Why is const-correctness specific to C++?

Question

Disclaimer: I am aware that there are two questions about the usefulness of const-correctness, however, none discussed how const-correctness is necessary in C++ as opposed to other programming languages. Also, I am not satisfied with the answers provided to these questions.

I've used a few programming languages now, and one thing that bugs me in C++ is the notion of const-correctness. There is no such notion in Java, C#, Python, Ruby, Visual Basic, etc., this seems to be very specific to C++.

Before you refer me to the C++ FAQ Lite, I've read it, and it doesn't convince me. Perfectly valid, reliable programs are written in Python all the time, and there is no const keyword or equivalent. In Java and C#, objects can be declared final (or const), but there are no const member functions or const function parameters. If a function doesn't need to modify an object, it can take an interface that only provides read access to the object. That technique can equally be used in C++. On the two real-world C++ systems I've worked on, there was very little use of const anywhere, and everything worked fine. So I'm far from sold on the usefulness of letting const contaminate a codebase.

I am wondering what is it in C++ that makes const necessary, as opposed to other programming languages.

So far, I've seen only one case where const must be used:

#include <iostream>

struct Vector2 {
    int X;
    int Y;
};

void display(/* const */ Vector2& vect) {
    std::cout << vect.X << " " << vect.Y << std::endl;
}

int main() {
    display(Vector2());
}

Compiling this with const commented out is accepted by Visual Studio, but with warning C4239, non-standard extension used. So, if you want the syntactic brevity of passing in temporaries, avoiding copies, and staying standard-compliant, you have to pass by const reference, no way around it. Still, this is more like a quirk than a fundamental reason.

Otherwise, there really is no situation where const has to be used, except when interfacing with other code that uses const. Const seems to me little else than a self-righteous plague that spreads to everything it touches :

The reason that const works in C++ is because you can cast it away. If you couldn't cast it away, then your world would suck. If you declare a method that takes a const Bla, you could pass it a non-const Bla. But if it's the other way around you can't. If you declare a method that takes a non-const Bla, you can't pass it a const Bla. So now you're stuck. So you gradually need a const version of everything that isn't const, and you end up with a shadow world. In C++ you get away with it, because as with anything in C++ it is purely optional whether you want this check or not. You can just whack the constness away if you don't like it.

Anders Hejlsberg (C# architect), CLR Design Choices

Answer 1

If you are writing programs for embedded devices with data in FLASH or ROM you can't live without const-correctness. It gives you the power to control the correct handling of data in different types of memory.

Answer 2

Const correctness provides two notable advantages to C++ that I can think of, one of which makes it rather unique.

• It allows pervasive notions of mutable/immutable data without requiring a bunch of interfaces. Individual methods can be annotated as to whether or not they can be run on const objects, and the compiler enforces this. Yes, it can be a hassle sometimes, but if you use it consistently and don't use const_cast you have compiler-checked safety with regards to mutable vs. immutable data.
• If an object or data item is const, the compiler is free to place it in read-only memory. This can particularly matter in embedded systems. C++ supports this; few other languages do. This also means that, in the general case, you cannot safely cast const away, although in practice you can do so in most environments.

C++ isn't the only language with const correctness or something like it. OCaml and Standard ML have a similar concept with different terminology — almost all data is immutable (const), and when you want something to be mutable you use a different type (a ref type) to accomplish that. So it's just unique to C++ within its neighboring languages.

Finally, coming the other direction: there have been times I have wanted const in Java. final sometimes doesn't go far enough as far as creating plainly immutable data (especially immutable views of mutable data), and don't want to create interfaces. Look at the Unmodifiable collection support in the Java API and the fact that it only checks at run time whether modification is allowed for an example of why const is useful (or at least the interface structure should be deepend to have List and MutableList) — there is no reason that attempting to mutate an immutable structure can't be a compile-type error.

Answer 3

You're right, const-correctness isn't necessary. You can certainly write all your code without the const keyword and get things to work, just as you do in Java and Python.

But if you do that, you'll no longer get the compiler's help in checking for const violations. Errors that the compiler would have told you about at compile-time will now be found only at run-time, if at all, and therefore will take you longer to diagnose and fix.

Therefore, trying to subvert or avoid the const-correctness feature is just making things harder for yourself in the long run.

Answer 4

Actually, it's not... not entirely, anyway.

In other languages, especially functional or hybrid languages, like Haskell, D, Rust, and Scala, you have the concept of mutability: variables can be mutable, or immutable, and are usually immutable by default.

This lets you (and your compiler/interpreter) reason better about functions: if you know that a function only takes immutable arguments, then you know that function isn't the one that's mutating your variable and causing a bug.

