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  2. How to easily create fully “variadic” functions with C++ 98 standard?

How to easily create fully “variadic” functions with C++ 98 standard?

由 我怕爱的太早我们不能终老 提交于 2019-12-18 09:37:28

问题


The library https://github.com/c42f/tinyformat/blob/2f9335afd9941688e42d60cae5166b9f0600b2d1/tinyformat.h#L1104-L1116, uses this awesome trick to do "variadic" templates on C++ 98:

inline void printfln(const char* fmt)
{
    format(std::cout, fmt);
    std::cout << '\n';
}

template<TINYFORMAT_ARGTYPES(n)>                                          \
void printfln(const char* fmt, TINYFORMAT_VARARGS(n))                     \
{                                                                         \
    format(std::cout, fmt, TINYFORMAT_PASSARGS(n));                       \
    std::cout << '\n';                                                    \
}

I am trying to improve it by eliminating the requirement to duplicate the function printfln twice, i.e., one time for the base case inline void printfln(const char* fmt), and a second time for the "variadic" part template<TINYFORMAT_ARGTYPES(n)> void printfln(const char* fmt, TINYFORMAT_VARARGS(n)).

They are required to split the printfln function in two parts because the "variadic" function can only accept one parameter, i.e., printfln("something"). On this case, the TINYFORMAT_VARARGS(n) would have to expand to nothing, however, when doing so, it will cause the code to have a trailing comma ,, leading to a invalid syntax on C++.

I could use the GNU GCC expansion trick with C macro Token-Pasting Operator ## to remove the trailing comma, however, this is not portable because it only works for GNU GCC. Then, my goal is to define the macros already containing a leading comma as the next example:

#include <stdio.h>

#define TINYFORMAT_ARGTYPES_0
#define TINYFORMAT_ARGTYPES_1 , class T1
#define TINYFORMAT_ARGTYPES_2 , class T1, class T2
#define TINYFORMAT_ARGTYPES_3 , class T1, class T2, class T3

#define TINYFORMAT_VARARGS_0
#define TINYFORMAT_VARARGS_1 , const T1& v1
#define TINYFORMAT_VARARGS_2 , const T1& v1, const T2& v2
#define TINYFORMAT_VARARGS_3 , const T1& v1, const T2& v2, const T3& v3

#define TINYFORMAT_PASSARGS_0
#define TINYFORMAT_PASSARGS_1 , v1
#define TINYFORMAT_PASSARGS_2 , v1, v2
#define TINYFORMAT_PASSARGS_3 , v1, v2, v3

#define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)

#define FACTORY(n) \
template<typename T0 TINYFORMAT_ARGTYPES(n)> \
inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
{ \
    fprintf(stderr, v0 TINYFORMAT_PASSARGS(n) ); \
}

TINYFORMAT_FOREACH_ARGNUM(FACTORY)

int main(int argc, char const *argv[])
{
    some( "Something %s.", "New" );
}

But, it does not work. The gcc compiler is going nuts when if finds a comma , right after the macro definition: g++ -o main -g -ggdb test_debugger.cpp --std=c++98 && ./main

