Difference between revisions of "C++0x: Variadic templates"

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Line 188: Line 188:
  
 
<pre>
 
<pre>
/* Base case for zero argument template */
+
/* Template definition for counting number of template type arguments */
 +
template<typename... PackTypes> struct count;
 +
 
 +
/* Full specialization for zero argument */
 
template <>
 
template <>
 
struct count<> {
 
struct count<> {
Line 194: Line 197:
 
};
 
};
  
 +
/* Partial specialization for peeling off the first argument */
 
template<typename T, typename... PackTypes>
 
template<typename T, typename... PackTypes>
struct count <T, PackTypes...> {
+
struct count<T, PackTypes...> {
 
   static const int value = 1 + count<PackTypes...>::value;
 
   static const int value = 1 + count<PackTypes...>::value;
 
};
 
};
  
 +
/* Template instantiation */
 
count<char, short, int> a;
 
count<char, short, int> a;
 +
</pre>
  
 +
The base case template:
 +
 +
<pre>
 +
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(...)
 
</pre>
 
</pre>
 +
 +
Compiler may instantiate these 3 types:
 +
 +
<pre>
 +
 +
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(...)
 +
</pre>
 +
  
 
== Change History ==
 
== Change History ==

Revision as of 09:58, 14 April 2009

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


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_type
  • DW_AT_sibling


3.3.7: Function 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.

Template parameter pack entries can have zero or more child entries. Each entry correspond to exactly one argument within the template parameter pack. This child entry must have the tag DW_TAG_template_type_parameter, and will have same form as other template type parameter entries.


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.

Template parameter pack entries can have zero or more child entries. Each entry correspond to exactly one argument within the template parameter pack. This child entry 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.


4.1: Data Object Entries

If a formal parameter is the first argument within the function parameter pack, the formal parameter entry has a DW_AT_name attribute, whose value is a null-terminated string containing the name of the function parameter pack. All other formal parameters within the function parameter pack does not have a DW_AT_name attribute.


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
                  DW_AT_type(reference to base type "char")
14$:          DW_TAG_template_type_parameter
                  DW_AT_type(reference to base type "int")
15$:      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$:      DW_TAG_formal_parameter
              DW_AT_name("args")
              DW_AT_type(reference to 13$)
18$:      DW_TAG_formal_parameter
              ! no DW_AT_name attribute
              DW_AT_type(reference to 14$)

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
                  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
              DW_AT_name("args")
              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$:       DW_TAG_formal_parameter
              DW_AT_name("value")
              DW_AT_type(reference to 31$)


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(...)


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.