The C++26 standard features a series of improvements to the format library. In this article, we will look at the most important of them.<\/p>\n\n\n\n
Prior to C++26, printing an empty line had to be done like this:<\/p>\n\n\n\n
std::print(\"\\n\");<\/pre>\n\n\n\nIn C++26,
std::println<\/code> has an overload without any parameters that prints a new line to the console.<\/p>\n\n\n\nstd::println();<\/pre>\n\n\n\nFormatting pointers<\/h2>\n\n\n\n
Formatting pointer types was not available directly, it required a hack: reinterpreting the pointer type as an integer type in order to print it.<\/p>\n\n\n\n
int i = 0;\nconst void* p = &i;\n\nstd::println(\"{:#018x}\", reinterpret_cast<uintptr_t>(p));<\/pre>\n\n\n\nIn C++26, the formatting library supports formatting of pointer types directly:<\/p>\n\n\n\n
\n
- implicitly, no specifier is required<\/li>\n\n\n\n
- explicitly, with wither the
p<\/code> (for lowercase) orP<\/code> (for uppercase) specifiers<\/li>\n<\/ul>\n\n\n\nNull pointers are formatted as
0x0<\/code> (unless padding is specified).<\/p>\n\n\n\nHere are several examples of formatting pointers:<\/p>\n\n\n\n
int i = 0;\nconst void* p = &i;\n \nstd::println(\"{}\", p); \/\/ lowercase, the default => 0x7fffb2715a54\nstd::println(\"{:p}\", p); \/\/ explicit, same as none => 0x7fffb2715a54\nstd::println(\"{:P}\", p); \/\/ uppercase => 0X7FFFB2715A54\nstd::println(\"{:018}\", p); \/\/ zero-padded to width 18 => 0x00007fffb2715a54\nstd::println(\"{:>20}\", p); \/\/ right-aligned in a 20-wide field => 0x7fffb2715a54\nstd::println(\"{}\", nullptr); \/\/ => 0x0\nstd::println(\"{:016}\", nullptr); \/\/ => 0x00000000000000<\/pre>\n\n\n\nFormatting paths<\/h2>\n\n\n\n
Another feature that required a workaround was printing paths from the
std::filesystem<\/code> namespace. You could usepath::string()<\/code> to get the string representation of a path.<\/p>\n\n\n\nnamespace fs = std::filesystem;\n\nfs::path p = \"\/usr\/local\/bin\/clang++\";\nstd::println(\"{}\", p.string());<\/pre>\n\n\n\nHowever, this prints the path unquoted. On the other hand, using the
<<<\/code> operator would print the path in quotes:<\/p>\n\n\n\nstd::cout << p << '\\n';<\/pre>\n\n\n\nC++26 adds a
std::formatter<\/code> forstd::filesystem::path<\/code>, which makes it easier to format paths.<\/p>\n\n\n\n\n
- by default, paths are formatted unquoted<\/li>\n\n\n\n
- the
?<\/code> option defines a debug form which gives an escaped representation (in quotes)<\/li>\n\n\n\n- the
g<\/code> option forces generic (forward-slash) separators (which mainly shows up on Windows)<\/li>\n<\/ul>\n\n\n\nfs::path p = \"\/usr\/local\/bin\/clang++\";\nstd::println(\"{}\", p); \/\/ \/usr\/local\/bin\/clang++\nstd::println(\"{:?}\", p); \/\/ \"\/usr\/local\/bin\/clang++\"<\/pre>\n\n\n\nfs::path p = R\"(C:\\Users\\marius\\file.txt)\";\nstd::println(\"{}\", w); \/\/ C:\\Users\\marius\\file.txt (native separators)\nstd::println(\"{:g}\", w); \/\/ C:\/Users\/marius\/file.txt (generic)\nstd::println(\"{:g?}\", w); \/\/ \"C:\/Users\/marius\/file.txt\" (generic + escaped)<\/pre>\n\n\n\nA related issue solved along was Windows string representation of paths.