test_debugger.cpp:21:19: error: expected nested-name-specifier before ‘T0’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                   ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:22: error: expected ‘>’ before ‘TINYFORMAT_ARGTYPES’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                      ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:24: error: ‘T0’ does not name a type
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                        ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:31: error: expected ‘,’ or ‘...’ before ‘TINYFORMAT_VARARGS’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                               ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:52: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                                                    ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp: In function ‘void some(const int&)’:
test_debugger.cpp:24:24: error: expected ‘)’ before ‘TINYFORMAT_PASSARGS’
     fprintf(stderr, v0 TINYFORMAT_PASSARGS(n) ); \
                        ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp: At global scope:
test_debugger.cpp:21:19: error: expected nested-name-specifier before ‘T0’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                   ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:22: error: expected ‘>’ before ‘TINYFORMAT_ARGTYPES’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                      ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:24: error: ‘T0’ does not name a type
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                        ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:31: error: expected ‘,’ or ‘...’ before ‘TINYFORMAT_VARARGS’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                               ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:52: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                                                    ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: error: redefinition of ‘template<<declaration error> > void some(const int&)’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:43: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                           ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: note: ‘template<<declaration error> > void some(const int&)’ previously declared here
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:19: error: expected nested-name-specifier before ‘T0’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                   ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:22: error: expected ‘>’ before ‘TINYFORMAT_ARGTYPES’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                      ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:24: error: ‘T0’ does not name a type
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                        ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:31: error: expected ‘,’ or ‘...’ before ‘TINYFORMAT_VARARGS’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                               ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:52: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                                                    ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: error: redefinition of ‘template<<declaration error> > void some(const int&)’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:48: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: note: ‘template<<declaration error> > void some(const int&)’ previously declared here
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:19: error: expected nested-name-specifier before ‘T0’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                   ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:21:22: error: expected ‘>’ before ‘TINYFORMAT_ARGTYPES’
 template<typename T0 TINYFORMAT_ARGTYPES(n)> \
                      ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:24: error: ‘T0’ does not name a type
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                        ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:31: error: expected ‘,’ or ‘...’ before ‘TINYFORMAT_VARARGS’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                               ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:52: error: default template arguments may not be used in function templates without -std=c++11 or -std=gnu++11
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
                                                    ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: error: redefinition of ‘template<<declaration error> > void some(const int&)’
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:53: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                                     ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: note: ‘template<<declaration error> > void some(const int&)’ previously declared here
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp: In function ‘int main(int, const char**)’:
test_debugger.cpp:31:34: error: no matching function for call to ‘some(const char [14], const char [4])’
     some( "Something %s.", "New" );
                                  ^
test_debugger.cpp:22:13: note: candidate: template<<declaration error> > void some(const int&)
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:22:13: note:   template argument deduction/substitution failed:
 inline void some(const T0& v0 TINYFORMAT_VARARGS(n)) \
             ^
test_debugger.cpp:18:38: note: in expansion of macro ‘FACTORY’
 #define TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3)
                                      ^
test_debugger.cpp:27:1: note: in expansion of macro ‘TINYFORMAT_FOREACH_ARGNUM’
 TINYFORMAT_FOREACH_ARGNUM(FACTORY)
 ^~~~~~~~~~~~~~~~~~~~~~~~~
test_debugger.cpp:31:34: note:   candidate expects 1 argument, 2 provided
     some( "Something %s.", "New" );
                                  ^

In my understanding my TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3) macro should create the following 4 valid C++ "variadic" functions:

template<typename T0> 
inline void some(const T0& v0) 
{ 
    fprintf(stderr, v0 ); 
}

template<typename T0, class T1> 
inline void some(const T0& v0, const T1& v1) 
{ 
    fprintf(stderr, v0, v1 ); 
}

template<typename T0, class T1, class T2> 
inline void some(const T0& v0, const T1& v1, const T1& v2) 
{ 
    fprintf(stderr, v0, v1, v2); 
}

template<typename T0, class T1, class T2, class T3> 
inline void some(const T0& v0, const T1& v1, const T1& v2, const T1& v3) 
{ 
    fprintf(stderr, v0, v1, v2, v3); 
}

Why the gcc preprocessor is not generating correctly my 4 "variadic" template functions as above?

For reference, I am using:

$ g++ --version
g++ (GCC) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Update

Output of g++ -o main -E -g -ggdb test_debugger.cpp --std=c++98

# 797 "/usr/include/stdio.h" 3 4
}
# 2 "test_debugger.cpp" 2
# 27 "test_debugger.cpp"

# 27 "test_debugger.cpp"
template<typename T0 TINYFORMAT_ARGTYPES(0)> 
inline void some(const T0& v0 TINYFORMAT_VARARGS(0)) { fprintf(
# 27 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 27 "test_debugger.cpp"
, v0 TINYFORMAT_PASSARGS(0) ); } template<typename T0 TINYFORMAT_ARGTYPES(1)> 
inline void some(const T0& v0 TINYFORMAT_VARARGS(1)) { fprintf(
# 27 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 27 "test_debugger.cpp"
, v0 TINYFORMAT_PASSARGS(1) ); } template<typename T0 TINYFORMAT_ARGTYPES(2)> 
inline void some(const T0& v0 TINYFORMAT_VARARGS(2)) { fprintf(
# 27 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 27 "test_debugger.cpp"
, v0 TINYFORMAT_PASSARGS(2) ); } template<typename T0 TINYFORMAT_ARGTYPES(3)> 
inline void some(const T0& v0 TINYFORMAT_VARARGS(3)) { fprintf(
# 27 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 27 "test_debugger.cpp"
, v0 TINYFORMAT_PASSARGS(3) ); }

int main(int argc, char const *argv[])
{
    some( "Something %s.", "New" );
}

Update 2

I tried to change the order of the definition in hope the C preprocessor would expand things right, but it just expanded exactly as before:

