Compile-time error for incorrect usage.

[Victor-Savu]

At the moment, the formatting crashes when the format string contains more placeholders than the number of arguments passed to the formatting operator():

std::cout << "Hello %!\n"_fmt();

However, it is perfectly possible to test at compile time if a given list of parameters would cause problems:

constexpr auto f = "Hello, %! How is % feeling today?"_fmt;

constexpr auto cnt_1 = f.check("Paul", "Chani");
static_assert(cnt_1.first == cnt.second,
             "Wrong number of arguments."); // Ok!

constexpr auto cnt_2 = f.check("Paul");
static_assert(cnt_2.first == cnt_2.second,
             "Wrong number of arguments."); // Compile-time ERROR!

The check() method returns a pair of numbers, one representing the number of placeholders '%' in the format string and another representing the number of parameters passed. Both are computed in a constexpr context.

Unfortunately, the static_assert cannot be performed inside the formatting operator() of the format string due to the c++14 rules for constexpr. Within the member operator, the object cannot be aware of its own "constexpr"-ness, so it is unable to compute the number of placeholders '%' in a constexpr context.

[Victor-Savu]

maybe c++17's if constexpr could help? Need to investigat further.