How to instantiate an fstream if you declare it as a member of a class?

问题

What constructor can you use to instantiate an fstream if you declare it as a member of a class?

#include <fstream>
class Foo {
Foo();
// not allowed
std::fstream myFile("\\temp\\foo.txt", fstream::in | fstream::out | fstream::trunc);

// allowed
std::fstream myFile;
}

---

// constructor
Foo::Foo() {
// what form of myFile("\\temp\\foo.txt", fstream::in | fstream::out | fstream::trunc)  can I use here?


myFile = ???
}

回答1:

In the new version of C++ (C++11), then the above code you have is perfectly fine; initializations are allowed inside the body of a class.

In C++03 (the previous version of C++), you can initialize the fstream by using the member initializer list like this:

Foo::Foo() : myFile("file-name", otherArguments) {
    // other initialization
}

Syntatically, this is done by adding a colon after the constructor name but before the brace, then listing the name of the field you want to initialize (here, myFile), and then in parentheses the arguments you want to use to initialize it. This will cause myFile to be initialized properly.

Hope this helps!

回答2:

Another option is:

Foo::Foo () {
    myFile.open("\\temp\\foo.txt", fstream::in | fstream::out | fstream::trunc);

    if(!myFile.is_open()) {
        printf("myFile failed to open!");
    }

    //other initialization
}

来源：https://stackoverflow.com/questions/8994157/how-to-instantiate-an-fstream-if-you-declare-it-as-a-member-of-a-class