31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.6 Output streams [output.streams]

31.7.6.1 General [output.streams.general]

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The header <ostream> defines a class template and several function templates that control output to a stream buffer, along with a function template that inserts into stream rvalues.

31.7.6.2 Class template basic_ostream [ostream]

31.7.6.2.1 General [ostream.general]

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When a function has a parameter type extended-floating-point-type, the implementation provides overloads for all cv-unqualified extended floating-point types ([basic.fundamental]).
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namespace std { template<class charT, class traits = char_traits<charT>> class basic_ostream : virtual public basic_ios<charT, traits> { public: // types (inherited from basic_ios) using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [ostream.cons], constructor/destructor explicit basic_ostream(basic_streambuf<char_type, traits>* sb); virtual ~basic_ostream(); // [ostream.sentry], prefix/suffix class sentry; // [ostream.formatted], formatted output basic_ostream& operator<<(basic_ostream& (*pf)(basic_ostream&)); basic_ostream& operator<<(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&)); basic_ostream& operator<<(ios_base& (*pf)(ios_base&)); basic_ostream& operator<<(bool n); basic_ostream& operator<<(short n); basic_ostream& operator<<(unsigned short n); basic_ostream& operator<<(int n); basic_ostream& operator<<(unsigned int n); basic_ostream& operator<<(long n); basic_ostream& operator<<(unsigned long n); basic_ostream& operator<<(long long n); basic_ostream& operator<<(unsigned long long n); basic_ostream& operator<<(float f); basic_ostream& operator<<(double f); basic_ostream& operator<<(long double f); basic_ostream& operator<<(extended-floating-point-type f); basic_ostream& operator<<(const void* p); basic_ostream& operator<<(const volatile void* p); basic_ostream& operator<<(nullptr_t); basic_ostream& operator<<(basic_streambuf<char_type, traits>* sb); // [ostream.unformatted], unformatted output basic_ostream& put(char_type c); basic_ostream& write(const char_type* s, streamsize n); basic_ostream& flush(); // [ostream.seeks], seeks pos_type tellp(); basic_ostream& seekp(pos_type); basic_ostream& seekp(off_type, ios_base::seekdir); protected: // [ostream.cons], copy/move constructor basic_ostream(const basic_ostream&) = delete; basic_ostream(basic_ostream&& rhs); // [ostream.assign], assignment and swap basic_ostream& operator=(const basic_ostream&) = delete; basic_ostream& operator=(basic_ostream&& rhs); void swap(basic_ostream& rhs); }; // [ostream.inserters.character], character inserters template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, charT); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, signed char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, unsigned char); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, wchar_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char8_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char16_t) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, char32_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char8_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char16_t) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, char32_t) = delete; template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, const charT*); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>&, const char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const signed char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const unsigned char*); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const wchar_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char8_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char16_t*) = delete; template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>&, const char32_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char8_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char16_t*) = delete; template<class traits> basic_ostream<wchar_t, traits>& operator<<(basic_ostream<wchar_t, traits>&, const char32_t*) = delete; }
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The class template basic_ostream defines a number of member function signatures that assist in formatting and writing output to output sequences controlled by a stream buffer.
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Two groups of member function signatures share common properties: the formatted output functions (or inserters) and the unformatted output functions. Both groups of output functions generate (or insert) output characters by actions equivalent to calling rdbuf()->sputc(int_type).
They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().
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If one of these called functions throws an exception, then unless explicitly noted otherwise the output function sets badbit in the error state.
If badbit is set in exceptions(), the output function rethrows the exception without completing its actions, otherwise it does not throw anything and proceeds as if the called function had returned a failure indication.
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[Note 1: 
The deleted overloads of operator<< prevent formatting characters as integers and strings as pointers.
— end note]

31.7.6.2.2 Constructors [ostream.cons]

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explicit basic_ostream(basic_streambuf<charT, traits>* sb);
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Effects: Initializes the base class subobject with basic_ios<charT, traits>​::​init(sb) ([basic.ios.cons]).
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Postconditions: rdbuf() == sb.
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basic_ostream(basic_ostream&& rhs);
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Effects: Move constructs from the rvalue rhs.
This is accomplished by default constructing the base class and calling basic_ios<charT, traits>​::​move(rhs) to initialize the base class.
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virtual ~basic_ostream();
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Remarks: Does not perform any operations on rdbuf().

31.7.6.2.3 Assignment and swap [ostream.assign]

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basic_ostream& operator=(basic_ostream&& rhs);
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Effects: Equivalent to: swap(rhs).
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Returns: *this.
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void swap(basic_ostream& rhs);
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Effects: Calls basic_ios<charT, traits>​::​swap(rhs).

