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std::basic_ifstream<CharT,Traits>::basic_ifstream

basic_ifstream();
 (1) 
explicit basic_ifstream( const char* filename,
                         std::ios_base::openmode mode
                             = std::ios_base::in );
 (2) 
explicit basic_ifstream( const std::filesystem::path::value_type* filename,
                         std::ios_base::openmode mode
                             = std::ios_base::in );
 (3) (since C++17) 
explicit basic_ifstream( const std::string& filename,
                         std::ios_base::openmode mode
                             = std::ios_base::in );
 (4) (since C++11) 
template< class FsPath >
explicit basic_ifstream( const FsPath& filename,
                         std::ios_base::openmode mode
                             = std::ios_base::in );
 (5) (since C++17) 
basic_ifstream( basic_ifstream&& other );
 (6) (since C++11) 
basic_ifstream( const basic_ifstream& rhs) = delete;
 (7) (since C++11)

Constructs new file stream.

1) Default constructor: constructs a stream that is not associated with a file: default-constructs the std::basic_filebuf and constructs the base with the pointer to this default-constructed std::basic_filebuf member.
2,3) First, performs the same steps as the default constructor, then associates the stream with a file by calling rdbuf()->open(filename, mode | std::ios_base::in) (see std::basic_filebuf::open for the details on the effects of that call). If the open() call returns a null pointer, sets setstate(failbit). Overload (3) is only provided if std::filesystem::path::value_type is not char. (since C++17)
4,5) Same as basic_ifstream(filename.c_str(), mode). (5) participates in overload resolution only if FsPath is std::filesystem::path. (since C++17)
6) Move constructor. First, move-constructs the base class from other (which does not affect the rdbuf() pointer), then move-constructs the std::basic_filebuf member, then calls this->set_rdbuf() to install the new basic_filebuf as the rdbuf() pointer in the base class.
7) The copy-constructor is deleted: this class is not copyable.

Parameters

filename - the name of the file to be opened
mode - specifies stream open mode. Following constants and bit-wise OR between them may be used:
Constant Explanation
app seek to the end of stream before each write
binary open in binary mode
in open for reading
out open for writing
trunc discard the contents of the stream when opening
ate seek to the end of stream immediately after open
noreplace (C++23) open in exclusive mode
other - another file stream to use as source

Example

#include <fstream>
#include <utility>
#include <string>
 
int main()
{
    std::ifstream f0;
    std::ifstream f1("test.bin", std::ios::binary);
    std::string name = "example.txt";
    std::ifstream f2(name);
    std::ifstream f3(std::move(f1));
}

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 3430 C++17 std::filesystem::path overload led to unwanted conversions avoided by making it a template

See also

opens a file and associates it with the stream
(public member function)
opens a file and configures it as the associated character sequence
(public member function of std::basic_filebuf<CharT,Traits>)
replaces the rdbuf without clearing its error state
(protected member function)
constructs the object
(public member function of std::basic_istream<CharT,Traits>)

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