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Showing content from https://en.cppreference.com/w/cpp/language/../named_req/../io/c/std_streams.html below:

stdin, stdout, stderr - cppreference.com

#define stdin  /* implementation-defined */

#define stdout /* implementation-defined */

#define stderr /* implementation-defined */

Three text streams are predefined. These streams are implicitly opened and unoriented at program startup.

1) Associated with the standard input stream, used for reading conventional input. At program startup, the stream is fully buffered if and only if the stream can be determined not to refer to an interactive device.

2) Associated with the standard output stream, used for writing conventional output. At program startup, the stream is fully buffered if and only if the stream can be determined not to refer to an interactive device.

3) Associated with the standard error stream, used for writing diagnostic output. At program startup, the stream is not fully buffered.

What constitutes an interactive device is implementation-defined.

These macros are expanded to expressions of type std::FILE*.

Although not mandated by POSIX, the UNIX convention is that stdin and stdout are line-buffered if associated with a terminal and stderr is unbuffered.

These macros may be expanded to modifiable lvalues. If any of these std::FILE* lvalue is modified, subsequent operations on the corresponding stream result in unspecified or undefined behavior.

This example shows a function similar to std::printf.

#include <concepts>
#include <cstdio>
#include <type_traits>
 
template<typename T>
concept IsPrintable = std::integral<T> or std::floating_point<T> or std::is_pointer_v<T>;
 
int my_printf(char const* const format, IsPrintable auto const ... arguments)
{
    return std::fprintf(stdout, format, arguments...);
}
 
int main(int argv, char*[])
{
    my_printf("Strings and chars:\t%s %c\n", "hello", 'x');
    my_printf("Rounding:\t\t%f %.0f %.32f\n", 1.5, 1.5, 1.3);
    my_printf("Padding:\t\t%05.2f %.2f %5.2f\n", 1.5, 1.5, 1.5);
    my_printf("Scientific:\t\t%E %e\n", 1.5, 1.5);
    my_printf("Hexadecimal:\t\t%a %A 0x%X\n", 1.5, 1.5, &argv);
}

Possible output:

Strings and chars:  hello x
Rounding:           1.500000 2 1.30000000000000004440892098500626
Padding:            01.50 1.50  1.50
Scientific:         1.500000E+00 1.500000e+00
Hexadecimal:        0x1.8p+0 0X1.8P+0 0x2CFB41BC

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