31.7.6.2.4 Class basic_ostream​::​sentry [ostream.sentry]

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namespace std { template<class charT, class traits> class basic_ostream<charT, traits>::sentry { bool ok_; // exposition only public: explicit sentry(basic_ostream& os); ~sentry(); explicit operator bool() const { return ok_; } sentry(const sentry&) = delete; sentry& operator=(const sentry&) = delete; }; }
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The class sentry defines a class that is responsible for doing exception safe prefix and suffix operations.
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explicit sentry(basic_ostream& os);
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If os.good() is nonzero, prepares for formatted or unformatted output.
If os.tie() is not a null pointer, calls os.tie()->flush().293
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If, after any preparation is completed, os.good() is true, ok_ == true otherwise, ok_ == false.
During preparation, the constructor may call setstate(failbit) (which may throw ios_base​::​​failure ([iostate.flags])).294
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~sentry();
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If (os.flags() & ios_base​::​unitbuf) && !uncaught_exceptions() && os.good() is true, calls os.rdbuf()->pubsync().
If that function returns , sets badbit in os.rdstate() without propagating an exception.
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explicit operator bool() const;
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Effects: Returns ok_.
293)293)
The call os.tie()->flush() does not necessarily occur if the function can determine that no synchronization is necessary.
294)294)
The sentry constructor and destructor can also perform additional implementation-dependent operations.

31.7.6.2.5 Seek members [ostream.seeks]

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Each seek member function begins execution by constructing an object of class sentry.
It returns by destroying the sentry object.
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pos_type tellp();
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Returns: If fail() != false, returns pos_type(-1) to indicate failure.
Otherwise, returns rdbuf()->​pubseekoff(​0, cur, out).
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basic_ostream& seekp(pos_type pos);
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Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base​::​out).
In case of failure, the function calls setstate(failbit) (which may throw ios_base​::​failure).
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Returns: *this.
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basic_ostream& seekp(off_type off, ios_base::seekdir dir);
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Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base​::​out).
In case of failure, the function calls setstate(failbit) (which may throw ios_base​::​failure).
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Returns: *this.

31.7.6.3 Formatted output functions [ostream.formatted]

31.7.6.3.1 Common requirements [ostream.formatted.reqmts]

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Each formatted output function begins execution by constructing an object of class sentry.
If that object returns true when converted to a value of type bool, the function endeavors to generate the requested output.
If the generation fails, then the formatted output function does setstate(ios_base​::​failbit), which can throw an exception.
If an exception is thrown during output, then ios_base​::​badbit is set295 in *this's error state.
If (exceptions()&badbit) != 0 then the exception is rethrown.
Whether or not an exception is thrown, the sentry object is destroyed before leaving the formatted output function.
If no exception is thrown, the result of the formatted output function is *this.
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The descriptions of the individual formatted output functions describe how they perform output and do not mention the sentry object.
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If a formatted output function of a stream os determines padding, it does so as follows.
Given a charT character sequence seq where charT is the character type of the stream, if the length of seq is less than os.width(), then enough copies of os.fill() are added to this sequence as necessary to pad to a width of os.width() characters.
If (os.flags() & ios_base​::​adjustfield) == ios_base​::​left is true, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence.
295)295)
This is done without causing an ios_base​::​failure to be thrown.

31.7.6.3.2 Arithmetic inserters [ostream.inserters.arithmetic]

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basic_ostream& operator<<(bool val); basic_ostream& operator<<(short val); basic_ostream& operator<<(unsigned short val); basic_ostream& operator<<(int val); basic_ostream& operator<<(unsigned int val); basic_ostream& operator<<(long val); basic_ostream& operator<<(unsigned long val); basic_ostream& operator<<(long long val); basic_ostream& operator<<(unsigned long long val); basic_ostream& operator<<(float val); basic_ostream& operator<<(double val); basic_ostream& operator<<(long double val); basic_ostream& operator<<(const void* val);
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Effects: The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing.
These inserter functions use the imbued locale value to perform numeric formatting.
When val is of type bool, long, unsigned long, long long, unsigned long long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), val).failed();
When val is of type short the formatting conversion occurs as if it performed the following code fragment: ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned short>(val)) : static_cast<long>(val)).failed();
When val is of type int the formatting conversion occurs as if it performed the following code fragment: ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield; bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), baseflags == ios_base::oct || baseflags == ios_base::hex ? static_cast<long>(static_cast<unsigned int>(val)) : static_cast<long>(val)).failed();
When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)).failed();
When val is of type float the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), static_cast<double>(val)).failed();
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The first argument provides an object of the ostreambuf_iterator<> class which is an iterator for class basic_ostream<>.
It bypasses ostreams and uses streambufs directly.
Class locale relies on these types as its interface to iostreams, since for flexibility it has been abstracted away from direct dependence on ostream.
The second parameter is a reference to the base class subobject of type ios_base.
It provides formatting specifications such as field width, and a locale from which to obtain other facets.
If failed is true then does setstate(badbit), which may throw an exception, and returns.
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Returns: *this.
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basic_ostream& operator<<(const volatile void* p);
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Effects: Equivalent to: return operator<<(const_cast<const void*>(p));
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basic_ostream& operator<<(extended-floating-point-type val);
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Effects: If the floating-point conversion rank of extended-floating-point-type is less than or equal to that of double, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), static_cast<double>(val)).failed();
Otherwise, if the floating-point conversion rank of extended-floating-point-type is less than or equal to that of long double, the formatting conversion occurs as if it performed the following code fragment: bool failed = use_facet< num_put<charT, ostreambuf_iterator<charT, traits>> >(getloc()).put(*this, *this, fill(), static_cast<long double>(val)).failed();
Otherwise, an invocation of the operator function is conditionally supported with implementation-defined semantics.
If failed is true then does setstate(badbit), which may throw an exception, and returns.
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Returns: *this.

