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add support for positional arguments as width and precision specifiers #4643

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Merged
vitaut merged 2 commits into from
Jan 5, 2026
Merged

add support for positional arguments as width and precision specifiers #4643

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90ad0fc
make positional arguments work as width and precision specifiers with…
KareemOtoum Jan 1, 2026
debde0a
fix test case failing ci tests
KareemOtoum Jan 2, 2026
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34 changes: 30 additions & 4 deletions include/fmt/printf.h
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Original file line number Diff line number Diff line change
Expand Up @@ -357,8 +357,21 @@ auto parse_header(const Char*& it, const Char* end, format_specs& specs,
if (specs.width == -1) report_error("number is too big");
} else if (*it == '*') {
++it;
specs.width = static_cast<int>(
get_arg(-1).visit(detail::printf_width_handler(specs)));
// Check for positional width argument like *1$
if (it != end && *it >= '0' && *it <= '9') {
int width_index = parse_nonnegative_int(it, end, -1);
if (it != end && *it == '$') {
++it;
specs.width = static_cast<int>(
get_arg(width_index).visit(detail::printf_width_handler(specs)));
} else {
// Invalid format, rewind and treat as non-positional
report_error("invalid format specifier");
}
} else {
specs.width = static_cast<int>(
get_arg(-1).visit(detail::printf_width_handler(specs)));
}
}
}
return arg_index;
Expand Down Expand Up @@ -439,8 +452,21 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
specs.precision = parse_nonnegative_int(it, end, 0);
} else if (c == '*') {
++it;
specs.precision =
static_cast<int>(get_arg(-1).visit(printf_precision_handler()));
// Check for positional precision argument like .*1$
if (it != end && *it >= '0' && *it <= '9') {
int precision_index = parse_nonnegative_int(it, end, -1);
if (it != end && *it == '$') {
++it;
specs.precision = static_cast<int>(
get_arg(precision_index).visit(printf_precision_handler()));
} else {
// Invalid format, rewind and treat as non-positional
report_error("invalid format specifier");
}
} else {
specs.precision =
static_cast<int>(get_arg(-1).visit(printf_precision_handler()));
}
} else {
specs.precision = 0;
}
Expand Down
29 changes: 29 additions & 0 deletions test/printf-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -296,6 +296,35 @@ TEST(printf_test, dynamic_precision) {
}
}

TEST(printf_test, positional_width) {
EXPECT_EQ(" 42", test_sprintf("%2$*1$d", 5, 42));
EXPECT_EQ("42 ", test_sprintf("%2$*1$d", -5, 42));
EXPECT_EQ(" abc", test_sprintf("%2$*1$s", 5, "abc"));
EXPECT_THROW_MSG(test_sprintf("%2$*1$d", 5.0, 42), format_error,
"width is not integer");
EXPECT_THROW_MSG(test_sprintf("%2$*1$d"), format_error, "argument not found");
EXPECT_THROW_MSG(test_sprintf("%2$*1$d", big_num, 42), format_error,
"number is too big");
}

TEST(printf_test, positional_precision) {
EXPECT_EQ("00042", test_sprintf("%2$.*1$d", 5, 42));
EXPECT_EQ("42", test_sprintf("%2$.*1$d", -5, 42));
EXPECT_EQ("Hell", test_sprintf("%2$.*1$s", 4, "Hello"));
EXPECT_THROW_MSG(test_sprintf("%2$.*1$d", 5.0, 42), format_error,
"precision is not integer");
EXPECT_THROW_MSG(test_sprintf("%2$.*1$d"), format_error, "argument not found");
EXPECT_THROW_MSG(test_sprintf("%2$.*1$d", big_num, 42), format_error,
"number is too big");
}

TEST(printf_test, positional_width_and_precision) {
EXPECT_EQ(" 00042", test_sprintf("%3$*1$.*2$d", 7, 5, 42));
EXPECT_EQ(" ab", test_sprintf("%3$*1$.*2$s", 7, 2, "abcdef"));
EXPECT_EQ(" 00042", test_sprintf("%3$*1$.*2$x", 7, 5, 0x42));
EXPECT_EQ("100.4400000", test_sprintf("%6$-*5$.*4$f%3$s%2$s%1$s", "", "", "", 7, 4, 100.44));
}

template <typename T> struct make_signed {
using type = T;
};
Expand Down