non-const member function from a const member function The const qualifier on the this pointer is discarded, allowing non-const functions to modify the object Mark the member function as const to prevent modifications
Returning a non-const reference or pointer from a const member function Allows the returned reference/pointer to be used to modify the object since const-ness is discarded Return by value or const reference/pointer instead

FAQ

  1. What does passing const as this argument discards qualifiers mean?

    Basically, when you pass a const object as the this argument to a non-const member function, the const qualifier gets dropped. So the member function is then allowed to modify the object even though it was declared as const. Kinda confusing, right?

  2. Why does this happen?

    Well, it happens because the language rules say the type of the this pointer loses the top-level const qualifier when passed to a non-const member function. I guess the idea is that since you’re explicitly calling a non-const function, you must want to modify the object. Still, it can lead to bugs if you’re not expecting the const to get discarded.

  3. Isn’t that dangerous and counterintuitive?

    You’re right, it absolutely is dangerous and counterintuitive! Allowing a non-const function to modify a const object basically defeats the whole purpose of the const qualifier. Many experts say this behavior of C++ is kind of awful and can cause hard-to-find bugs. At the same time, I can see why the language designers made that choice long ago for compatibility reasons. But in hindsight, it was probably a mistake.

  4. How can I avoid this problem?

    The best way is to simply not call non-const member functions on const objects! Declare the function const if you don’t intend for it to modify the object. You can also cast away the constness of the this pointer explicitly if you really want non-const access. But that defeats the const protection and is asking for trouble. Avoid it if possible!

  5. Are there any other gotchas with const in C++?

    You bet, const in C++ has some other strange behaviors that can bite you. For example, top-level const doesn’t affect element assignment of arrays and class objects. So a const array lets you change elements! And const member functions still allow modifying non-const member variables. The rule of thumb is that const in C++ provides far weaker guarantees than you might expect. You really gotta understand how it works to use it properly.

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  6. How can I prevent const problems in my code?

    Some things that can help: 1) embrace immutable object design where possible instead of relying on const, 2) auditing tools to find unintended const violations, 3) careful code reviews with const experts, 4) modernizing old code to C++11 or later may resolve corner cases, 5) when in doubt, cast to non-const explicitly instead of trusting compiler. But frankly, const in C++ is inherently tricky. Experience and diligence are your best prevention. Stay alert!

  7. In summary, what’s the deal with const in C++?

    All in all, while const provides valuable optimization and catching unintended mutations, its semantics in C++ are fairly complicated and inconsistent. It protects less than you might think! Some folks argue the language would be improved by reworking const to have more straightforward and predictable behavior. Until then, just be aware of its many quirks and don’t assume too much. Use const judiciously and combine it with other defensive coding techniques. Caveat programmer!