std::print(std::ostream)
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< cpp | io | basic ostream
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std::basic_ostream
Member functions Formatted output Unformatted output Positioning Miscellaneous Member classes Non-member functions
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print(std::ostream)
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Defined in header
<ostream>
template< class... Args >
void print( std::ostream & os, std::format_string <Args...> fmt, Args&&... args );
(since C++23)
void print( std::ostream & os, std::format_string <Args...> fmt, Args&&... args );
Formats args according to the format string fmt, and inserts the result into os stream.
If the ordinary literal encoding is UTF-8, equivalent to:
- std::vprint_unicode (os, fmt.get(), std::make_format_args (args...));. Otherwise,
- std::vprint_nonunicode (os, fmt.get(), std::make_format_args (args...));.
The behavior is undefined if std::formatter <Ti, char> does not meet the BasicFormatter requirements for any Ti in Args (as required by std::make_format_args ).
Contents
[edit] Parameters
os
-
output stream to insert data into
fmt
-
an object that represents the format string. The format string consists of
args...
-
arguments to be formatted
outputs formatted representation of the arguments with appended '\n'
(function template) [edit]
- ordinary characters (except { and }), which are copied unchanged to the output,
- escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
- replacement fields.
Each replacement field has the following format:
{ arg-id (optional) }
(1)
{ arg-id (optional) : format-spec }
(2)
1) replacement field without a format specification
2) replacement field with a format specification
arg-id
-
specifies the index of the argument in
args whose value is to be used for formatting; if it is omitted, the arguments are used in order.
The arg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.
format-spec
-
the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.
- For basic types and standard string types, the format specification is interpreted as standard format specification.
- For chrono types, the format specification is interpreted as chrono format specification.
- For range types, the format specification is interpreted as range format specification.
- For std::pair and std::tuple , the format specification is interpreted as tuple format specification.
- For std::thread::id and std::stacktrace_entry, see thread id format specification and stacktrace entry format specification.
- For std::basic_stacktrace, no format specifier is allowed.
(since C++26)
- For other formattable types, the format specification is determined by user-defined
formatterspecializations.
[edit] Exceptions
- std::bad_alloc on allocation failure.
- Propagate any exception thrown by any formatter, e.g. std::format_error , without regard to the value of os.exceptions() and without turning on ios_base::badbit in the error state of os.
- May throw ios_base::failure caused by os.setstate(ios_base::badbit) which is called if an insertion into os fails.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_print |
202207L |
(C++23) | Formatted output |
__cpp_lib_format |
202207L |
(C++23) | Exposing std::basic_format_string |
[edit] Example
Run this code
#include <array> #include <cctype> #include <cstdio> #include <format> #include <numbers> #include <ranges> #include <sstream> int main() { std::array <char, 24> buf; std::format_to (buf.begin(), "{:.15f}", std::numbers::sqrt2 ); unsigned num{}, sum{}; for (auto n : buf | std::views::filter (isdigit) | std::views::transform ([](char x) { return x - '0'; }) | std::views::take_while ([&sum](char) { return sum < 42; })) sum += n, ++num; std::stringstream stream; #ifdef __cpp_lib_print std::print (stream, #else stream << std::format ( #endif "√2 \N{ALMOST EQUAL TO} {0}.\n" "The sum of its first {1} digits is {2}.", std::numbers::sqrt2, num, sum ); std::puts (stream.str().data()); }
Output:
√2 ≈ 1.4142135623730951. The sum of its first 13 digits is 42.
[edit] See also
(C++23)
(function template) [edit]