Error: std::endl is of Unknown Type when Overloading Operator <<

In C++, the << operator is commonly overloaded for custom output for user-defined types using streams. However, an issue may arise when using std::endl with an overloaded operator<<. It is shown as the type of std::endl being unknown, leading to compilation errors.

In this article, we will learn the cause of this issue and how can we resolve it.

What is std::endl in C++?

std::endl is a manipulator in the <iostream> library which used to insert a newline character into the output stream and flush the stream. It is not a simple constant or function, but rather an instance of a function template.

Error with std::endl and << Operator Overloading

When overloading the << operator for a user-defined type, you might encounter a compilation error stating that the type of std::endl is unknown. This happens because the compiler cannot deduce the appropriate overload for the manipulator. The main case where it happens can be:

1. Incorrect Return Type in operator<<

One of the most common causes is an incorrect return type for the overloaded operator<<. For chaining operations, including manipulators like std::endl, the operator must return a reference to std::ostream.

2. Incorrect Parameter Type in operator<<

The parameter for the overloaded operator<< must be a reference to std::ostream. If the parameter type is incorrect, the manipulator will not work properly.

3. Incorrect Usage or Lack of Namespace

If the std:: namespace is not used properly, the compiler might not recognize std::endl.

How to Fix the std::endl Unknown Type Error?

There are three approaches to resolve the issue of std::endl being of unknown type when overloading operator<<:

1. Fixing std::endl Unknown Type Error Using std::ostream& as the Return type
2. Fixing std::endl Unknown Type Error Using std::ostream as a Parameter

1. Fixing std::endl Unknown Type Error Using std::ostream& as the Return type

When overloading operator<<, the return type should be std::ostream& to allow chaining of output operations, including manipulators like std::endl.

Example

#include <iostream>

using std::cout;
using std::endl;
using std::ostream;

class MyClass {
public:
    MyClass(int val) : data(val) {}

    // Friend function to overload the << operator for
    // MyClass
    friend ostream& operator<<(ostream& os,
                               const MyClass& obj)
    {
        os << obj.data;
        // Return the output stream to allow chaining
        return os;
    }

private:
    int data;
};

int main()
{
    MyClass obj(10);

    // Print the MyClass object followed by a newline
    cout << obj << endl;

    return 0;
}

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2. Fixing std::endl Unknown Type Error Using std::ostream as a Parameter

Make sure that the parameter type of the overloaded operator<< is std::ostream&.

Example

#include <iostream>

using std::cout;
using std::endl;
using std::ostream;

class MyClass {
public:
    MyClass(int val): data(val){}

    // Member function to print the data to the given output
    // stream
    void print(ostream& os) const { os << data; }

private:
    int data;
};

// Overloaded operator<< to print MyClass objects
ostream& operator<<(ostream& os, const MyClass& obj)
{
    obj.print(os);
    // Return the output stream to allow chaining
    return os;
}

int main()
{
    MyClass obj(10);

    // Print the MyClass object followed by a newline
    cout << obj << endl;

    return 0;
}

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Conclusion

In this article, we learned why the compiler shows the unknown type error when overloading << operator. We also looked at the ways which we can resolve this error. This basically tells us to know the return value and the parameter of the any operator while overloading so that for providing one functionality, we do not sacrifice the already implemented ones.