I have a C++ class that is the frontend for a logging system. Its logging function is implemented using C++11's variadic templates:
template <typename... Args>
void Frontend::log(const char *fmt, Args&&... args) {
backend->true_log(fmt, std::forward<Args>(args)...);
}
Each logging backend implements its own version of true_log
, that, among other things, uses the forwarded parameters to call vsnprintf
. E.g.:
void Backend::true_log(const char *fmt, ...) {
// other stuff..
va_list ap;
va_start(ap, fmt);
vsnprintf(buffer, buffer_length, fmt, ap);
va_end(ap);
// other stuff..
}
Everything works great, and I am happy.
Now, I want to add a static check on the log()
parameters: specifically, I would like to use GCC's printf format attribute.
I started by tagging the log()
function with __attribute__ ((format (printf, 2, 3)))
(as this
is the first "hidden" parameter, I need to shift parameter indices by one). This does not work, because if fails with a compilation error:
error: args to be formatted is not ‘...’
Then, I tried to add the same attribute to the true_log()
function. It compiles, but no error checking is actually performed: I tried to pass to log()
some invalid format/variable combinations, and no warning was issued. Maybe this kind of check is "too late", or, in other words, the information about the variable has been lost in the chain of calls?
As a last resort, if I annotated log()
with __attribute__ ((format (printf, 2, 0)))
, I would receive warnings about wrong format strings, but no diagnostic would be issued for invalid format/variable combinations.
Summarizing the problem: how can I have full format checking from GCC if I use C++11's variadic templates?
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