Mixing extern and const

Can I mix extern and const, as extern const? If yes, does the const qualifier impose it's reign only within the scope it's declared in or should it exactly match the declaration of the translational unit it's declared in? I.e. can I declare say extern const int i; even when the actual i is not a const and vice versa?

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Yes, you can use them together.

If you declare "extern const int i", then i is const over its full scope. It is impossible to redefine it as non-const. Of course you can bypass the const flag by casting it away (using const_cast).

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You can use them together and you can do all sorts of things which ignore the const keyword, because that's all it is; a keyword. It tells the compiler that you won't be changing a variable which in turn allows the compiler to do some useful optomisations and stops you from changing things you didn't mean to.

Possibility.com has a decent article with some more background.

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You can use them together. But you need to be consistent on your use of const because when C++ does name decoration, const is included in the type information that is used to decorate the symbol names. so extern const int i will refer to a different variable than extern int i

Unless you use extern "C" {}. C name decoration doesn't pay attention to const.

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最佳答案
  • Yes, you can use them together.
  • And yes, it should exactly match the declaration in the translation unit it's actually declared in. Unless of course you are participating in the Underhanded C Programming Contest :-)

The usual pattern is:

  • file.h:
    extern const int a_global_var;
  • file.c:
    #include "file.h"
    const int a_global_var = /* some const expression */;

Edit: Incorporated legends2k's comment. Thanks.

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C++17 inline variables

If you think you want an extern const, then it is more likely that you would actually want to use C++17 inline variables.

This awesome C++17 feature allow us to:

  • conveniently use just a single memory address for each constant
  • store it as a constexpr: How to declare constexpr extern?
  • do it in a single line from one header

main.cpp

#include <cassert>


#include "notmain.hpp"


int main() {
// Both files see the same memory address.
assert(&notmain_i == notmain_func());
assert(notmain_i == 42);
}

notmain.hpp

#ifndef NOTMAIN_HPP
#define NOTMAIN_HPP


inline constexpr int notmain_i = 42;


const int* notmain_func();


#endif

notmain.cpp

#include "notmain.hpp"


const int* notmain_func() {
return &notmain_i;
}

Compile and run:

g++ -c -o notmain.o -std=c++17 -Wall -Wextra -pedantic notmain.cpp
g++ -c -o main.o -std=c++17 -Wall -Wextra -pedantic main.cpp
g++ -o main -std=c++17 -Wall -Wextra -pedantic main.o notmain.o
./main

GitHub upstream.

See also: How do inline variables work?

C++ standard on inline variables

The C++ standard guarantees that the addresses will be the same. C++17 N4659 standard draft 10.1.6 "The inline specifier":

6 An inline function or variable with external linkage shall have the same address in all translation units.

cppreference https://en.cppreference.com/w/cpp/language/inline explains that if static is not given, then it has external linkage.

Inline variable implementation

We can observe how it is implemented with:

nm main.o notmain.o

which contains:

main.o:
U _GLOBAL_OFFSET_TABLE_
U _Z12notmain_funcv
0000000000000028 r _ZZ4mainE19__PRETTY_FUNCTION__
U __assert_fail
0000000000000000 T main
0000000000000000 u notmain_i


notmain.o:
0000000000000000 T _Z12notmain_funcv
0000000000000000 u notmain_i

and man nm says about u:

"u" The symbol is a unique global symbol. This is a GNU extension to the standard set of ELF symbol bindings. For such a symbol the dynamic linker will make sure that in the entire process there is just one symbol with this name and type in use.

so we see that there is a dedicated ELF extension for this.

Pre-C++ 17: extern const

extern const does work as in the example below, but the downsides over inline are:

  • it is not possible to make the variable constexpr with this technique, only inline allows that: How to declare constexpr extern?
  • it is less elegant as you have to declare and define the variable separately in the header and cpp file

main.cpp

#include <cassert>


#include "notmain.hpp"


int main() {
// Both files see the same memory address.
assert(&notmain_i == notmain_func());
assert(notmain_i == 42);
}

notmain.cpp

#include "notmain.hpp"


const int notmain_i = 42;


const int* notmain_func() {
return &notmain_i;
}

notmain.hpp

#ifndef NOTMAIN_HPP
#define NOTMAIN_HPP


extern const int notmain_i;


const int* notmain_func();


#endif

GitHub upstream.

Pre-C++17 header only alternatives

These are not as good as the extern solution, but they work and only take up a single memory location:

A constexpr function, because ABC0 implies inline and inline allows (forces) the definition to appear on every translation unit:

constexpr int shared_inline_constexpr() { return 42; }

and I bet that any decent compiler will inline the call.

You can also use a const or constexpr static integer variable as in:

#include <iostream>


struct MyClass {
static constexpr int i = 42;
};


int main() {
std::cout << MyClass::i << std::endl;
// undefined reference to `MyClass::i'
//std::cout << &MyClass::i << std::endl;
}

but you can't do things like taking its address, or else it becomes odr-used, see also: https://en.cppreference.com/w/cpp/language/static "Constant static members" and Defining constexpr static data members

Any way to fully inline it?

TODO: is there any way to fully inline the variable, without using any memory at all?

Much like what the preprocessor does.

This would require somehow:

  • forbidding or detecting if the address of the variable is taken
  • add that information to the ELF object files, and let LTO optimize it up

Related:

Tested in Ubuntu 18.10, GCC 8.2.0.

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An additional note to this answer from https://en.cppreference.com/w/cpp/language/storage_duration

Names at the top-level namespace scope (file scope in C) that are const and not extern have external linkage in C, but internal linkage in C++.

Internal linkage means the name is only accessible within the current translation unit. You cannot use extern in another file to refer to names with internal linkage. To explicitly mark a const name with explicit linkage, use extern.

// foo.cpp (refers to bar from bar.cpp)
extern const bool bar;


// bar.cpp
extern const bool bar = true;

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