Parsing strings at compile-time - if possible - c++11

Question

I am trying to make compile time checks over the format of the printf. Here is the code.
#include <type_traits>
#include <iostream>
template <typename CHAR, typename ...ARGS>
constexpr size_t trace_cond(CHAR fmt, ARGS&&... args) {
//always needs to pass
return 1;
}
template <size_t N, typename ...ARGS>
constexpr size_t trace_cond(const char (&fmt)[N], ARGS&&... args) {
//return parse(fmt) == args;
return sizeof...(args) != 0;
}
#define TRACE(fmt, ...) { \
static_assert(trace_cond(fmt, __VA_ARGS__), "Wrong ARGS"); \
printf(fmt, ##__VA_ARGS__); \
}
int main(int argc, char* argv[]) {
//working fine
TRACE("%d %d\n", 2, 3);
const char* format = "%d %d\n";
//error
TRACE(format, 2, 3);
}
So when the format is known at compile time I want to have a check(using static_assert) and if it is not known then the check should be not called or always to pass
Obs:
Currently the code is not compiling because const char *format is not declared constexpr
The main should not be change because TRACE macro is used in a large codebase but changes to TRACE are more than welcome
So my question is:
Is there a way to skip static_assert or make it pass when fmt type is const char *

Answer 1

I think you can do what you want by moving the static_assert to the trace_cond
#include <type_traits>
#include <iostream>
template <size_t C, typename CHAR>
size_t trace_cond(CHAR fmt) {
//always needs to pass
return 1;
}
template <typename ...ARGS>
constexpr size_t count_args(ARGS&&... args) {
return sizeof...(args);
}
template <size_t C, size_t N>
constexpr size_t trace_cond(const char (&fmt)[N]) {
// TODO, parse fmt and do the needed checks
static_assert(C == 3, "");
return 1;
}
#define TRACE(fmt, ...) { \
trace_cond<count_args(__VA_ARGS__)>(fmt); \
printf(fmt, ##__VA_ARGS__); \
}
int main(int argc, char* argv[]) {
// Static check - hard coded to check for 3 arguments
TRACE("%d %d %d\n", 2, 3, 4);
const char* format = "%d %d\n";
// No checks performed
TRACE(format, 2, 3);
}