Issue 250516.1: Support C++0x Variadic templates
| Author: | Kendrick Wong |
|---|---|
| Champion: | Jason Merrill |
| Date submitted: | 2025-05-16 |
| Date revised: | 2025-09-15 |
| Date closed: | 2025-08-04 |
| Type: | Enhancement |
| Status: | Accepted |
| DWARF version: | 6 |
Comparing original with latest. [ Return to the latest version ]
This proposal was originally submitted as [090107.2][090107.2].
[090107.2]: https://dwarfstd.org/issues/090107.2.html
In the DWARF commmittee meeting on June 2, 2009, four options were presented:
1. `DW_TAG_template_parameter_pack`/`DW_TAG_formal_parameter_pack` is
used as marker.
2. Make `DW_TAG_template_type_parameter` DIEs children of
`DW_TAG_template_parameter_pack`. And make `DW_TAG_formal_parameter`
DIEs children of `DW_TAG_formal_parameter_pack`.
3. Create new children DIEs under
`DW_TAG_template_parameter_pack`/`DW_TAG_formal_parameter_pack` DIE
and add references to
`DW_TAG_template_type_parameter`/`DW_TAG_formal_parameter`
respectively.
4. Reject Variadic templates proposal.
The consensus was to reject the proposal and wait for a vendor to produce
the information first.
## Background
For a detailed description of the feature, please refer to:
- [A Brief Introduction to Variadic Templates][n2087]
- [Variadic Templates (Revision 3)][n2080]
[n2087]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2087.pdf
[n2080]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2080.pdf
## Overview
C++0x will allow function and class templates to take an arbitrary
number of arguments. The motivation behind this language extension is to
allow TR1 facilities such as tuple to accept variable number of
parameters.
template<typename... Elements> class tuple;
tuple<int, long> tuple_int_long; // Elements is a template type parameter pack
// containing 2 arguments, int and long type
tuple<float> tuple_float; // Elements is a template type parameter pack
containing 1 argument: float type
template<typename T, unsigned PrimaryDimension, unsigned... Dimensions> class array;
array<double, 3, 3> t1; // Dimensions is a template value parameter pack
// containing a single argument: unsigned int 3.
array<double, 3, 3, 4, 5> t2; // Dimensions is a template value parameter pack
// containing three arguments: unsigned int 3, 4 and 5.
template<typename... Args> void eat(Args... args);
// The eat() function is a variadic template with template parameter pack Args.
// The ellipsis to the left of the identifier args indicates that args is a
// function parameter pack.
## Proposed change to DWARF
## Proposed changes to DWARF
### Purpose
- DWARF does not represent the generic template definition, but does
represent each template instantiation.
- Template type parameter pack contains only template type parameters
- Template value parameter pack contains only template value
parameters
- Debug information should show where the template parameter pack is
used within the template definition.
- If possible, debug information should also show the arguments within
the template parameter pack.
### New DWARF tags
Add the following new tags to Table 2.1, "Tag names", to Table 7.3, "Tag encodings", and to Table A.1, "Attributes by tag value":
- `DW_TAG_template_parameter_pack`
Allowable attributes:
- DECL
- `DW_AT_name`
- `DW_AT_sibling`
- `DW_TAG_formal_parameter_pack`
Allowable attributes:
- DECL
- `DW_AT_name`
- `DW_AT_sibling`
### 2.23: Template Parameters
Replace the third and fourth paragraphs with the following:
A template type parameter is represented by a debugging information entry with
> A template type parameter is represented by a debugging information entry with
the tag `DW_TAG_template_type_parameter`. A template value parameter is
> the tag `DW_TAG_template_type_parameter`. A template value parameter is
represented by a debugging information entry with the tag
> represented by a debugging information entry with the tag
`DW_TAG_template_value_parameter`. <ins>A template parameter pack is represented by a
> `DW_TAG_template_value_parameter`. <ins>A template parameter pack is represented by a
debugging information entry with the tag `DW_TAG_template_parameter_pack`.</ins> The
> debugging information entry with the tag `DW_TAG_template_parameter_pack`.</ins> The
actual template parameter entries appear in the same order as the corresponding
> actual template parameter entries appear in the same order as the corresponding
template formal parameter declarations in the source program.
> template formal parameter declarations in the source program.
>
<ins>Each element of a parameter pack is represented by a debugging information
> <ins>Each element of a parameter pack is represented by a debugging information
entry for a type or value parameter. These entries are children of the
> entry for a type or value parameter. These entries are children of the
debugging information entry with the tag `DW_TAG_template_parameter_pack`.
> debugging information entry with the tag `DW_TAG_template_parameter_pack`.
