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130412.1 Paul Robinson Non-integral non-type template parameters Clarification Accepted Paul Robinson


Section 5.5.8, pg 93
C++11 14.1 [temp.param]p4 says:
A non-type template-parameter shall have one of the following (optionally
cv-qualified) types:
- integral or enumeration type,
- pointer to object or pointer to function,
- lvalue reference to object or lvalue reference to function,
- pointer to member,
- std::nullptr_t.

C++11 14.3.2 [temp.arg.nontype]p1 says:
A template-argument for a non-type, non-template template-parameter
shall be one of:
- for a non-type template-parameter of integral or enumeration type,
  a converted constant expression (5.19) of the type of the
  template-parameter; or
- the name of a non-type template-parameter; or
- 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; or
- a constant expression that evaluates to a null pointer value (4.10); or
- a constant expression that evaluates to a null member pointer value
  (4.11); or
- a pointer to member expressed as described in 5.3.1.

The first kind is a "normal" constant value; all the rest are
addresses that are load-time constant but not necessarily compile-time
constant.

DWARF's existing DW_TAG_template_value_parameter handles the first
case, integral/enumeration type, by specifying that the actual value
is described using DW_AT_const_value.  The descriptions of that
attribute allow it to be a string, to have a constant data form, or
a data block form.  None of these are appropriate for describing the
load-time constant address of a non-type template argument.

I tried compiling some examples with GCC 4.7.2, and it uses
DW_AT_location (DW_OP_addr, DW_OP_stack_value) to describe arguments
that are object addresses, and DW_AT_const_value=0 to describe an
argument that is a null pointer.  (Although I didn't try the
pointer-to-member case as I don't really understand them.)


Proposal:

Revise section 5.5.8 to include non-type template parameters
that are run-time constant but not compile-time constant.

Section 5.5.8, non-normative text at the beginning, second sentence
was:
    The generic description of the class may include both
    parameterized types and parameterized constant values.

Replace this with:
    The generic description of the class may include
    parameterized types, parameterized compile-time constant
    values, and/or parameterized run-time constant addresses.


Section 5.5.8, item 2, last sentence was:
    Finally, the template value parameter entry has a
    DW_AT_const_value attribute, whose value is the actual constant
    value of the value parameter for this instantiation as
    represented on the target architecture.

Replace that sentence with:
    Finally, the template value parameter entry has an attribute
    giving the actual compile-time or run-time constant value of
    the value parameter for this instantiation.  This can be a
    DW_AT_const_value attribute, whose value is the compile-time
    constant value as represented on the target architecture; or,
    a DW_AT_location attribute, whose value is a single location
    description giving the run-time constant address.

---

Revised 7/22/13.
Accepted 7/23/13.  


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