Why aren't C++ types const by default?

Question

Gurus everywhere tell us to const everything unless we need to modify it, yet the standard makes everything mutable until we declare it const .

Am I missing something here, or is this a contradiction? Why aren't C++ types const by default when, according to the experts (some of whom presumably designed the standard), they should be made const by default?

Answer 1

Gurus everywhere tell us to const everything unless we need to modify it,

This makes sense to me . Immutability is a very noble idea.

^{Rust is a new language (developed by Mozilla) in which variables, by default, are immutable . If you want your variables to be mutable, then you've to make it mutable explicitly by using the keyword mut which is exactly opposite of what C++ does — in C++, you've to make things immutable explicitly by using the keyword const , otherwise they're mutable by default.}

Especially with the advent of multicore processors and more multi-threading software than ever before, it makes more sense to me. I think by default there should be more restriction and the restriction should be lifted (when you need to) consciously and explicitly (as opposed to implicitly). For example, members of a class are private by default; the inheritance is private by default. You make them public or protected explicitly. So should be the case with the-right-to-modify a variable. You should not have right to modify a variable, by default (in my opinion). It requires a bit of maturity to appreciate immutability/restriction — by putting restriction, you avoid a whole class of bugs in your software. That is my thought, in support of the statement.

Now coming back to the actual question,

Why aren't C++ types const by default when, according to the const-people they should be made const by default?

It is because

• Not many programmers realized that immutability (as the default) makes their life easy, as it is a sort of new trend, at least it is catching up very recently.
• backward-compatibility. C has mutable types by default.

Answer 2

Gurus everywhere tell us to const everything unless we need to modify it,

That's been conventional wisdom for a decade or two, yes.

yet the standard makes everything mutable until we declare it const.

The standard has evolved from languages developed around fifty years ago, when const-correctness, and even type checking, were largely regarded as being only of academic interest.

Am I missing something here, or is this a contradiction?

It's not a contradiction; just a language that doesn't work how some would say it should. You still can (and should) declare things const where appropriate, and we just have to put up with a language that doesn't push us towards safe practices.

Why aren't C++ types const by default?

Because changing such a fundamental aspect of the language will break just about all existing code. That's a much bigger concern than slightly reducing the scope for mistakes.

Answer 3

I had the same question a few years ago, and I wrote some helper functions to make the use of const variable more terse.

You can see it here : https://github.com/pthom/cpp_terse_const

And my post on codereview at stackechange : https://codereview.stackexchange.com/questions/106074/const-by-default

Beware, this was just an exercice, I do not use this in production.