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

You want to use const in methods as well in order to take advantage of return value optimization. See Scott Meyers More Effective C++ item 20.

Answer 2

For example you have a funcion:

void const_print(const char* str)
{
    cout << str << endl;
}

Another method

void print(char* str)
{
    cout << str << endl;
}

In main:

int main(int argc, char **argv)
{
    const_print("Hello");
    print("Hello");        // syntax error
}

This because "hello" is a const char pointer, the (C-style) string is put in read only memory. But it's useful overall when the programmer knows that the value will not be changed.So to get a compiler error instead of a segmentation fault. Like in non-wanted assignments:

const int a;
int b;
if(a=b) {;} //for mistake

Since the left operand is a const int.