31.7.6.3.3 basic_ostream​::​operator<< [ostream.inserters]

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basic_ostream& operator<<(basic_ostream& (*pf)(basic_ostream&));
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Effects: None.
Does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).
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Returns: pf(*this).296
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basic_ostream& operator<<(basic_ios<charT, traits>& (*pf)(basic_ios<charT, traits>&));
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Effects: Calls pf(*this).
This inserter does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).
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Returns: *this.297
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basic_ostream& operator<<(ios_base& (*pf)(ios_base&));
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Effects: Calls pf(*this).
This inserter does not behave as a formatted output function (as described in [ostream.formatted.reqmts]).
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Returns: *this.
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basic_ostream& operator<<(basic_streambuf<charT, traits>* sb);
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Effects: Behaves as an unformatted output function ([ostream.unformatted]).
After the sentry object is constructed, if sb is null calls setstate(badbit) (which may throw ios_base​::​failure).
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Gets characters from sb and inserts them in *this.
Characters are read from sb and inserted until any of the following occurs:
  • (8.1)
    end-of-file occurs on the input sequence;
  • (8.2)
    inserting in the output sequence fails (in which case the character to be inserted is not extracted);
  • (8.3)
    an exception occurs while getting a character from sb.
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If the function inserts no characters, it calls setstate(failbit) (which may throw ios_base​::​​failure ([iostate.flags])).
If an exception was thrown while extracting a character, the function sets failbit in the error state, and if failbit is set in exceptions() the caught exception is rethrown.
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Returns: *this.
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basic_ostream& operator<<(nullptr_t);
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Effects: Equivalent to: return *this << s; where s is an implementation-defined NTCTS.
296)296)
See, for example, the function signature endl(basic_ostream&) ([ostream.manip]).
297)297)
See, for example, the function signature dec(ios_base&) ([basefield.manip]).

31.7.6.3.4 Character inserter function templates [ostream.inserters.character]

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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, charT c); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char c); // specialization template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, char c); // signed and unsigned template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, signed char c); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, unsigned char c);
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Effects: Behaves as a formatted output function of out.
Constructs a character sequence seq.
If c has type char and the character type of the stream is not char, then seq consists of out.widen(c); otherwise seq consists of c.
Determines padding for seq as described in [ostream.formatted.reqmts].
Inserts seq into out.
Calls os.width(0).
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Returns: out.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const charT* s); template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const char* s); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const char* s); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const signed char* s); template<class traits> basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const unsigned char* s);
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Preconditions: s is not a null pointer.
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Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of out.
Creates a character sequence seq of n characters starting at s, each widened using out.widen() ([basic.ios.members]), where n is the number that would be computed as if by:
  • (4.1)
    traits​::​length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*,
  • (4.2)
    char_traits<char>​::​length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*,
  • (4.3)
    traits​::​length(reinterpret_cast<const char*>(s)) for the other two overloads.
Determines padding for seq as described in [ostream.formatted.reqmts].
Inserts seq into out.
Calls width(0).
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Returns: out.

