Format to `char*` works for built-in types, but not user types

[wkenyon]

#include <fmt/format.h>

enum class ExampleEnum : char
{
    A = '0',
    B = '1',
    C = '3',
    D = '?'
};


template <>
struct fmt::formatter<ExampleEnum> : fmt::formatter<fmt::string_view>
{
    using base = fmt::formatter<fmt::string_view>;

    template <typename FormatContext>
    auto format(const ExampleEnum& e, FormatContext& ctx)
        {
            switch (e)
            {
            case ExampleEnum::A:
                return base::format("A", ctx);
            case ExampleEnum::B:
                return base::format("B", ctx);
            case ExampleEnum::C:
                return base::format("C", ctx);
            case ExampleEnum::D:
                return base::format("D", ctx);
            };
            return base::format("NOT_RECOGNISED", ctx);
        }
};

int main() {
    char buf[80];
    memset(buf, 0, sizeof(buf));
    fmt::format_to(buf, "1: {}\n", "A");
    fmt::print(buf);

    char buf2[80];
    memset(buf2, 0, sizeof(buf2));
    fmt::format_to(buf2, "2: {}\n", ExampleEnum::A);
    fmt::print(buf2);

    fmt::print("3: {}\n", ExampleEnum::A);
    return 0;
}

As you can see, I'm trying to use format_to to write to a raw pointer. I know that you support 'iterators'. But a raw pointer is an iterator so this should work right? It seems to work for built-in types, but not this user defined formatter that inherits from string_view, as suggested in #787. I tested this on the latest master.

$ clang++ --std=c++14 -isystem external/fmt/include/  FmtBug.cpp -L bazel-bin/third_party/ -lfmt_lib
$ ./a.out
1: A
2: 
3: A

[eliaskosunen]

Using format_to inside the format member function fixes this for me:

    template <typename FormatContext>
    auto format(const ExampleEnum& e, FormatContext& ctx)
        {
            switch (e)
            {
            case ExampleEnum::A:
                return format_to(ctx.begin(), "A");
            case ExampleEnum::B:
                return format_to(ctx.begin(), "B");
            case ExampleEnum::C:
                return format_to(ctx.begin(), "C");
            case ExampleEnum::D:
                return format_to(ctx.begin(), "D");
            };
            return format_to(ctx.begin(), "NOT_RECOGNISED");
        }

I'm not sure if this could cause some issues, though.

[wkenyon]

Yes, thank you, however, that isn't an option for me because I want padding and alignment to work like the formatter for string_view.

[wkenyon]

If I just use format_to as you suggest, then padding and alignment are ignored.

[vitaut]

Fixed in c04fb91. Thanks for reporting!

[wkenyon]

Thanks,

Very kind