What is ios::in|ios::out?

问题

I was reading some project code and I found this,here MembersOfLibrary() is a constructor of class MenberOfLibrary

    class MembersOfLibrary {

  public:
    MembersOfLibrary();
    ~MembersOfLibrary() {}
    void addMember();
    void removeMember();
    unsigned int searchMember(unsigned int MembershipNo);
    void searchMember(unsigned char * name);
    void displayMember();
  private:
    Members    libMembers;

};

MembersOfLibrary::MembersOfLibrary() {

    fstream memberData;
    memberData.open("member.txt", ios::in|ios::out);
    if(!memberData) {
    cout<<"\nNot able to create a file. MAJOR OS ERROR!! \n";
    }
    memberData.close();
}

I m not able to understand the meaning of --> ios::in|ios::out <-- Please Help out ! Thank You

回答1:

ios::in allows input (read operations) from a stream.
ios::out allows output (write operations) to a stream.
| (bitwise OR operator) is used to combine the two ios flags,
meaning that passing ios::in | ios::out to the constructor
of std::fstream enables both input and output for the stream.

Important things to note:

std::ifstream automatically has the ios::in flag set.
std::ofstream automatically has the ios::out flag set.
std::fstream has neither ios::in or ios::out automatically
set. That's why they're explicitly set in your example code.

回答2:

ios::in and ios::out are openmode flags, and in your case combined with a binary or (|) operation. Thus the file is opened for reading and writing.

回答3:

 memberData.open("member.txt", ios::in|ios::out);

ios::in is used when you want to read from a file

ios::out is used when you want to write to a file

ios::in|ios::out means ios::in or ios::out, that is whichever is required is used

Here's a useful link

http://www.cplusplus.com/doc/tutorial/files/

来源：https://stackoverflow.com/questions/28338775/what-is-iosiniosout

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c++

file-handling