Does const reference have external linkage in C++?

Question

According to clause 3 of section 3.5 of C++ 1998 standard, a const reference has internal linkage.

A name having namespace scope (3.3.5) has internal linkage if it is the name of

an object, reference, function or function template that is explicitly declared static or,

an object or reference that is explicitly declared const and neither explicitly declared extern nor previously declared to have external linkage; or

a data member of an anonymous union.

But why multiple definition conflict is generated when compiling the following code?

// a.cpp const int& a = 1; int main() { return 0; } // b.cpp const int& a = 1;

And then compile the code.

$ g++ a.cpp b.cpp /tmp/ccb5Qi0M.o:(.bss+0x0): multiple definition of `a' /tmp/ccD9vrzP.o:(.bss+0x0): first defined here collect2: error: ld returned 1 exit status

If the const reference is changed to const, as follows

// a.cpp const int a = 1; int main() { return 0; } // b.cpp const int a = 1;

It is OK to compile.

Mike Seymour · Accepted Answer · 2014-03-31 16:05:21Z

2

The reference itself isn't const, just the object it refers to; so (arguably) this rule doesn't give the reference internal linkage.

It doesn't make sense for a reference to be declared const. The C++11 standard clarifies the wording:

a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage

with no mention of the nonsensical concept of references declared const.

edited Mar 31, 2014 at 16:05

answered Mar 31, 2014 at 15:30

Mike Seymour

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5 Comments

spockwang Over a year ago

You mean the wording in the C++ 1998 standard is misleading?

Mike Seymour Over a year ago

@spockwang: I don't know about "misleading", but it's certainly confusing, and has now been clarified.

Deduplicator Over a year ago

Does anyone have a good example for using such a reference, and wanting it not file-scoped by default?

spockwang Over a year ago

@MikeSeymour Isn't the reference a variable in the C++11 standard?

Mike Seymour Over a year ago

@spockwang: Yes. But it can't be explicitly declared const, so this rule doesn't apply to it.

http8086 · Accepted Answer · 2014-04-02 11:05:26Z

I think @Mike is correct, here is no new thing, just a little tip.

There is reference and the object that referenced to, the object can be constant then comes internal linkage, but the reference itself can never be constant since it has no CV concept(reference was initialized during declaration, then never shift to some object else, I remember GCC complains if you give a constant reference, mean for int const& r = o; although VS was not complaining, that just makes no sense), since the reference is neither constant nor static, then the declaration crosses source files.

Dr. Gut · Accepted Answer · 2020-06-01 01:01:51Z

This is a program with the most const reference ever. It compiles, and links with C++17 (try it):

// a.cpp extern const int& ir; int main() { return ir; }

// b.cpp constexpr const int& ir = 42;

As you can see, ir can be referenced from another translation unit, i.e. it has external linkage.

Conclusion: global reference variables have external linkage, unless declared as static or in anonymous namespace. Also the constexpr keyword on references does not make the reference a constant view to the referree (try it).

And constexpr references are not implicitly inline. This is redefinition of ir in C++17 (try it):

// a.cpp constexpr const int& ir = 42; int main() { return ir; }

// b.cpp constexpr const int& ir = 42;

If you put inline before the declaration of ir in both files, it compiles and links (try it).

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