#include <stdio.h>

#define TINYFORMAT_ARGTYPES_0
#define TINYFORMAT_ARGTYPES_1 , class T1
#define TINYFORMAT_ARGTYPES_2 , class T1, class T2
#define TINYFORMAT_ARGTYPES_3 , class T1, class T2, class T3

#define TINYFORMAT_VARARGS_0
#define TINYFORMAT_VARARGS_1 , const T1& v1
#define TINYFORMAT_VARARGS_2 , const T1& v1, const T2& v2
#define TINYFORMAT_VARARGS_3 , const T1& v1, const T2& v2, const T3& v3

#define TINYFORMAT_PASSARGS_0
#define TINYFORMAT_PASSARGS_1 , v1
#define TINYFORMAT_PASSARGS_2 , v1, v2
#define TINYFORMAT_PASSARGS_3 , v1, v2, v3

#define TINYFORMAT_FOREACH_ARGNUM(m) \
    m(TINYFORMAT_ARGTYPES(0),TINYFORMAT_VARARGS(0),TINYFORMAT_PASSARGS(0)) \
    m(TINYFORMAT_ARGTYPES(1),TINYFORMAT_VARARGS(1),TINYFORMAT_PASSARGS(1)) \
    m(TINYFORMAT_ARGTYPES(2),TINYFORMAT_VARARGS(2),TINYFORMAT_PASSARGS(2)) \
    m(TINYFORMAT_ARGTYPES(3),TINYFORMAT_VARARGS(3),TINYFORMAT_PASSARGS(3))

#define FACTORY(argtypes,varargs,passargs) \
template<typename T0 argtypes> \
inline void some(const T0& v0 varargs) \
{ \
    fprintf(stderr, v0 passargs); \
}
TINYFORMAT_FOREACH_ARGNUM(FACTORY)

int main(int argc, char const *argv[])
{
    some( "Something %s.", "New" );
}

Update 3

As @aschepler commented, I was missing the definition of TINYFORMAT_ARGTYPES_ ## n, this is a fixed version:

#include <stdio.h>

#define TINYFORMAT_ARGTYPES_0(...)
#define TINYFORMAT_ARGTYPES_1(...) __VA_ARGS__ class T1
#define TINYFORMAT_ARGTYPES_2(...) __VA_ARGS__ class T1, class T2
#define TINYFORMAT_ARGTYPES_3(...) __VA_ARGS__ class T1, class T2, class T3

#define TINYFORMAT_VARARGS_0(...)
#define TINYFORMAT_VARARGS_1(...) __VA_ARGS__ const T1& v1
#define TINYFORMAT_VARARGS_2(...) __VA_ARGS__ const T1& v1, const T2& v2
#define TINYFORMAT_VARARGS_3(...) __VA_ARGS__ const T1& v1, const T2& v2, const T3& v3

#define TINYFORMAT_PASSARGS_0(...)
#define TINYFORMAT_PASSARGS_1(...) __VA_ARGS__ v1
#define TINYFORMAT_PASSARGS_2(...) __VA_ARGS__ v1, v2
#define TINYFORMAT_PASSARGS_3(...) __VA_ARGS__ v1, v2, v3

#define TINYFORMAT_ARGTYPES(n,...) TINYFORMAT_ARGTYPES_ ## n (__VA_ARGS__)
#define TINYFORMAT_VARARGS(n,...)  TINYFORMAT_VARARGS_  ## n (__VA_ARGS__)
#define TINYFORMAT_PASSARGS(n,...) TINYFORMAT_PASSARGS_ ## n (__VA_ARGS__)

#define TINYFORMAT_FOREACH_ARGNUM(m,...) \
        m(0) m(1,__VA_ARGS__) m(2,__VA_ARGS__) m(3,__VA_ARGS__)