Each such entry does not have a `DW_AT_name` attribute.</ins>
> Each such entry does not have a `DW_AT_name` attribute.</ins>
>
<ins>Except as above,</ins> a type or value parameter <ins>or parameter
> <ins>Except as above,</ins> a type or value parameter <ins>or parameter
pack</ins> entry may have a DW_AT_name attribute, whose value is a
> pack</ins> entry may have a `DW_AT_name` attribute, whose value is a
null-terminated string containing the name of the corresponding formal
> null-terminated string containing the name of the corresponding formal > parameter. <del>The</del> <ins>A type or value parameter</ins> entry
parameter. The entry may also have a DW_AT_default_value attribute, which is a
> may also have a `DW_AT_default_value` attribute, which is a
flag indicating that the value corresponds to the default argument for the
> flag indicating that the value corresponds to the default argument for the
template parameter.
> template parameter.
### 3.3.7: Function Template Instantiations
Add the following numbered item following item #1 (renumbering items 2 and 3):
4\. Each function parameter pack declaration appearing in the template
> 2\. Each function parameter pack declaration appearing in the template
definition is represented by a debugging information entry with the tag
> definition is represented by a debugging information entry with the tag
`DW_TAG_formal_parameter_pack`. Each such entry may have a `DW_AT_name`
> `DW_TAG_formal_parameter_pack`. Each such entry may have a `DW_AT_name`
attribute, whose value is a null-terminated string containing the name
> attribute, whose value is a null-terminated string containing the name
of the function parameter pack as it appears in the source program.
> of the function parameter pack as it appears in the source program.
>
5\. Each element of the function parameter pack is represented by a
> Each element of the function parameter pack is represented by a
debugging information entry with the tag `DW_TAG_formal_parameter`.
> debugging information entry with the tag `DW_TAG_formal_parameter`.
These entries are children of the debugging information entry with the
> These entries are children of the debugging information entry with the
tag `DW_TAG_formal_parameter_pack`. Each such entry does not have a
> tag `DW_TAG_formal_parameter_pack`. Each such entry does not have a
`DW_AT_name` attribute. Each such entry has a `DW_AT_type` attribute
> `DW_AT_name` attribute. Each such entry has a `DW_AT_type` attribute
describing the type of the formal parameter in this instantiation.
> describing the type of the formal parameter in this instantiation.
## APPENDIX
## APPENDIX D: Examples
### D.8 Variadic Templates Examples
#### D.8.1 Function Template Instantiations
/* Base case for printf */
void printf(const char* s) {
while (*s) {
std::cout << *s++;
}
}
template<typename T, typename... PackTypes>
void printf(const char* s, T value, PackTypes... args) {
while (*s) {
if (*s == '%' && *++s != '%') {
std::cout << value;
return printf (++s, args...);
}
std::cout << *s++;
}
}
int x;
printf<int, char, int> ("%c %d", x, 'x', 3);
The base case for printf is not a template instantiation:
printf (const char* s);
1$: DW_TAG_subprogram
DW_AT_name("printf")
2$: DW_TAG_formal_parameter
DW_AT_name("s")
DW_AT_type(reference to type "const char*")
Compiler may instantiate these 3 functions:
# printf<int,char,int> (const char* s, int value, char, int); // args => char, int
# printf<char,int> (const char* s, char value, int); // args => int
# printf<int> (const char* s, int value); // args => (empty)
1) printf<int,char,int> (const char* s, int value, char, int); // args => char, int
10$: DW_TAG_subprogram
DW_AT_name("printf")
11$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "int")
12$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
13$: DW_TAG_template_type_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "char")
14$: DW_TAG_template_type_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "int")
15$: DW_TAG_formal_parameter
DW_AT_name("s")
DW_AT_type(reference to type "const char*")
16$: DW_TAG_formal_parameter
DW_AT_name("value")
DW_AT_type(reference to base type "int")
17$: DW_TAG_formal_parameter_pack
DW_AT_name("args")
18$: DW_TAG_formal_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "char")
19$: DW_TAG_formal_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "int")
2) printf<char,int> (const char* s, char value, int); // args => int
20$: DW_TAG_subprogram
DW_AT_name("printf")
21$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "int")
22$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
23$: DW_TAG_template_type_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "int")
24$: DW_TAG_formal_parameter
DW_AT_name("s")
DW_AT_type(reference to type "const char*")
25$: DW_TAG_formal_parameter
DW_AT_name("value")
DW_AT_type(reference to base type "char")
26$: DW_TAG_formal_parameter_pack
DW_AT_name("args")
27$: DW_TAG_formal_parameter
! no DW_AT_name attribute
DW_AT_type(reference to base type "int")
3) printf<int> (const char* s, int value); // args => (empty)
30$: DW_TAG_subprogram
DW_AT_name("printf")
31$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "int")
32$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
33$: DW_TAG_formal_parameter
DW_AT_name("s")
DW_AT_type(reference to type "const char*")
34$: DW_TAG_formal_parameter
DW_AT_name("value")
DW_AT_type(reference to base type "int")
35$: DW_TAG_formal_parameter_pack
DW_AT_name("args")
#### D.8.2 Class Template Instantiations
/* Template definition for counting number of template type arguments */
template<typename... PackTypes> struct count;
/* Full specialization for zero argument */
template <>
struct count<> {
static const int value = 0;
};
/* Partial specialization for peeling off the first argument */
template<typename T, typename... PackTypes>
struct count<T, PackTypes...> {
static const int value = 1 + count<PackTypes...>::value;
};
/* Template instantiation */
count<char, short, int> a;
The base case template:
1$: DW_TAG_structure_type
DW_AT_name("count")
2$: DW_TAG_member
DW_AT_name("value")
DW_AT_declaration(yes)
DW_AT_type(reference to type "const int")
3$: DW_TAG_variable
DW_AT_specification(reference to 2$)
DW_AT_location(...)