std::filesystem::path<\/code> stores its text inwchar_t<\/code> encoded as UTF-16 (Windows native). Butp.string()<\/code> narrows it down to the active code page, rather than UTF-8 which is what the formatting library expects. The result was a non-ASCII path could get transcoded to gibberish. The C++26std::formatter<std::filesystem::path><\/code> converts Windows native UTC-16 to UTF-8 using Unicode transcoding and avoiding code pages, therefore solving the problem. Ill-formed UTF-16 is replaced withU+FFFD<\/code> by default, or escaped under{:?}<\/code>.<\/p>\n\n\n\nconstexpr std::format<\/h2>\n\n\n\n
In C++26, the formatting functions
std::format<\/code>,std::vformat<\/code>,std::format_to<\/code>,std::format_to_n<\/code>,std::formatted_size<\/code>, and their wide variants, plus the underlying pieces (the format context,std::basic_format_arg<\/code>,std::basic_format_string<\/code>, and the format member of the standard formatters) areconstexpr<\/code>.<\/p>\n\n\n\nThis makes it possible to use
static_assert<\/code> for instance withstd::format<\/code> such as in the following examples:<\/p>\n\n\n\nstatic_assert(std::format(\"{} {}\", 1, 2) == \"1 2\");\n\nstatic_assert(sizeof(void*) == 8,\n std::format(\"expected 64-bit, pointer is {} bytes\", sizeof(void*)));<\/pre>\n\n\n\nThis works because it relies on
to_chars()<\/code> overloads which have been madeconstexpr<\/code>, but only for integral types. So it can be used with strings, integer types,bool<\/code>,char<\/code>, andpointers<\/code>. But there are several limitations to this feature. The following are not supported:<\/p>\n\n\n\n\n
- floating-point types<\/li>\n\n\n\n
- chrono types<\/li>\n\n\n\n
- locale-aware formatting (using the
L<\/code> specifier – as in{:L}<\/code>, makes the call non-constant)<\/li>\n<\/ul>\n\n\n\nFor now, compile-time
std::format<\/code> covers integers, strings, and diagnostics well, with floating-point support waiting on a separate paper (P3652) to make the floating-point<charconv><\/code> functionsconstexpr<\/code>.<\/p>\n\n\n\nstd::runtime_format becomes std::dynamic_format<\/h2>\n\n\n\n
The format string of
std::format<\/code> orstd::print<\/code> must be a constant expression. For instance, you can write this:<\/p>\n\n\n\nstd::println(\"{} = {}\", \"x\", 13);<\/pre>\n\n\n\nBut you cannot write the following:<\/p>\n\n\n\n
std::string strf = \"{} = {}\";\nstd::println(strf, \"x\", 13);<\/pre>\n\n\n\nThis is ill-formed because
strf<\/code> is only known at runtime, and therefore is not a constant expression. The workaround is to usestd::vformat<\/code>:<\/p>\n\n\n\nconst char* key = \"y\";\nint val = 13;\nstd::string strf = \"{} = {}\";\n\nstd::string s = std::vformat(strf, std::make_format_args(key, val));\nstd::println(\"{}\", s);<\/pre>\n\n\n\nThere is a workaround for the workaround (it’s basically syntactic sugar for
std::vformat<\/code>), the formally known asstd::runtime_format<\/code> function, that returns an object that stores a dynamic format string directly usable in user-oriented formatting functions and can be implicitly converted tostd::basic_format_string<\/code>.<\/p>\n\n\n\nstd::string strf = \"{} = {}\";\nstd::println(std::runtime_format(strf), \"x\", 13);<\/pre>\n\n\n\nIn the final version of C++26, this has been simply renamed to
std::dynamic_format<\/code> (although at this time the compilers that do support it still use the previous name). Therefore, the snippet above now becomes:<\/p>\n\n\n\nstd::string strf = \"{} = {}\";\nstd::println(std::dynamic_format(strf), \"x\", 13);<\/pre>\n\n\n\n
std::dynamic_format<\/code> is constexpr, which means it can be used in constant-evaluation contexts, such as in the following example:<\/p>\n\n\n\nconstexpr auto make_string = [](std::string_view f) {\n return std::format(std::dynamic_format(f), 1, 2);\n};\nstatic_assert(make_string(\"{}+{}\") == \"1+2\");<\/pre>\n\n\n\nSee more<\/h2>\n\n\n\n
You can learn more about these changes from the following articles:<\/p>\n\n\n\n
\n
- C++26: std::format improvement (Part 1)<\/a><\/li>\n\n\n\n
- C++26: std::format improvements (Part 2)<\/a><\/li>\n\n\n\n
constexpr std::format<\/a><\/code><\/li>\n<\/ul>\n\n\n\n<\/p>\n","protected":false},"excerpt":{"rendered":"
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