Is there a way to use fmt with a spec that is computed at runtime.
Noticed that with gcc 10 this code worked fine but not with modern gcc.
#include <fmt/format.h>
const char* get_spec(bool test)
{ // image this is implemented in some other .cpp file
return test ? "->{}" : "{}<-";
}
int main() {
const char* non_constexpr = get_spec(true);
fmt::print(non_constexpr,1);
}
You need to use fmt::runtime():
#include <fmt/format.h>
const char* get_spec(bool test)
{ // image this is implemented in some other .cpp file
return test ? "->{}" : "{}<-";
}
int main() {
const char* non_constexpr = get_spec(true);
fmt::print(fmt::runtime(non_constexpr),1);
}
Like always, gcc has bad error descriptions. The problem is that the normal constructor for format patterns is consteval in C++20, so you cannot pass a runtime format to it.
For that purpose, just use fmt::runtime() around your runtime format pattern.
In case of C++20's std::format problem, you need to use std::vformat to resolve the problem as mentioned in cpp ref.
You can use fmt::vprint (std::vprint_unicode in C++23) for runtime format string
const char* non_constexpr = get_spec(true);
int arg = 1;
fmt::vprint(non_constexpr, fmt::make_format_args(arg));