#define FACTORY(n,...) \
template<TINYFORMAT_ARGTYPES(n,__VA_ARGS__)> \
inline void some(TINYFORMAT_VARARGS(n,__VA_ARGS__)) \
{ \
    fprintf(stderr, "variadic" TINYFORMAT_PASSARGS(n,,) ); \
}
TINYFORMAT_FOREACH_ARGNUM(FACTORY,)

int main(int argc, char const *argv[]) {
    some();
}

Expanding to: g++ -o main -E -g -ggdb test_debugger.cpp --std=c++98

# 797 "/usr/include/stdio.h" 3 4
}
# 2 "test_debugger.cpp" 2
# 31 "test_debugger.cpp"

# 31 "test_debugger.cpp"
template<> 
inline void some() { fprintf(
# 31 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 31 "test_debugger.cpp"
, "variadic" ); } template< class T1> 
inline void some( const T1& v1) { fprintf(
# 31 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 31 "test_debugger.cpp"
, "variadic" , v1 ); } template< class T1, class T2> 
inline void some( const T1& v1, const T2& v2) { fprintf(
# 31 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 31 "test_debugger.cpp"
, "variadic" , v1, v2 ); } template< class T1, class T2, class T3> 
inline void some( const T1& v1, const T2& v2, const T3& v3) { fprintf(
# 31 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 31 "test_debugger.cpp"
, "variadic" , v1, v2, v3 ); }

int main(int argc, char const *argv[]) {
    some();
}

Now the problem is when the macro template expands to something without any template parameters, i.e., template<> inline void some(), it generates the following code:

template<>
inline void some()
{
    fprintf(stderr, "variadic");
}

int main(int argc, char const *argv[])
{
    some();
}

Which causes C++ to compiler to throw this error:

test_debugger.cpp: error: ‘some’ is not a template function
 inline void some()
                  ^
test_debugger.cpp: In function ‘int main(int, const char**)’:
test_debugger.cpp: error: ‘some’ was not declared in this scope
     some();
     ^~~~

回答1:


A template can't have zero template parameters. The syntax that starts with template <> instead is used for an explicit specialization: a declaration to be used instead of the template for a specific set of template arguments.

So your zero-argument version will need to skip the template <> part. You might do something like:

#define TINYFORMAT_TEMPLATE_HEAD_0(...)
#define TINYFORMAT_TEMPLATE_HEAD_1(...) template < TINYFORMAT_ARGTYPES_1(__VA_ARGS__) >
#define TINYFORMAT_TEMPLATE_HEAD_2(...) template < TINYFORAMT_ARGTYPES_2(__VA_ARGS__) >
#define TINYFORMAT_TEMPLATE_HEAD_3(...) template < TINYFORMAT_ARGTYPES_3(__VA_ARGS__) >

#define TINYFORMAT_TEMPLATE_HEAD(n, ...) TINYFORMAT_TEMPLATE_HEAD_ ## n (__VA_ARGS__)

#define FACTORY(n,...) \
TINYFORMAT_TEMPLATE_HEAD(n,__VA_ARGS__) \
inline void some(TINYFORMAT_VARARGS(n,__VA_ARGS__)) \
{ \
    fprintf(stderr, "variadic" TINYFORMAT_PASSARGS(n,,) ); \
}



回答2:


Thanks to @aschepler help, I also managed to build this more general solution/example:

#include <stdio.h>

#define TINYFORMAT_ARGTYPES_0(...)
#define TINYFORMAT_ARGTYPES_1(begin,end,...) begin __VA_ARGS__ class T1 end
#define TINYFORMAT_ARGTYPES_2(begin,end,...) begin __VA_ARGS__ class T1, class T2 end
#define TINYFORMAT_ARGTYPES_3(begin,end,...) begin __VA_ARGS__ class T1, class T2, class T3 end

#define TINYFORMAT_VARARGS_0(...)
#define TINYFORMAT_VARARGS_1(...) __VA_ARGS__ const T1& v1
#define TINYFORMAT_VARARGS_2(...) __VA_ARGS__ const T1& v1, const T2& v2
#define TINYFORMAT_VARARGS_3(...) __VA_ARGS__ const T1& v1, const T2& v2, const T3& v3

#define TINYFORMAT_PASSARGS_0(...)
#define TINYFORMAT_PASSARGS_1(...) __VA_ARGS__ v1
#define TINYFORMAT_PASSARGS_2(...) __VA_ARGS__ v1, v2
#define TINYFORMAT_PASSARGS_3(...) __VA_ARGS__ v1, v2, v3

