How to Define a Virtual `

Overcoming Virtual << Operator Error

When attempting to define a virtual << operator as a free function, developers may encounter the compiler error "operator <<: only member functions and bases can be virtual." This error arises because free functions cannot be virtual.

To resolve this issue, consider adding an intermediary layer by introducing a new virtual member function within the class:

<code class="cpp">class MyClass {
public:
    virtual void print(ostream&amp; where) const;
};</code>

This virtual function provides a means to customize the output behavior in subclasses.

Next, define the operator<< as a free function with the correct parameter order:

<code class="cpp">ostream&amp; operator<< (ostream&amp; out, const MyClass&amp; mc) {
    mc.print(out);
    return out;
}</code>

This approach ensures that the operator<< has the desired parameter order while allowing for the virtual behavior to be defined in the subclasses.

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