如何轻松地创建完全“可变"的符合C ++ 98标准的功能? [英] How to easily create fully "variadic" functions with C++ 98 standard?
本文介绍了如何轻松地创建完全“可变"的符合C ++ 98标准的功能?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
图书馆 https://github.com/c42f/tinyformat/blob/2f9335afd9941688e42d60cae5166b9f0600b2d1/tinyformat.h#L1104-L1116 ，使用这个很棒的技巧在C ++ 98上做可变"模板:
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'; \
}
我正在尝试通过消除重复两次复制功能 printfln 的要求(即对于基本情况 inline void printfln(const char* fmt) 一次)和一个第二次变体"部分 template<TINYFORMAT_ARGTYPES(n)> void printfln(const char* fmt, TINYFORMAT_VARARGS(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)).
由于"variadic"功能只能接受一个参数，即 printfln("something") ，因此必须将 printfln 函数分为两部分.在这种情况下， TINYFORMAT_VARARGS(n) 不能扩展为任何内容，但是这样做时，它将导致代码后缀逗号为 , ，从而导致C ++上的语法无效.
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++.
我可以将 GNU GCC 扩展技巧与C宏令牌粘贴操作符 ## 一起使用，以删除结尾的逗号，但是，此操作不可移植，因为它仅适用于 GNU GCC .然后，我的目标是定义已经包含前导逗号的宏作为下一个示例:
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" );
}
但是，它不起作用.如果在宏定义之后紧随其后找到 comma ， gcc 编译器就会发疯: g++ -o main -g -ggdb test_debugger.cpp --std=c++98 && ./main
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" );
^
据我了解，我的 TINYFORMAT_FOREACH_ARGNUM(m) m(0) m(1) m(2) m(3) 宏应创建以下4个有效的C ++可变"函数:
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);
}
为什么 gcc 预处理器无法正确生成我的4个可变"模板函数，如上所述?
Why the gcc preprocessor is not generating correctly my 4 "variadic" template functions as above?
供参考，我正在使用:
$ 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.
更新
g++ -o main -E -g -ggdb test_debugger.cpp --std=c++98
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" );
}
更新2
我试图更改定义的顺序，希望C预处理器可以正确扩展内容，但是它只是像以前一样扩展:
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" );
}
更新3
正如 @aschepler 所评论的那样，我缺少了 TINYFORMAT_ARGTYPES_ ## n 的定义，这是一个固定版本:
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();
}
展开至: g++ -o main -E -g -ggdb test_debugger.cpp --std=c++98
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();
}
现在的问题是，当宏模板扩展为没有任何模板参数的东西时，即 template<> inline void some() ，它会生成以下代码:
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();
}
这会导致C ++编译器抛出此错误:
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();
^~~~
推荐答案
模板不能包含零个模板参数.相反，以template <>开头的语法用于显式专业化:声明用于代替特定模板参数集的模板.
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.
因此，您的零参数版本将需要跳过template <>部分.您可能会执行以下操作:
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,,) ); \
}
这篇关于如何轻松地创建完全“可变"的符合C ++ 98标准的功能?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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