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std::filesystem::path::concat, std::filesystem::path::operator+=

path& operator+=( const path& p );	(1)	(since C++17)
path& operator+=( const string_type& str ); path& operator+=( std::basic_string_view<value_type> str );	(2)	(since C++17)
path& operator+=( const value_type* ptr );	(3)	(since C++17)
path& operator+=( value_type x );	(4)	(since C++17)
template< class CharT > path& operator+=( CharT x );	(5)	(since C++17)
template< class Source > path& operator+=( const Source& source );	(6)	(since C++17)
template< class Source > path& concat( const Source& source );	(7)	(since C++17)
template< class InputIt > path& concat( InputIt first, InputIt last );	(8)	(since C++17)

Concatenates the current path and the argument.

1-3,6-7) Appends path(p).native() to the pathname stored in *this in the native format. This directly manipulates the value of native() and may not be portable between operating systems.

4-5) Same as return *this += std::basic_string_view(&x, 1);

8) Same as return *this += path(first, last);

(6) and (7) 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	-	path to append
str	-	string or string view to append
ptr	-	pointer to the beginning of a null-terminated string to append
x	-	single character 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`)
-`CharT` 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

Unlike with append() or operator/=, additional directory separators are never introduced.

Example

#include <iostream>
#include <filesystem>
#include <string>
int main() {
    std::filesystem::path p1; // an empty path
    p1 += "var"; // does not insert a separator
    std::cout << R"("" + "var" --> )" << p1 << '\n';
    p1 += "lib"; // does not insert a separator
    std::cout << R"("var" + "lib" --> )" << p1 << '\n';
    auto str = std::string{"1234567"};
    p1.concat(std::begin(str)+3, std::end(str)-1);
    std::cout << "p1.concat --> " << p1 << '\n';
}

Output:

"" + "var" --> "var"
"var" + "lib" --> "varlib"
p1.concat --> "varlib456"

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 3055	C++17	the specification of concatenating a single character was ill-formed	made well-formed
LWG 3244	C++17	constraint that `Source` cannot be `path` was missing	added

appendoperator/=	appends elements to the path with a directory separator (public member function)
operator/ (C++17)	concatenates two paths with a directory separator (function)