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std::filesystem::path::append, std::filesystem::path::operator/=

path& operator/=( const path& p );	(1)	(since C++17)
template< class Source > path& operator/=( const Source& source );	(2)	(since C++17)
template< class Source > path& append( const Source& source );	(3)	(since C++17)
template< class InputIt > path& append( InputIt first, InputIt last );	(4)	(since C++17)

1) If p.is_absolute() || (p.has_root_name() && p.root_name() != root_name()), then replaces the current path with p as if by operator=(p) and finishes.

* Otherwise, if p.has_root_directory(), then removes any root directory and the entire relative path from the generic format pathname of *this

* Otherwise, if has_filename() || (!has_root_directory() && is_absolute()), then appends path::preferred_separator to the generic format of *this

* Either way, then appends the native format pathname of p, omitting any root-name from its generic format, to the native format of *this.

// where "//host" is a root-name
path("//host")  / "foo" // the result is      "//host/foo" (appends with separator)
path("//host/") / "foo" // the result is also "//host/foo" (appends without separator)
 
// On POSIX,
path("foo") / ""      // the result is "foo/" (appends)
path("foo") / "/bar"; // the result is "/bar" (replaces)
 
// On Windows,
path("foo") / "C:/bar";  // the result is "C:/bar" (replaces)
path("foo") / "C:";      // the result is "C:"     (replaces)
path("C:") / "";         // the result is "C:"     (appends, without separator)
path("C:foo") / "/bar";  // yields "C:/bar"        (removes relative path, then appends)
path("C:foo") / "C:bar"; // yields "C:foo/bar"     (appends, omitting p's root-name)

2,3) Same as (1), but accepts any std::basic_string, std::basic_string_view, null-terminated multicharacter string, or an input iterator pointing to a null-terminated multicharacter sequence. Equivalent to return operator/=(path(source));.

4) Same as (1), but accepts any iterator pair that designates a multicharacter string. Equivalent to return operator/=(path(first, last));

(2) and (3) participate in overload resolution only if Source and path are not the same type, and either:

Source is a specialization of std::basic_string or std::basic_string_view, or
std::iterator_traits<std::decay_t<Source>>::value_type is valid and denotes a possibly const-qualified encoding character type (char, char8_t, (since C++20)char16_t, char32_t, or wchar_t).

Parameters

p	-	pathname to append
source	-	`std::basic_string`, `std::basic_string_view`, null-terminated multicharacter string, or an input iterator pointing to a null-terminated multicharacter sequence, which represents a path name (either in portable or in native format)
first, last	-	pair of LegacyInputIterators that specify a multicharacter sequence that represents a path name
Type requirements
-`InputIt` must meet the requirements of LegacyInputIterator.
-The value type of `InputIt` must be one of the encoded character types (`char`, `wchar_t`, `char16_t` and `char32_t`)

Return value

*this.

Exceptions

May throw std::bad_alloc if memory allocation fails.

Notes

These functions effectively yield an approximation of the meaning of the argument path p in an environment where *this is the starting directory.

Example

The output is produced on Windows.

#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;
int main() {
    fs::path p1 = "C:";
    p1 /= "Users"; // does not insert a separator
    std::cout << "\"C:\" / \"Users\" == " << p1 << '\n';
    p1 /= "batman"; // inserts fs::path::preferred_separator, '\' on Windows
    std::cout << "\"C:\" / \"Users\" / \"batman\" == " << p1 << '\n';
}

Possible output:

"C:" / "Users" == "C:Users"
"C:" / "Users" / "batman" == "C:Users\\batman"

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3244	C++17	constraint that `Source` cannot be `path` was missing	added

concatoperator+=	concatenates two paths without introducing a directory separator (public member function)
operator/ (C++17)	concatenates two paths with a directory separator (function)