函数的返回类型中的“＆”符号是什么意思？

Question

I'm reading a C++ book that explains the following function:

istream& read_hw(istream& in, vector<double>& hw) {
    if (in) {
        hw.clear() ;
        double x;
        while (in >> x)
            hw.push_back(x);
        in.clear();
    }
    return in;
}

The book explained those "&" in the arguments list mean they are passed by reference, but there is no explanation about that symbol in istream& : in the return type of the function.
Removing it causes many compilation errors. Can someone clarify?

Answer 1

The function returns by reference as well. In this case, the object you pass in is returned from the function, so you can "chain" calls to this function:

in.read_hw(hw1).read_hw(hw2);

This is a common pattern in C++, especially when in with the IOstreams library.

Answer 2

This is returning a reference to the istream. Note that this is probably the same as the istream& reference passed as an argument.

From learncpp.com :

Return by reference is typically used to return arguments passed by reference to the function back to the caller. In the following example, we return (by reference) an element of an array that was passed to our function by reference:

// This struct holds an array of 25 integers
struct FixedArray25
{
    int anValue[25];
};

// Returns a reference to the nIndex element of rArray
int& Value(FixedArray25 &rArray, int nIndex)
{
    return rArray.anValue[nIndex];
}

int main()
{
    FixedArray25 sMyArray;

    // Set the 10th element of sMyArray to the value 5
    Value(sMyArray, 10) = 5;

    cout << sMyArray.anValue[10] << endl;
    return 0;
}

Answer 3

in "istream& in" the & operator means "the reference of"

just know that whatever variable you pass into here, the original value will be modified

Answer 4

It is a reference. It's like a pointer but it can't be NULL.

So your function is returning a reference to an istream object. Notice that you also pass the same data type as the first parameter to your function.

It is quite common to do this with streams so you can use the stream test operator to check for error conditions:

if( !read_hw(in, hw) ) cerr << "Read failed\n";