[ifstream] # 31 Input/output library [[input.output]](./#input.output) ## 31.10 File-based streams [[file.streams]](file.streams#ifstream) ### 31.10.4 Class template basic_ifstream [ifstream] #### [31.10.4.1](#general) General [[ifstream.general]](ifstream.general) [🔗](#lib:basic_ifstream) namespace std {template>class basic_ifstream : public basic_istream {public:using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; using native_handle_type = typename basic_filebuf::native_handle_type; // [[ifstream.cons]](#cons "31.10.4.2 Constructors"), constructors basic_ifstream(); explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in);// wide systems only; see [[fstream.syn]](fstream.syn "31.10.1 Header synopsis")explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); templateexplicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); basic_ifstream(const basic_ifstream&) = delete; basic_ifstream(basic_ifstream&& rhs); basic_ifstream& operator=(const basic_ifstream&) = delete; basic_ifstream& operator=(basic_ifstream&& rhs); // [[ifstream.swap]](#swap "31.10.4.3 Swap"), swapvoid swap(basic_ifstream& rhs); // [[ifstream.members]](#members "31.10.4.4 Member functions"), members basic_filebuf* rdbuf() const; native_handle_type native_handle() const noexcept; bool is_open() const; void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [[fstream.syn]](fstream.syn "31.10.1 Header synopsis")void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in); void close(); private: basic_filebuf *sb*; // *exposition only*};} [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11821) The classbasic_ifstream supports reading from named files[.](#general-1.sentence-1) It uses abasic_filebuf<​charT, traits> object to control the associated sequence[.](#general-1.sentence-2) For the sake of exposition, the maintained data is presented here as: - [(1.1)](#general-1.1) *sb*, the filebuf object[.](#general-1.sentence-3) #### [31.10.4.2](#cons) Constructors [[ifstream.cons]](ifstream.cons) [🔗](#lib:basic_ifstream,constructor) `basic_ifstream(); ` [1](#cons-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11843) *Effects*: Initializes the base class withbasic_istream(addressof(*sb*)) ([[istream.cons]](istream.cons "31.7.5.2.2 Constructors")) and *sb* withbasic_filebuf() ([[filebuf.cons]](filebuf.cons "31.10.3.2 Constructors"))[.](#cons-1.sentence-1) [🔗](#lib:basic_ifstream,constructor_) `explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [[fstream.syn]](fstream.syn "31.10.1 Header synopsis") ` [2](#cons-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11860) *Effects*: Initializes the base class withbasic_istream(addressof(*sb*)) ([[istream.cons]](istream.cons "31.7.5.2.2 Constructors")) and *sb* withbasic_filebuf() ([[filebuf.cons]](filebuf.cons "31.10.3.2 Constructors")), then callsrdbuf()->open(s, mode | ios_base​::​in)[.](#cons-2.sentence-1) If that function returns a null pointer, callssetstate(failbit)[.](#cons-2.sentence-2) [🔗](#lib:basic_ifstream,constructor__) `explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); ` [3](#cons-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11879) *Effects*: Equivalent to basic_ifstream(s.c_str(), mode)[.](#cons-3.sentence-1) [🔗](#lib:basic_ifstream,constructor___) `template explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); ` [4](#cons-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11891) *Constraints*: is_same_v is true[.](#cons-4.sentence-1) [5](#cons-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11895) *Effects*: Equivalent to basic_ifstream(s.c_str(), mode)[.](#cons-5.sentence-1) [🔗](#lib:basic_ifstream,constructor____) `basic_ifstream(basic_ifstream&& rhs); ` [6](#cons-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11906) *Effects*: Move constructs the base class, and the contained basic_filebuf[.](#cons-6.sentence-1) Then calls basic_istream​::​set_rdbuf(​addressof(*sb*)) to install the contained basic_filebuf[.](#cons-6.sentence-2) #### [31.10.4.3](#swap) Swap [[ifstream.swap]](ifstream.swap) [🔗](#lib:swap,basic_ifstream) `void swap(basic_ifstream& rhs); ` [1](#swap-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11921) *Effects*: Exchanges the state of *this and rhs by callingbasic_istream​::​swap(rhs) and*sb*.swap(rhs.*sb*)[.](#swap-1.sentence-1) [🔗](#lib:swap,basic_ifstream_) `template void swap(basic_ifstream& x, basic_ifstream& y); ` [2](#swap-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11936) *Effects*: Equivalent to x.swap(y)[.](#swap-2.sentence-1) #### [31.10.4.4](#members) Member functions [[ifstream.members]](ifstream.members) [🔗](#lib:rdbuf,basic_ifstream) `basic_filebuf* rdbuf() const; ` [1](#members-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11949) *Returns*: const_cast*>(addressof(*sb*))[.](#members-1.sentence-1) [🔗](#lib:native_handle,basic_ifstream) `native_handle_type native_handle() const noexcept; ` [2](#members-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11960) *Effects*: Equivalent to: return rdbuf()->native_handle(); [🔗](#lib:is_open,basic_ifstream) `bool is_open() const; ` [3](#members-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11971) *Returns*: rdbuf()->is_open()[.](#members-3.sentence-1) [🔗](#lib:open,basic_ifstream) `void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [[fstream.syn]](fstream.syn "31.10.1 Header synopsis") ` [4](#members-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L11984) *Effects*: Callsrdbuf()->open(s, mode | ios_base​::​in)[.](#members-4.sentence-1) If that function does not return a null pointer calls clear(), otherwise callssetstate(failbit) (which may throwios_base​::​failure) ([[iostate.flags]](iostate.flags "31.5.4.4 Flags functions"))[.](#members-4.sentence-2) [🔗](#lib:open,basic_ifstream_) `void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in); ` [5](#members-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12003) *Effects*: Calls open(s.c_str(), mode)[.](#members-5.sentence-1) [🔗](#lib:close,basic_ifstream) `void close(); ` [6](#members-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/iostreams.tex#L12014) *Effects*: Callsrdbuf()->close() and, if that function returns a null pointer, callssetstate(failbit) (which may throwios_base​::​failure) ([[iostate.flags]](iostate.flags "31.5.4.4 Flags functions"))[.](#members-6.sentence-1)