Compiler may instantiate these 3 types:
1) count<char, short, int> // PackTypes => short, int
11$: DW_TAG_structure_type
DW_AT_name("count")
12$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "char")
13$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
14$: DW_TAG_template_type_parameter
DW_AT_type(reference to base type "short")
15$: DW_TAG_template_type_parameter
DW_AT_type(reference to base type "int")
16$: DW_TAG_member
DW_AT_name("value")
DW_AT_declaration(yes)
DW_AT_type(reference to type "const int")
17$: DW_TAG_variable
DW_AT_specification(reference to 16$)
DW_AT_location(...)
2) count<short, int> // PackTypes => int
21$: DW_TAG_structure_type
DW_AT_name("count")
22$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "short")
23$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
24$: DW_TAG_template_type_parameter
DW_AT_type(reference to base type "int")
25$: DW_TAG_member
DW_AT_name("value")
DW_AT_declaration(yes)
DW_AT_type(reference to type "const int")
26$: DW_TAG_variable
DW_AT_specification(reference to 25$)
DW_AT_location(...)
3) count<int> // PackTypes => (empty)
31$: DW_TAG_structure_type
DW_AT_name("count")
32$: DW_TAG_template_type_parameter
DW_AT_name("T")
DW_AT_type(reference to base type "int")
33$: DW_TAG_template_parameter_pack
DW_AT_name("PackTypes")
34$: DW_TAG_member
DW_AT_name("value")
DW_AT_declaration(yes)
DW_AT_type(reference to type "const int")
35$: DW_TAG_variable
DW_AT_specification(reference to 34$)
DW_AT_location(...)
4) count<char, short, int> a;
41$: DW_TAG_variable
DW_AT_name("a")
DW_AT_type(reference to 11$)
DW_AT_location(...)
#### D.8.3 Multiple template packs in template definition
template <class X,int I>
struct pair {
void showself () {
std::cout << I << std::endl;
X local;
local.showself();
}
};
struct a1 { void showself () { std::cout << "a1" << std::endl; }};
struct a2 { void showself () { std::cout << "a2" << std::endl; }};
template <class headX, int headI, class... restX, int... restI>
void showself (pair<headX, headI> headArg, pair<restX, restI>... restArg) {
headArg.showself();
showself (restArg...);
}
/* Instantiations */
pair<a1,1> t1;
pair<a2,2> t2;
showself(t1,t2);
First the simple structures:
struct a1 { void showself () { std::cout << "a1" << std::endl; }};
11$: DW_TAG_structure_type
DW_AT_name("a1")
12$: DW_TAG_subprogram
DW_AT_name("showself")
DW_AT_type(reference to type "void")
struct a2 { void showself () { std::cout << "a2" << std::endl; }};
21$: DW_TAG_structure_type
DW_AT_name("a2")
22$: DW_TAG_subprogram
DW_AT_name("showself")
DW_AT_type(reference to type "void")
DWARF for 'pair' structures:
pair<a1,1> t1;
31$: DW_TAG_structure_type
DW_AT_name("pair")
32$: DW_TAG_template_type_parameter
DW_AT_name("X")
DW_AT_type(reference to 11$)
33$: DW_TAG_template_value_parameter
DW_AT_name("I")
DW_AT_type(reference to base type "int")
DW_AT_const_value(1)
33$: DW_TAG_subprogram
DW_AT_name("showself")
DW_AT_type(reference to type "void")
34$: DW_TAG_variable
DW_AT_name("t1")
DW_AT_type(reference to 31$)
DW_AT_location(...)
pair<a2,2> t2;
41$: DW_TAG_structure_type
DW_AT_name("pair")
42$: DW_TAG_template_type_parameter
DW_AT_name("X")
DW_AT_type(reference to 21$)
43$: DW_TAG_template_value_parameter
DW_AT_name("I")
DW_AT_type(reference to base type "int")
DW_AT_const_value(2)
43$: DW_TAG_subprogram
DW_AT_name("showself")
DW_AT_type(reference to type "void")
44$: DW_TAG_variable
DW_AT_name("t2")
DW_AT_type(reference to 41$)
DW_AT_location(...)
