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Showing content from https://en.cppreference.com/w/cpp/algorithm/../io/ios_base/../basic_fstream/open.html below:

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

Opens and associates the file with name filename with the file stream.

Calls clear() on success. Calls setstate(failbit) on failure.

1,2)

Effectively calls

rdbuf()->open(filename, mode)

(see

std::basic_filebuf::open

for the details on the effects of that call).

Overload (2) is only provided if std::filesystem::path::value_type is not char.(since C++17)

3,4) Effectively calls (1,2) as if by open(filename.c_str(), mode).

[edit] Parameters filename - the name of the file to be opened mode - specifies stream open mode. It is a BitmaskType, the following constants are defined: 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 [edit] Example

#include <fstream>
#include <iostream>
#include <string>
 
int main()
{
    std::string filename = "example.123";
 
    std::fstream fs;
    fs.open(filename);
 
    if (!fs.is_open())
    {
        fs.clear();
        fs.open(filename, std::ios::out); // create file
        fs.close();
        fs.open(filename);
    }
 
    std::cout << std::boolalpha;
    std::cout << "fs.is_open() = " << fs.is_open() << '\n';
    std::cout << "fs.good() = " << fs.good() << '\n';
}

[edit] 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 22 C++98 it was unclear how error state changes upon a successful open the error state is unchanged LWG 409 C++98 the error state was unchanged upon a successful open it is cleared^[1] LWG 460 C++98 the default argument of mode in overload (1)
was missing (it is present in the synopsis) added

â The resolution of LWG issue #22 is overridden.

[edit] See also checks if the stream has an associated file
(public member function) [edit] closes the associated file
(public member function) [edit] opens a file and configures it as the associated character sequence
(public member function of std::basic_filebuf<CharT,Traits>) [edit]

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