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    <p>Add <code>line</code> as a <code>friend</code>, not just a method:</p> <pre><code> friend class line; </code></pre> <p>Other remarks:</p> <ul> <li>separate declarations from the implementations in header and implementation files.</li> <li>prefer full qualification over <code>using</code> directives (i.e. remove <code>using namespace std;</code> and use <code>std::cout</code> instead. </li> <li>prefer pass-by-reference for complex types - change <code>line(point,point);</code> to <code>line(const point&amp;, const point&amp;);</code></li> </ul> <p><strong>EDIT</strong> For educational purposes - You can't declare that specific function as friend as the code is now because there's no full definition of the <code>line</code> class. Ergo, the following is the only approach:</p> <pre><code>class point; class line { point *p1,*p2; public: line(point,point); float slope(); }; class point { int x,y; public: point(int,int); point(); friend float line::slope(); }; </code></pre> <p>You forward-declare <code>point</code> and change the <code>point</code> members in <code>line</code> to <code>point*</code> (because point isn't a complete type yet). In <code>point</code> you now have the full definition of the <code>line</code> class, so you can declare the method as friend.</p> <p>EDIT 2: For this particular scenario, it's not possible using <code>point</code> objects inside <code>line</code> because you'd need the full type. But <code>line</code> would also have to be fully defined in order to declare its member as <code>friend</code>.</p>
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    1. POProblems with forward declaration - Friend functions and line / point classes
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    1. USLuchian Grigore
    1. USLuchian Grigore
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    1. POProblems with forward declaration - Friend functions and line / point classes
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    1. COI needed this very program to explain the friend functions in particular. Friend classes are next topic, so I need to fix this for friend functions only.
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      1. USRaj Pawan Gumdal
    2. CO@Raj ok, see edit.
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      1. USLuchian Grigore
    3. CO@Raj there's some changes you need to make to the rest of the code (like replacing `.` with `->` where applicable). Also, this is just for educational purposes. In practice, you usually declare a full class as `friend`, and even that should be done seldom as it breaks encapsulation.
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