If the associated file was already open (is_open()!=false), returns a null pointer right away.

Otherwise, opens the file with the given name (s,p.c_str()(since C++17) orstr.c_str(), depending on the overload).std::ios_base::openmode values may be written as, e.g.,std::ios_base::out|std::ios_base::app.

The file is opened as if by callingstd::fopen with the second argument (file access mode) determined by the result ofmode& ~std::ios_base::ate as follows,open() fails if the result is not some combination of flags shown in the table:

If the open operation succeeds and(openmode&std::ios_base::ate)!=0 (theate bit is set), repositions the file position to the end of file, as if by callingstd::fseek(file,0,SEEK_END), wherefile is the pointer returned by callingstd::fopen. If the repositioning fails, callsclose() and returns a null pointer to indicate failure.

[edit]Parameters

[edit]Return value

this on success, a null pointer on failure.

[edit]Notes

open() is typically called through the constructor or theopen() member function ofstd::basic_fstream.

[edit]Example

#include <fstream>#include <iostream> int main(){std::string filename="Test.b";std::filebuf fb; // prepare a file to readdouble d=3.14;if(!fb.open(filename, std::ios::binary| std::ios::out)){std::cout<<"Open file "<< filename<<" for write failed\n";return1;}     fb.sputn(reinterpret_cast<char*>(&d), sizeof d);    fb.close(); // open file for readingdouble d2=0.0;if(!fb.open(filename, std::ios::binary| std::ios::in)){std::cout<<"Open file "<< filename<<" for read failed\n";return1;} auto got= fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);if(sizeof(d2)!= got)std::cout<<"Read of "<< filename<<" failed\n";elsestd::cout<<"Read back from file: "<< d2<<'\n';}

Read back from file: 3.14

[edit]Defect reports

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

[edit]See also

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