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

Inserts data into the stream.

Parameters

Return value

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)

• 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

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)
• 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

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.
}

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

Defect reports

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

See also