std::basic_ostream<CharT,Traits>::operator<<

Since C++11

Inserts data into the stream.

# Declarations

basic_ostream& operator<<( bool value );

basic_ostream& operator<<( long value );

basic_ostream& operator<<( unsigned long value );

basic_ostream& operator<<( long long value );

(since C++11)

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

(since C++11)

basic_ostream& operator<<( double value );

basic_ostream& operator<<( long double value );

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

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

(since C++23)

basic_ostream& operator<<( std::nullptr_t );

(since C++17)

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 );

(since C++23)

basic_ostream& operator<<( std::basic_streambuf<CharT, Traits>* sb );

basic_ostream& operator<<(
std::ios_base& (*func)(std::ios_base&) );

basic_ostream& operator<<(
std::basic_ios<CharT, Traits>& (*func)(std::basic_ios<CharT, Traits>&) );

basic_ostream& operator<<(
std::basic_ostream<CharT, Traits>& (*func)
(std::basic_ostream<CharT, Traits>&) );

# Parameters

value: integer, floating-point, boolean, or pointer value to insert
func: function to call
sb: pointer to the stream buffer to read the data from

# Notes

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).

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

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.

# Example

#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.
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 117	C++98	overloads (1-8,11-15) delegated the insertion tonum_put::put, but it does not have overloads for short,unsigned short, int, unsigned int, and float	they are convertedbefore being passedto num_put::put
LWG 567	C++98	overload (17) behaved as a FormattedOutputFunctionbecause of the resolution of LWG issue 60	it behaves as anUnformattedOutputFunction