#define TINYFORMAT_ARGTYPES(n,begin,end,...) TINYFORMAT_ARGTYPES_ ## n (begin,end,__VA_ARGS__)
#define TINYFORMAT_VARARGS(n,...)  TINYFORMAT_VARARGS_  ## n (__VA_ARGS__)
#define TINYFORMAT_PASSARGS(n,...) TINYFORMAT_PASSARGS_ ## n (__VA_ARGS__)

#define TINYFORMAT_FOREACH_ARGNUM(m,...) \
        m(0) m(1,__VA_ARGS__) m(2,__VA_ARGS__) m(3,__VA_ARGS__)

#define FACTORY_FULLY_OPTIONAL(n,...) \
TINYFORMAT_ARGTYPES(n,template<,>,__VA_ARGS__) \
inline void some_optional(TINYFORMAT_VARARGS(n,__VA_ARGS__)) \
{ \
    fprintf( stderr, "some_optional\n" TINYFORMAT_PASSARGS(n,,) ); \
}

#define FACTORY_WITH_EXISTENT_ARGS(n,...) \
template<class T0 TINYFORMAT_ARGTYPES(n,,,__VA_ARGS__)> \
inline void some_existent(const T0& v0 TINYFORMAT_VARARGS(n,__VA_ARGS__)) \
{ \
    fprintf( stderr, "some_existent %s\n", v0 TINYFORMAT_PASSARGS(n,__VA_ARGS__) ); \
}

TINYFORMAT_FOREACH_ARGNUM(FACTORY_FULLY_OPTIONAL,)
TINYFORMAT_FOREACH_ARGNUM(FACTORY_WITH_EXISTENT_ARGS,,)

int main(int argc, char const *argv[])
{
    some_optional();
    some_existent( "varing" );
}

This is the generated code: g++ -o main -E -g -ggdb test_debugger.cpp --std=c++98

# 797 "/usr/include/stdio.h" 3 4
}
# 2 "test_debugger.cpp" 2
# 38 "test_debugger.cpp"

# 38 "test_debugger.cpp"
inline void some_optional() { fprintf(
# 38 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 38 "test_debugger.cpp"
, "some_optional\n" ); } template< class T1 > 
inline void some_optional( const T1& v1) { fprintf(
# 38 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 38 "test_debugger.cpp"
, "some_optional\n" , v1 ); } template< class T1, class T2 > 
inline void some_optional( const T1& v1, const T2& v2) { fprintf(
# 38 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 38 "test_debugger.cpp"
, "some_optional\n" , v1, v2 ); } template< class T1, class T2, class T3 > 
inline void some_optional( const T1& v1, const T2& v2, const T3& v3) { fprintf(
# 38 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 38 "test_debugger.cpp"
, "some_optional\n" , v1, v2, v3 ); }

template<class T0 > 
inline void some_existent(const T0& v0 ) { fprintf(
# 39 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 39 "test_debugger.cpp"
, "some_existent %s\n", v0 ); } template<class T0 , class T1 > 
inline void some_existent(const T0& v0 , const T1& v1) { fprintf(
# 39 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 39 "test_debugger.cpp"
, "some_existent %s\n", v0 , v1 ); } template<class T0 , class T1, class T2 > 
inline void some_existent(const T0& v0 , const T1& v1, const T2& v2) { fprintf(
# 39 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 39 "test_debugger.cpp"
, "some_existent %s\n", v0 , v1, v2 ); } template<class T0 , class T1, class T2, class T3 > 
inline void some_existent(const T0& v0 , const T1& v1, const T2& v2, const T3& v3) { fprintf(
# 39 "test_debugger.cpp" 3 4
((__getreent())->_stderr)
# 39 "test_debugger.cpp"
, "some_existent %s\n", v0 , v1, v2, v3 ); }

int main(int argc, char const *argv[])
{
    some_optional();
    some_existent("varing");
}

Running it: ./main

some_optional
some_existent varing

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来源：https://stackoverflow.com/questions/59331919/how-to-easily-create-fully-variadic-functions-with-c-98-standard

标签
macros
variadic-templates
variadic-functions
c++98
variadic-macros
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