Specializing std::formatter not working?

Runtimes C++
1

It seems like this code compiles with libstdc++, but not libc++. I got it (with minor changes) from the book C++20 The Complete Guide, by Josuttis. (godbolt code link)

Is this supposed to work?

struct Foo { 
    int value = 123;
}; 

#include <format> 
#include <iostream> 

template<> 
struct std::formatter<Foo> 
{ 
  constexpr auto parse(std::format_parse_context& ctx) { 
    return ctx.begin(); 
  } 
  auto format(const Foo& obj, std::format_context& ctx) const { 
    return std::format_to(ctx.out(), "{}", obj.value); 
  } 
}; 

int main() {
    std::cout << std::format("{}\n", Foo());
}
2

At first sight this looks like a libc++ bug. I don’t have time to look at it today, but I’ll try to look at it tomorrow.

3

I’ve verified it and libc++ is indeed wrong, however I’m not convinced whether this is worth fixing:

  • C++20 has a textual requirement for what a proper std::formatter specialization is.
  • C++23 ranges has a formattable concept.

Your formatter satisfies the C++20 requirements, but not the C++23 concept as implemented by libstdc++ and libc++. (Compiler Explorer) So when you want to format a std::vector<Foo> it correctly fails on libc++. (On libstdc++ it fails, because they don’t have a formatter for std::vector yet.)

The easiest fix for you would be to use auto format(const Foo& obj, auto& ctx) const as format member function.

I will give this some more thoughts, to see what the best solution is. (There is a bit of an issue with the concept, so your formatter might be formattable on MSVC STL if they make different design choices. In fact it’s possible to create a formatter that’s formattable for libc++ but not for libstdc++.)

1 Like
4

Thank you. I’m not far enough along teaching myself C++ to understand why it’s failing and have an opinion on this, but your quick fix lets me get started making formatters for my own simple types.

5

It’s failing since formatters are expected to work with different format_context’s. So creating a formatter only for std::format_context is not very generic. The context depends the type of output iterator used.

Have fun learning C++!

6

I was having the same problem. Looked at how libc++ was implemented internally and found they use
auto instead of std::format_context in the format function. Make that change and the specialization works.

template<> 
struct std::formatter<Foo> 
{ 
  constexpr auto parse(std::format_parse_context& ctx) { 
    return ctx.begin(); 
  } 

  auto format(const Foo& obj, auto& ctx) const { // Change 'std::format_context' to 'auto'
    return std::format_to(ctx.out(), "{}", obj.value); 
  } 
};
7

Thanks, this is indeed the way to fix it. Did you also read Josuttis’ book?

8

Not C++20, but I am the proud owner of another one he co-authored - C++ Templates.