What is the difference between the const qualifier in C and the const qualifier in C++?

Question

I´ve found a comment of user R..:

C and C++ are not the same language. In particular, C const has nothing to do with C++ const.

I know, that one difference between the const qualifier in C and the const qualifier in C++ is its default linkage.

An object declared at namespace scope with const qualifier in C++ has internal linkage, while in C an object with const qualifier declared at global scope (without having a static qualifier before const) has external linkage.

But how else do they both differ between the languages of C and C++? I´ve thought both have the same kind of concept and purpose in both languages.

My Question:

• What is the difference between the const qualifier in C and the const qualifier in C++?

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The answers to How does "const" differ in C and C++? do not point an exact difference between the languages of C and C++ in the context of the const qualifier. Only what you can´t do or can do with it in a certain language.

Answer 1

• The most important difference is that in C++ a const variable is a constant expression (even prior the introduction of C++11 constexpr), but a const variable in C is not.

  Meaning that C++ allows you to do things like const size_t n = 1; static int array[n]; but C does not allow that, supposedly for historical reasons.

• In C++, const plays part in determining linkage. This is different between C++ versions. According to cppreference.com (emphasis mine):

  Any of the following names declared at namespace scope have internal linkage:

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  • non-volatile non-template (since C++14) non-inline (since C++17) non-exported (since C++20) const-qualified variables (including constexpr) that aren't declared extern and aren't previously declared to have external linkage;

  Whereas in C, const does not play part in determining linkage at all - only declaration scope and storage class specifiers matter.

• In C++, you can const qualify member functions. This isn't possible in C since it doesn't have syntax support for member functions.

• C allows const-qualified variables to be declared without an initializer. In C, we can write const int x; without initializers, but C++ does not allow that. At a glance, this may seem like a senseless language bug in C, but the rationale is that computers have read-only hardware registers with values set by hardware, not software. Meaning that C remains suitable for hardware-related programming.

Answer 2

From cppreference.com:

The const qualifier used on a declaration of a non-local non-volatile non-template (since C++14) non-inline (since C++17) variable that is not declared extern gives it internal linkage. This is different from C where const file scope variables have external linkage.

Other than that const has the same semantics in C and C++ and C headers with const are often compiled as C++ headers with conditional "extern C".