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Showing content from https://en.cppreference.com/w/cpp/language/../io/basic_streambuf/../basic_filebuf/open.html below:

std::basic_filebuf<CharT,Traits>::open - cppreference.com

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) or str.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.

Overload (4) is only provided if std::filesystem::path::value_type is not char.

The file is opened as if by calling std::fopen with the second argument (file access mode) determined by the result of mode & ~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 (the ate bit is set), repositions the file position to the end of file, as if by calling std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling std::fopen. If the repositioning fails, calls close() and returns a null pointer to indicate failure.

this on success, a null pointer on failure.

open() is typically called through the constructor or the open() member function of std::basic_fstream.

#include <fstream>
#include <iostream>
 
int main()
{
    std::string filename = "Test.b";
    std::filebuf fb;
 
    // prepare a file to read
    double d = 3.14;
    if (!fb.open(filename, std::ios::binary | std::ios::out))
    {
        std::cout << "Open file " << filename << " for write failed\n";
        return 1;
    } 
    fb.sputn(reinterpret_cast<char*>(&d), sizeof d);
    fb.close();
 
    // open file for reading
    double d2 = 0.0;
    if (!fb.open(filename, std::ios::binary | std::ios::in))
    {
        std::cout << "Open file " << filename << " for read failed\n";
        return 1;
    }
 
    auto got = fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);
    if (sizeof(d2) != got)
        std::cout << "Read of " << filename << " failed\n";
    else
        std::cout << "Read back from file: " << d2 << '\n';
}

Output:

Read back from file: 3.14

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

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