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up vote 1 down vote favorite

Many of you may know the following in C++:

cout << 1 << 2 << 3 << 4; // would produce 1234

I'm attempting to recreate the same thing - but instead encapsulate it into a class and increment the values into an integer variables.

I get the error:

error: 'int operator<<(const int&, const int&)' must have an argument of class or enumerated type|

class Test
{
private:
    int data;

public:
    Test() { data = 0; }
    void print() { cout << data; }

    friend int operator<<(const int &x, const int &y)
    {
        data += x;
        data += y;
    }
};

int main()
{
    Test a;
    a << 50 << 20;
    a.print();   //if I had only 1 parameter - it worked and printed out only 50
    return 0;
}
share|improve this question
    
Your operator is for your test class not just for ints. it must be a member not a friend. – aggsol Jun 12 '13 at 11:30
2  
Take one parameter. Return reference to *this; – Simon G. Jun 12 '13 at 11:31
up vote 7 down vote accepted

cout << 1 << 2 << 3 << 4;

This way this works is as a series of calls with two arguments e.g.

(((cout << 1) << 2) << 3) << 4;

Which is roughly equivalent to this:

cout << 1;
cout << 2;
cout << 3;
cout << 4;

So you don't write an operator<< taking multiple parameters, it always takes two operands, the left operand and the right operand. The left operand in the example above is cout i.e. the ostream and the right operand is the int being written to it. The operator returns the left operand, allowing it to be used again in the next operation, and so on for as many << operations as you chain together.

So cout << 1 returns cout again, so that (cout << 1) << 2 invokes the operator to write 1 to the stream and return the stream, then invokes the operator again on the return value to write 2 to the stream and then returns the stream again.

This is just nonsense:

friend int operator<<(const int &x, const int &y)
{
    data += x;
    data += y;
}

Where is data supposed to come from? A friend function is not a member of the class, so there is no this pointer, so no this->data, also you claim to return int but don't return anything, and this class is completely unrelated to Test. What you've written is an operator<< for two ints i.e. for doing 1 << 2 but that operator already exists, it's the bitshift operator, and you can't overload it for built-in types like int.

You want:

class Test
{
private:
    int data;

public:
    Test() { data = 0; }
    void print() { cout << data; }

    Test& operator<<(int y)
    {
        data += x;
        return *this;
    }
};

Or as a friend:

class Test
{
private:
    int data;

public:
    Test() { data = 0; }
    void print() { cout << data; }

    friend Test& operator<<(Test& t, int y)
    {
        t.data += x;
        return t;
    }
};
share|improve this answer
    
+1 for completeness - although I would not bother mentioning friend – Arne Mertz Jun 12 '13 at 11:44
up vote 1 down vote

In a nutshell, the operator needs to return the Test instance:

class Test
{
    ...
    friend Test& operator<<(Test& test, int val);
};

Test& operator<<(Test& test, int val)
{
    test.data += val;
    return test;
}
share|improve this answer
    
It works, thank you so much. Could you point me to some docs where I can understand what happens? – Andy Carter Jun 12 '13 at 11:40
    
No need to make it a friend and public function, since a simple member function with one parameter will do. – Arne Mertz Jun 12 '13 at 11:41
1  
@AndyCarter any explanation of how operator<< works for ostreams from your C++ textbook of choice should do. – Arne Mertz Jun 12 '13 at 11:41
    
Cheers. I'll go back on that chapter and read it again. – Andy Carter Jun 12 '13 at 11:44

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