The variadic template function showself():
template <class headX, int headI, class... restX, int... restI>
void showself (pair<headX, headI> headArg, pair<restX, restI>... restArg) {
headArg.showself();
showself (restArg...);
}
showself (pair<struct a1,1>, pair<struct a2,2>):
51$: DW_TAG_subprogram
DW_AT_name("showself")
52$: DW_TAG_template_type_parameter
DW_AT_name("headX")
DW_AT_type(reference to 11$)
53$: DW_TAG_template_value_parameter
DW_AT_name("headI")
DW_AT_type(reference to base type "int")
DW_AT_const_value(1)
54$: DW_TAG_template_parameter_pack
DW_AT_name("restX")
55$: DW_TAG_template_type_parameter
DW_AT_type(reference to 21$)
56$: DW_TAG_template_parameter_pack
DW_AT_name("restI")
DW_AT_type(reference to base type "int")
57$: DW_TAG_template_value_parameter
DW_AT_const_value(2)
58$: DW_TAG_formal_parameter
DW_AT_name("headArg")
DW_AT_type(reference to 31$)
59$: DW_TAG_formal_parameter_pack
DW_AT_name("restArg")
60$: DW_TAG_formal_parameter
! no DW_AT_name attribute
DW_AT_type(reference to 41$)
showself (pair<struct a2,2>):
71$: DW_TAG_subprogram
DW_AT_name("showself")
72$: DW_TAG_template_type_parameter
DW_AT_name("headX")
DW_AT_type(reference to 21$)
73$: DW_TAG_template_value_parameter
DW_AT_name("headI")
DW_AT_type(reference to base type "int")
DW_AT_const_value(2)
74$: DW_TAG_template_parameter_pack
DW_AT_name("restX")
75$: DW_TAG_template_parameter_pack
DW_AT_name("restI")
76$: DW_TAG_formal_parameter
DW_AT_name("headArg")
DW_AT_type(reference to 41$)
77$: DW_TAG_formal_parameter_pack
DW_AT_name("restArg")
## Change History
March 17, 2009.
- Define template parameter pack and function parameter pack.
- Add `DW_AT_parameter_pack` to indicate that a parameter is a part of
template parameter pack.
- Add naming convention for parameters which are part of template
parameter pack.
April 13, 2009.
- Place Example section in Appendix D
- Re-format DWARF description in Example section to match current
documentation.
- Remove `DW_AT_parameter_pack` attribute
- Add example of class template instantiation with template parameter
pack.
May 5, 2009.
- Add `DW_TAG_formal_parameter_pack` to indicate the name of the
function parameter pack.
- `DW_TAG_template_type_parameter` are not longer children of
`DW_TAG_template_parameter_pack`. This is made so that consumer
don't need to understand the new TAG, and still be able to process
the other information.
- Add debug information representation for variable 'a' in D.8.2.
June 2, 2009.
- Add example for multiple template parameter pack in template
definition.
September 11, 2009.
- Dodji Seketeli points out that function parameter packs do not
have to be located at the end of a parameter-declaration-list [[n2939#US45][us45]]
This provides the argument that `DW_TAG_formal_parameter_pack` can
not be used as marker. And for consistency,
`DW_TAG_template_parameter_pack` can follow the same behavior as
`DW_TAG_formal_parameter_pack`.
- Adjust examples to make `DW_TAG_template_type_parameter` and
`DW_TAG_formal_parameter` children of
`DW_TAG_template_parameter_pack` and `DW_TAG_formal_parameter_pack`
respectively.
July 6, 2025.
- Correct formal parameter types in examples to match the specification.
- Refactor to change 2.23 and not 5.6.8.
[us45]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2939.html#US45
---
2025-08-04: Accepted, with possible editorial changes.
2025-09-15: [Editorial changes][diff1]. List tables needing change due to new DWARF tags; Clarify scope of changes to Sections 2.23 and 3.3.7; Clarified text to disallow `DW_AT_default_value` attribute on parameter pack entries. In Section 3.3.7, combined items 4 and 5 and inserted between old items 1 and 2. Of the examples given, only the printf example will be added to Appendix D.
