Quelle fmt-in-gecko.md
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# `{fmt}` in Gecko
[`{fmt}`](https://fmt.dev/) is a library implementation of C++20's [`std::format`](https://en.cppreference.com/w/cpp/header/format) formatting API which enables type-informed string formatting. Unlike `printf` this format string style does not require specifying the types of each format parameter in the format string. For example, instead of:
```c++
#include <mozilla/Sprintf.h>
#include <inttypes.h>
// ...
char buf[1024];
int64_t a = 123;
uint64_t a = 456;
auto literal = "A literal"_ns;
mozilla::SprintfBuf(buf, 1024,
"Formatting a number: %" PRId64
" and another one: " PRIu64
", and finally a string: %s",
a, b, literal.get());
```
one can do:
```c++
#include <fmt/format.h>
// ...
char buf[1024];
int64_t a = 123;
uint64_t a = 456;
auto literal = "A literal"_ns;
fmt::format_to_n(res, 1024,
FMT_STRING("Formatting a number: {} and another one: {} "
"and finally a string: {}"),
a, b, literal.get());
```
# User-defined types
Formatting a [user-defined type](https://fmt.dev/11.0/api/#formatting-user-defined-types) can be done once, and then used with all sorts of formatting function in `{fmt}`. Given an example object:
```c++
struct POD {
double mA;
uint64_t mB;
};
```
one can write a custom formatter like so:
```c++
auto format_as(POD aInstance) -> std::string {
return fmt::format(FMT_STRING("POD: mA: {}, mB: {}"), aInstance.mA,
aInstance.mB);
}
```
and use it as expected in a variety of ways:
```c++
char bufFmt[1024] = {};
POD p{4.3, 8};
auto [out, size] = fmt::format_to(bufFmt, "{}", p);
*out = 0; // Write the null terminator
assert(!strcmp("POD: mA: 4.3, mB: 8", bufFmt));
fmt::println(FMT_STRING("### debug: {}"), p);
fmt::print(stderr, FMT_STRING("### debug to stderr {}\n"), p);
MOZ_LOG_FMT(gLogModule, "Important: {}", p);
```
# Formatting sequences
Containers that can work with with range-based for-loop can be formatted easily:
```c++
nsTArray<uint8_t> array(4);
for (uint32_t i = 0; i < 4; i++) {
array.AppendElement((123 * 5 * (i+1)) % 255);
}
auto [out, size] = fmt::format_to(bufFmt, FMT_STRING("{:#04x}"), fmt::join(array, ", "));
*out = 0; // Write the null terminator
ASSERT_STREQ("0x69, 0xd2, 0x3c, 0xa5", bufFmt);
```
# `MOZ_LOG` integration
`MOZ_LOG_FMT` is like `MOZ_LOG`, but takes an `{fmt}`-style format string:
```c++
MOZ_LOG_FMT(gLogModule, "{}x{} = {}", 3, 3, 3*3);
```
Unlike with `MOZ_LOG`, it is unnecessary to put an extra pair of parenthesis around the format and argument list.
# `ns*String` integration
It is possible to append an `{fmt}`-style format string to an `nsString` like so:
```c++
nsCString aLovelyString("Here is a value: ");
aLovelyString.AppendFmt(FMT_SRING("{}"), 4);
nsString aLovelyWideString(u"Here are two values: ");
aLovelyString.AppendFmt(FMT_SRING(u"{}, {}"), 4, u"wide");
```
Or directly use `nsFmt[C]String`:
```c++
nsFmtCString str(FMT_STRING("{},{},{},{}"), 1, 1, 2, 3);
nsFmtString str(FMT_STRING(u"{},{},{},{}"), 1, 1, 2, u"wide string");
// use it as usual
```
# Firefox Profiler marker integration
```
PROFILER_MARKER_FMT("Marker Name", MEDIA_PLAYBACK, {},
"Three numbers: {} {} {}", 1, 2, 3);
```
Crucially, the formatting isn't executed if markers aren't being collected (e.g.
the profiler isn't running).
# Useful links
- The syntax of `{fmt}` format string: <https://fmt.dev/latest/syntax/>
- The complete API of the library: <https://fmt.dev/latest/api/>
[ Dauer der Verarbeitung: 0.21 Sekunden
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2026-03-28
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