C++0x: Variadic templates

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For detail description of the feature, please refer to:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2087.pdf

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2080.pdf

README

In the June 2 DWARF meeting, four proposals were presented for voting:

  1. DW_TAG_template_parameter_pack/DW_TAG_formal_parameter_pack is used as marker (i.e. current proposal)
  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 is to reject the proposal and wait for a vendor to produce the information first. We can revisit the vendor implementation at a later time and re-evaluate the proposal then.

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

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 tag

DW_TAG_template_parameter_pack 0x43 template parameters pack

Allowable attributes:

  • DECL
  • DW_AT_name
  • DW_AT_sibling


DW_TAG_formal_parameter_pack 0x44 function parameters pack

Allowable attributes:

  • DECL
  • DW_AT_name
  • DW_AT_sibling


3.3.7: Function Template Instantiations

4. Each template parameter pack declaration appearing in the template definition is represented by a debugging information entry with the tag DW_TAG_template_parameter_pack. Each such entry may have a DW_AT_name attribute, whose value is a null-terminated string containing the name of the template parameter pack as it appears in the source program.

5. Each argument within the template parameter pack is represented by a debugging information entry with the tag DW_TAG_template_type_parameter. These entries follow immediately after the debugging information entry with the tag DW_TAG_template_parameter_pack, and are owned by the same parent entry. Each such entry does not have a DW_AT_name attribute. Each such entry has a DW_AT_type attribute describing the actual type by which the formal is replaced for this instantiation.

6. Each function parameter pack declaration appearing in the template 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 attribute, whose value is a null-terminated string containing the name of the template parameter pack as it appears in the source program.

7. Each argument within the function parameter pack is represented by a debugging information entry with the tag DW_TAG_formal_parameter. These entries follow immediately after the debugging information entry with the tag DW_TAG_formal_parameter_pack, and are owned by the same parent entry. Each such entry does not have a DW_AT_name attribute. Each such entry has a DW_AT_type attribute describing the actual type by which the formal is replaced for this instantiation.


5.6.8: Class Template Instantiations

Each template parameter pack declaration appearing in the template definition is represented by a debugging information entry with the tag DW_TAG_template_parameter_pack. Each such entry may have a DW_AT_name attribute, whose value is a null-terminated string containing the name of the template parameter pack as it appears in the source program.

If the template parameter pack entry represent a template value parameter pack, it may contain a DW_AT_type attribute to denote the type of the arguments within the template value parameter pack.

Each argument within the template parameter pack can have one of the tags DW_TAG_template_type_parameter or DW_TAG_template_value_parameter, and will have same form as other template parameter entries. These entries follow immediately after the debugging information entry with the tag DW_TAG_template_parameter_pack, and are owned by the same parent entry. Each such entry does not have a DW_AT_name attribute. Each such entry has a DW_AT_type attribute describing the actual type by which the formal is replaced for this instantiation.


APPENDIX

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*")
<pre>

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)

<pre>
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$:      DW_TAG_formal_parameter
              DW_AT_name("value")
              DW_AT_type(reference to 11$)
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 13$)
19$:      DW_TAG_formal_parameter
              ! no DW_AT_name attribute
              DW_AT_type(reference to 14$)

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 21$)
26$:      DW_TAG_formal_parameter_pack
              DW_AT_name("args")
27$:      DW_TAG_formal_parameter
              ! no DW_AT_name attribute
              DW_AT_type(reference to 23$)

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$:      DW_TAG_formal_parameter
              DW_AT_name("value")
              DW_AT_type(reference to 31$)
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 a function might have multiple parameter packs not necessarily located at the end of the parameter list:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2939.html#US45