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std::ios_base:: openmode

From cppreference.net
C++
Input/output library
std::ios_base
typedef /* implementation defined */ openmode ;
static constexpr openmode app = /* implementation defined */ ;

static constexpr openmode binary = /* implementation defined */ ;
static constexpr openmode in = /* implementation defined */ ;
static constexpr openmode out = /* implementation defined */ ;
static constexpr openmode trunc = /* implementation defined */ ;

static constexpr openmode ate = /* implementation defined */ ;
static constexpr openmode noreplace = /* implementation defined */ ;
(since C++23)

Specifies available file open flags. 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

Example

Run this code 
#include <fstream>
#include <iostream>
#include <string>
int main()
{
    const char* fname = "unique_name.txt";
    // write to a temporary stream object
    std::fstream(fname, std::ios::out | std::ios::trunc) << "Hi";
    std::string s;
    std::fstream(fname, std::ios::in) >> s;
    std::cout << s << '\n';
}

Output: 

Hi

See also

opens a file and configures it as the associated character sequence
(public member function of std::basic_filebuf<CharT,Traits> )
constructs a basic_stringbuf object
(public member function of std::basic_stringbuf<CharT,Traits,Allocator> )