31.7.6.3.5 Print [ostream.formatted.print]

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template<class... Args> void print(ostream& os, format_string<Args...> fmt, Args&&... args);
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Effects: If the ordinary literal encoding ([lex.charset]) is UTF-8, equivalent to: vprint_unicode(os, fmt.str, make_format_args(std::forward<Args>(args)...));
Otherwise, equivalent to: vprint_nonunicode(os, fmt.str, make_format_args(std::forward<Args>(args)...));
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template<class... Args> void println(ostream& os, format_string<Args...> fmt, Args&&... args);
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Effects: Equivalent to: print(os, "{}\n", format(fmt, std::forward<Args>(args)...));
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void vprint_unicode(ostream& os, string_view fmt, format_args args); void vprint_nonunicode(ostream& os, string_view fmt, format_args args);
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Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]) of os, except that:
  • (3.1)
    failure to generate output is reported as specified below, and
  • (3.2)
    any exception thrown by the call to vformat is propagated without regard to the value of os.exceptions() and without turning on ios_base​::​badbit in the error state of os.
After constructing a sentry object, the function initializes an automatic variable via string out = vformat(os.getloc(), fmt, args);
If the function is vprint_unicode and os is a stream that refers to a terminal capable of displaying Unicode which is determined in an implementation-defined manner, writes out to the terminal using the native Unicode API; if out contains invalid code units, the behavior is undefined and implementations are encouraged to diagnose it.
If the native Unicode API is used, the function flushes os before writing out.
Otherwise (if os is not such a stream or the function is vprint_nonunicode), inserts the character sequence [out.begin(), out.end()) into os.
If writing to the terminal or inserting into os fails, calls os.setstate(ios_base​::​badbit) (which may throw ios_base​::​failure).
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Recommended practice: For vprint_unicode, if invoking the native Unicode API requires transcoding, implementations should substitute invalid code units with U+fffd replacement character per the Unicode Standard, Chapter 3.9 U+fffd Substitution in Conversion.

31.7.6.4 Unformatted output functions [ostream.unformatted]

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Each unformatted output function begins execution by constructing an object of class sentry.
If that object returns true, while converting to a value of type bool, the function endeavors to generate the requested output.
If an exception is thrown during output, then ios_base​::​badbit is set298 in *this's error state.
If (exceptions() & badbit) != 0 then the exception is rethrown.
In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the unformatted output function.
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basic_ostream& put(char_type c);
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Effects: Behaves as an unformatted output function (as described above).
After constructing a sentry object, inserts the character c, if possible.299
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Otherwise, calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
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Returns: *this.
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basic_ostream& write(const char_type* s, streamsize n);
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Effects: Behaves as an unformatted output function (as described above).
After constructing a sentry object, obtains characters to insert from successive locations of an array whose first element is designated by s.300
Characters are inserted until either of the following occurs:
  • (5.1)
    n characters are inserted;
  • (5.2)
    inserting in the output sequence fails (in which case the function calls setstate(badbit), which may throw ios_base​::​failure ([iostate.flags])).
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Returns: *this.
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basic_ostream& flush();
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Effects: Behaves as an unformatted output function (as described above).
If rdbuf() is not a null pointer, constructs a sentry object.
If that object returns true when converted to a value of type bool the function calls rdbuf()->pubsync().
If that function returns calls setstate(badbit) (which may throw ios_base​::​failure ([iostate.flags])).
Otherwise, if the sentry object returns false, does nothing.
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Returns: *this.
298)298)
This is done without causing an ios_base​::​failure to be thrown.
299)299)
Note that this function is not overloaded on types signed char and unsigned char.
300)300)
Note that this function is not overloaded on types signed char and unsigned char.

31.7.6.5 Standard manipulators [ostream.manip]

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Each instantiation of any of the function templates specified in this subclause is a designated addressable function ([namespace.std]).
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template<class charT, class traits> basic_ostream<charT, traits>& endl(basic_ostream<charT, traits>& os);
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Effects: Calls os.put(os.widen('\n')), then os.flush().
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& ends(basic_ostream<charT, traits>& os);
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Effects: Inserts a null character into the output sequence: calls os.put(charT()).
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& flush(basic_ostream<charT, traits>& os);
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Effects: Calls os.flush().
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& emit_on_flush(basic_ostream<charT, traits>& os);
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Effects: If os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls buf->set_emit_on_sync(true).
Otherwise this manipulator has no effect.
[Note 1: 
To work around the issue that the Allocator template argument cannot be deduced, implementations can introduce an intermediate base class to basic_syncbuf that manages its emit_on_sync flag.
— end note]
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& noemit_on_flush(basic_ostream<charT, traits>& os);
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Effects: If os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, calls buf->set_emit_on_sync(false).
Otherwise this manipulator has no effect.
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Returns: os.
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template<class charT, class traits> basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);
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Effects: Calls os.flush().
Then, if os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, behaves as an unformatted output function ([ostream.unformatted]) of os.
After constructing a sentry object, calls buf->emit().
If that call returns false, calls os.setstate(ios_base​::​badbit).
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Returns: os.

31.7.6.6 Rvalue stream insertion [ostream.rvalue]

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template<class Ostream, class T> Ostream&& operator<<(Ostream&& os, const T& x);
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Constraints: The expression os << x is well-formed when treated as an unevaluated operand and Ostream is publicly and unambiguously derived from ios_base.
2
#
Effects: As if by: os << x;
3
#
Returns: std​::​move(os).