Variadic template with printf argument checking - c++

Question

I have a printf-style function that takes a variable number of arguments. Here is my starting point:
#include <stdio.h>
#include <stdarg.h>
void MyPrint (const char* fmt,...)
{
va_list arglist ;
va_start (arglist, fmt) ;
vprintf (fmt, arglist) ;
va_end (arglist) ;
}
int main()
{
MyPrint ("Hello, %s\n", "world") ;
}
This prints Hello, world as expected.
Now I want to make two changes. First, I want to check the format string using the format attribute of g++. So I declare the MyPrint function first (I have to declare it first, because for some reason g++ doesn't let you assign attributes to a function definition):
void MyPrint (const char* fmt,...) __attribute__ ((format (printf, 1, 2))) ;
Now if I try e.g. MyPrint ("Hello, %d\n", "world") ; I get a nice error message.
The second change I want to make is to use a variadic template parameter. Like this:
#include <utility> // for std::forward
template<typename...Params>
void MyPrint (Params&&... fmt)
{
printf (std::forward<Params> (fmt)...) ;
}
This works too. So I combine the two, by adding the format-checking attribute to the variadic function template with this forward declaration:
template<typename...Params>
void MyPrint (Params&&... fmt) __attribute__ ((format (printf, 1, 2))) ;
But now I get this error message (gcc 10.2):
<source>: In substitution of 'template<class ... Params> void MyPrint(Params&& ...) [with Params = {const char (&)[11], const char (&)[6]}]':
<source>:15:38: required from here
<source>:8:6: error:
'format' attribute argument 2 value '1' refers to parameter type
'const char (&)[11]'
This has got me completely baffled. Can anybody tell me what I'm doing wrong?
Here is the complete program:
#include <stdio.h>
#include <utility> // for std::forward
template<typename...Params>
void MyPrint (Params&&... fmt) __attribute__ ((format (printf, 1, 2))) ;
template<typename...Params>
void MyPrint (Params&&... fmt) // <-- Line 8
{
printf (std::forward<Params> (fmt)...) ;
}
int main()
{
MyPrint ("Hello, %s\n", "world") ; // <-- Line 15
}

Answer 1

You can make the first error go away by adding a fixed const char * argument as the format string and pointing the attribute to that.
template<typename...Params>
void MyPrint (const char * format, Params&&... fmt) __attribute__ ((format (printf, 1, 2))) ;
template<typename...Params>
void MyPrint (const char * format, Params&&... fmt) // <-- Line 9
{
printf (format, std::forward<Params> (fmt)...) ;
}
Which reveals another error:
test.cc:8:6: error: ‘format’ attribute argument 3 value ‘2’ does not refer to a variable argument list
8 | void MyPrint (const char * format, Params&&... fmt) // <-- Line 9
| ^~~~~~~
It seems that the attribute for checking the printf archetype relies on one const char * argument and a variable argument list and is not willing to work without them. So you have to give up either the C++ template magic or the compile-time format string checking.