static constexpr pointer-to-function, difference between compilers

Question

When answering this question, I tried the following code with gcc (code compiled) and clang (code rejected):

typedef long (*func)(int);

long function(int) { return 42; }

struct Test
{
    static constexpr func f = &function;
};

template<func c>
struct Call
{
    static void f()
    {
        c(0);
    }
};

int main()
{
    Call<Test::f>::f();
}

I am not sure which compiler is right, although I think the constexpr initialization of Test::f is ok. The error clang outputs is:

error: non-type template argument for template parameter of pointer type 'func'
       (aka 'long (*)(int)') must have its address taken

Which compiler is right ?
If clang is right, why , and what does this error really means ?

EDIT: for the "why", see DyP's question.

I'm wondering whether or not &function is actually a valid initializer for constexpr. Both compilers seem to think so. But it seems iffy to me. That isn't known at compile time, only at link time. — Omnifarious
– Omnifarious, Commented Apr 7, 2013 at 20:34
&function is the address of a function, so for me it is known at compile time... For example, you can pass a pointer to function as a template argument. — Synxis
– Synxis, Commented Apr 7, 2013 at 20:37
A pointer to a function is one thing... if this was part of a shared library, for example, how could the address possibly qualify as a constexpr? — Brett Hale
– Brett Hale, Commented Apr 7, 2013 at 20:46
@BrettHale I don't know exactly, but I suspect the pointer to function to be an offset to the beginning of the dll code, and it is relocated when the dll is loaded. Anyway, I know that it works for libraries, since I already used it in template parameters (it was in C++03, though). — Synxis
– Synxis, Commented Apr 7, 2013 at 21:01

dyp · Accepted Answer · 2013-04-08 17:15:41Z

9

14.3.2 Template non-type arguments [temp.arg.nontype]

A template-argument for a non-type, non-template template-parameter shall be one of:

[...]

— a constant expression (5.19) that designates the address of an object with static storage > duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; [...]

(n3485, emphasis mine)

I don't know exactly why it's been limited, but I think it might be related to the fact that the function address is not available at compile time (there might be a substitute for template instantiation purposes).

Edit: Enhanced answer due to a follow-up question (comment) of Synxis

constexpr func = &function;

^ this is well-formed; you can use the address of a function to initialize a constexpr object. The problem is that it's explicitly forbidden to use pointers as non-type template arguments other than of the form &identifier:

using My_Call     = Call < &function >;  // fine

constexpr func mypointer = &function;    // fine
using My_Ind_Call = Call < func >;       // forbidden, argument not of form `&id`

edited Apr 8, 2013 at 17:15

answered Apr 7, 2013 at 20:37

dyp

39.4k13 gold badges119 silver badges190 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Omnifarious Over a year ago

This seems like an oversight. If you assign a constexpr thing to a constexpr and then assign that to another constexpr, it should be transitive.

dyp Over a year ago

@Omnifarious It is a constexpr. But it's explicitly forbidden as a template non-type argument.

Omnifarious Over a year ago

Well, that's interesting. Explicitly not treated orthogonally. I wonder what the reasoning is. Though, I have a strong suspicion. My guess is that objects that can be relocated by the linker are referred to by name in the mangling rules for templates. Requiring compilers to carry around the original name for constexpr things like that is rather onerous and possibly impossible in some scenarios.

Synxis Over a year ago

@DyP In the sentence you put in bold, I don't see where it explicitely say constexpr func f = &function; to be illegal (maybe I missed a part of your reasoning). Also I think the function can always be available at compile-time, because you can use offsets to start of machine code as function address for example (and also I made it working on several compilers, though not using constexpr nor static: template<void (*f)()> struct A {}; void func() {} A<&func> object;).

dyp Over a year ago

@Synxis I enhanced my answer, but couldn't answer the "why", therefore asked it as a separate question

|

Collectives™ on Stack Overflow

static constexpr pointer-to-function, difference between compilers

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related