C++ – Should ‘const’ and ‘constexpr’ Variables in Headers Be ‘inline’?

c++c++17language-lawyerone-definition-rule

Consider the following header and assume it is used in several TUs:

static int x = 0;

struct A {
    A() {
        ++x;
        printf("%d\n", x);
    }
};

As this question explains, this is an ODR violation and, therefore, UB.

Now, there is no ODR violation if our inline function refers to a non-volatile const object and we do not odr-use it within that function (plus the other provisions), so this still works fine in a header:

constexpr int x = 1;

struct A {
    A() {
        printf("%d\n", x);
    }
};

But if we do happen to odr-use it, we are back at square one with UB:

constexpr int x = 1;

struct A {
    A() {
        printf("%p\n", &x);
    }
};

Thus, given we have now inline variables, should not the guideline be to mark all namespace-scoped variables as inline in headers to avoid all problems?

constexpr inline int x = 1;

struct A {
    A() {
        printf("%p\n", &x);
    }
};

This also seems easier to teach, because we can simply say "inline-everything in headers" (i.e. both function and variable definitions), as well as "never static in headers".

Is this reasoning correct? If yes, are there any disadvantages whatsoever of always marking const and constexpr variables in headers as inline?

Best Answer

As you have pointed out, examples one and third does indeed violate ODR as per [basic.def.odr]/12.2.1

[..] in each definition of D, corresponding names, looked up according to [basic.lookup], shall refer to an entity defined within the definition of D, or shall refer to the same entity, after overload resolution and after matching of partial template specialization, except that a name can refer to

a non-volatile const object with internal or no linkage if the object

  • is not odr-used in any definition of D, [..]

Is this reasoning correct?

Yes, inline variables with external linkage are guaranteed to refer to the same entity even when they are odr-used as long all the definitions are the same:

[dcl.inline]/6

An inline function or variable shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case ([basic.def.odr]). [..] An inline function or variable with external linkage shall have the same address in all translation units.

The last example is OK because it meets and don't violate the bold part of the above.

are there any disadvantages whatsoever of always marking const and constexpr variables in headers as inline?

I can't think of any, because if we keep the promise of having the exact same definition of an inline variable with external linkage through TU's, the compiler is free to pick any of them to refer to the variable, this will be the same, technically, as having just one TU and have a global variable declared in the header with appropriate header guards

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