Fmt: Question: How to elegantly format vector of integer

[prehistoric-penguin]

I have read through the doc, but have found an elegant solution.

Input:

std::vector vec{1, 2, 16};

Expected output:

"0x1 0x2 0x10"

I think it may be achieved by combine join and format, but it seems that the current API doesn't support this kind of usage, thanks!

[OnurKader]

Not really elegant but this works, doesn't do uppercase formatting eg. 0XA1, but I think
it can be achieved with an enum + switch

#include <fmt/format.h>
#include <vector>

template<>
struct fmt::formatter<std::vector<int>>
{
    bool hex {false};

    constexpr auto parse(format_parse_context& ctx)
    {
        auto it = ctx.begin();
        const auto end = ctx.end();

        if(it != end && *it == 'x')
        {
            hex = true;
            ++it;
        }

        if(it != end && *it != '}')
            throw format_error("invalid format");

        return it;
    }

    template<typename FormatContext>
    auto format(const std::vector<int>& vec, FormatContext& fc)
    {
        format_to(fc.out(), "[");
        for(auto it = vec.cbegin(); it != vec.cend() - 1; ++it)
        {
            format_to(fc.out(), hex ? "{:#x}, " : "{}, ", *it);
        }
        return format_to(fc.out(), hex ? "{:#x}]" : "{}]", vec.back());
    }
};

int main()
{
    std::vector nums {18, 47, 128, 10};
    fmt::print("{}\n", nums);
    fmt::print("{:x}\n", nums);
}

Output:

[18, 47, 128, 10]
[0x12, 0x2f, 0x80, 0xa]

The int may be templated in the struct definition.

Also replacing the ternary with an if-else might look better

[vitaut]

It can be easily done using join:

#include <vector>
#include <fmt/format.h>

int main () {
  std::vector vec{1, 2, 16};
  fmt::print("{:#x}", fmt::join(vec, " "));
}

Output:

0x1 0x2 0x10

Godbolt: https://godbolt.org/z/7Y3xs5