C and C++ do something similar using const, except that it's a much less firm guarantee: the immutability isn't enforced; a function further down the call stack could cast away the constness, and mutate your data, but that would be a deliberate violation of the API contract. So the intention or best practice is for it to work quite like immutability in other languages.

All that said, C++ 11 now has an actual mutable keyword, alongside the more limited const keyword.

Answer 5

The const keyword in C++ (as applied to parameters and type declarations) is an attempt to keep programmers from shooting off their big toe and taking out their whole leg in the process.

The basic idea is to label something as "cannot be modified". A const type can't be modified (by default). A const pointer can't point to a new location in memory. Simple, right?

Well, that's where const correctness comes in. Here are some of the possible combinations you can find yourself in when you use const:

A const variable Implies that the data labeled by the variable name cannot be modified.

A pointer to a const variable Implies that the pointer can be modified, but the data itself cannot.

A const pointer to a variable Implies that the pointer cannot be modified (to point to a new memory location), but that the data to which the pointer points can be modified.

A const pointer to a const variable Implies that neither the pointer nor the data to which it points can be modified.

Do you see how some things can be goofy there? That's why when you use const, it's important to be correct in which const you are labeling.

The point is that this is just a compile-time hack. The labeling just tells the compiler how to interpret instructions. If you cast away from const, you can do whatever you want. But you'll still have to call methods that have const requirements with types that are cast appropriately.

Answer 6

Programming is writing in a language that will be ultimately processed by the computer, but that is both a way of communicating with the computer and other programmers in the same project. When you use a language, you are restricted to the concepts that can be expressed in it, and const is just one more concept you can use to describe your problem, and your solution.

Constantness enables you to express clearly from the design board to the code one concept that other languages lack. As you come from a language that does not have it, you may seem puzzled by a concept you have never used --if you never used it before, how important can it be?

Language and thought are tightly coupled. You can only express your thoughts in the language you speak, but the language also changes the way you think. The fact that you did not have the const keyword in the languages you worked with implies that you have already found other solutions to the same problems, and those solutions are what seems natural to you.

In the question you argued that you can provide a non mutating interface that can be used by functions that do not need to change the contents of the objects. If you think about it for a second, this same sentence is telling you why const is a concept you want to work with. Having to define a non-mutating interface and implement it in your class is a work around the fact that you cannot express that concept in your language.

Constantness allows you to express those concepts in a language that the compiler (and other programers) can understand. You are establishing a compromise on what you will do with the parameters you receive, the references you store, or defining limits on what the users of your class are allowed to do with the references you provide. Pretty much each non-trivial class can have a state represented by attributes, and in many cases there are invariants that must be kept. The language lets you define functions that offer access to some internal data while at the same time limits the access to a read-only view that guarantees no external code will break your invariants.

This is the concept I miss more when moving to other languages. Consider an scenario where you have a class C that has among others an attribute a of type A that must be visible to external code (users of your class must be able to query for some information on a). If the type of A has any mutating operation, then to keep user code from changing your internal state, you must create a copy of a and return it. The programmer of the class must be aware that a copy must be performed and must perform the (possibly expensive) copy. On the other hand, if you could express constantness in the language, you could just return a constant reference to the object (actually a reference to a constant view of the object) and just return the internal element. This will allow the user code to call any method of the object that is checked as non-mutating, thus preserving your class invariants.

The problem/advantage, all depends on the point of view, of constantness is that it is viral. When you offer a constant reference to an object, only those methods flagged as non-mutating can be called, and you must tell the compiler which of the methods have this property. When you declare a method constant, you are telling the compiler that user code that calls that method will keep the object state. When you define (implement) a method that has a constant signature, the compiler will remind you of your promise and actually require that you do not internally modify the data.

The language enables you to tell the compiler properties of your methods that you cannot express any other way, and at the same time, the compiler will tell you when you are not complying with your design and try to modify the data.

In this context, const_cast<> should never be used, as the results can take you into the realm of undefined behavior (both from a language point of view: the object could be in read-only memory, and from a program point of view: you might be breaking invariants in other classes). But that, of course, you already know if you read the C++FAQ lite.

As a side note, the final keyword in Java has really nothing to do with the const keyword in C++ when you are dealing with references (in C++ references or pointers). The final keyword modifies the local variable to which it refers, whether a basic type or a reference, but is not a modifier of the referred object. That is, you can call mutating methods through a final reference and thus provide changes in the state of the object referred. In C++, references are always constant (you can only bind them to an object/variable during construction) and the const keyword modifies how the user code can deal with the referred object. (In case of pointers, you can use the const keyword both for the datum and the pointer: X const * const declares a constant pointer to a constant X)