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std::basic_ostream<CharT,Traits>::operator<< - cppreference.com

basic_ostream& operator<<( bool value );

basic_ostream& operator<<( long value );

basic_ostream& operator<<( unsigned long value );

basic_ostream& operator<<( long long value );

basic_ostream& operator<<( unsigned long long value );

basic_ostream& operator<<( double value );

basic_ostream& operator<<( long double value );

basic_ostream& operator<<( const void* value );

basic_ostream& operator<<( const volatile void* value );

basic_ostream& operator<<( short value );

basic_ostream& operator<<( int value );

basic_ostream& operator<<( unsigned short value );

basic_ostream& operator<<( unsigned int value );

basic_ostream& operator<<( float value );

basic_ostream& operator<<( /* extended-floating-point-type */ value );

Inserts data into the stream.

1-8) Inserts value.

This function behaves as a

. After constructing and checking the sentry object, inserts a value by calling

. If the end of file condition was encountered during output (

), sets

.

9) Equivalent to return operator<<(const_cast<const void*>(p));.

10) Equivalent to return *this << s;, where s is an implementation-defined null-terminated character type string.

11) Inserts a value from short value.

12) Inserts a value from int value.

13,14) Inserts a value from unsigned short or unsigned int value.

This function behaves as a

. After constructing and checking the sentry object, inserts

as in

.

15) Inserts a value from float value.

This function behaves as a

. After constructing and checking the sentry object, inserts

as in

.

Inserts a value from

. The library provides overloads for all cv-unqualified

as the type of the parameter value.

This function behaves as a

. After constructing and checking the sentry object, checks the

of

:

• If the rank is less than or equal to that of double, inserts static_cast<double>(value) as in (6).
• Otherwise, if the rank is less than or equal to that of long double, inserts static_cast<long double>(value) as in (7).
• Otherwise, an invocation of this overload is conditionally supported with implementation-defined semantics.

This function behaves as an

. After constructing and checking the sentry object, checks if

is a null pointer. If it is, executes

and exits. Otherwise, extracts characters from the input sequence controlled by

and inserts them into

until one of the following conditions are met:

• end-of-file occurs on the input sequence;
• inserting in the output sequence fails (in which case the character to be inserted is not extracted);
• an exception occurs (in which case the exception is caught).

If no characters were inserted, executes

. If an exception was thrown while extracting, sets

and, if

is set in

, rethrows the exception.

Calls

. These overloads are used to implement output I/O manipulators such as

.

1-19) *this

20) func(*this)

There are no overloads for pointers to non-static members, pointers to volatiles,(until C++23) or function pointers (other than the ones with signatures accepted by the (18-20) overloads).

• Attempting to output such objects invokes implicit conversion to bool, and, for any non-null pointer value, the value 1 is printed (unless boolalpha was set, in which case true is printed).

Character and character string arguments (e.g., of type char or const char*) are handled by the non-member overloads of operator<<.

• Attempting to output a character using the member function call syntax (e.g., std::cout.operator<<('c');) will call one of the overloads in (2-5) or (11-14) and output the numerical value.
• Attempting to output a character string using the member function call syntax will call overload (8) and print the pointer value instead.

Overload (10) was added by the resolution of LWG issue 2221, but it is never implemented in any standard library implementation under C++11/14 modes.

#include <iomanip>
#include <iostream>
#include <sstream>
 
int fun() { return 42; }
 
int main()
{
    std::istringstream input(" \"Some text.\" ");
    double f = 3.14;
    bool b = true;
 
    std::cout
        << fun()          // int overload (12)
        << ' '            // non-member overload
        << std::boolalpha // function overload (18)
        << b              // bool overload (1)
        << " "            // non-member overload
        << std::fixed     // function overload (18) again
        << f              // double overload (6)
        << input.rdbuf()  // streambuf overload
        << fun            // bool overload (1): there's no overload for int(*)()
        << std::endl;     // function overload (18) again
 
    int x = 0;
    int* p1 = &x;
    volatile int* p2 = &x;
    std::cout
        << "p1: " << p1 << '\n'  // `const void*` overload, prints address
        << "p2: " << p2 << '\n'; // before C++23 (P1147): bool overload :), because
            // operator<<(const void*) is not a match, as it discards the `volatile`
            // qualifier. To fix this, C++23 adds `const volatile void*` overload (9),
            // that prints the address as expected.
}

Possible output:

42 true 3.140000 "Some text." true
p1: 0x7ffcea766600
p2: 0x7ffcea766600

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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