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copied!<p>A copy constructor is a constructor with a specific signature. It takes a single parameter, of const reference to the same type being constructed. So for a <code>class Foo</code>, the copy constructor looks like this:</p> <pre><code>Foo::Foo(const Foo&) </code></pre> <p>There are 2 other kinds of constructors: default and convert. The default constructor takes no parameters (or can be called as if it took no parameters, eg, all parameters have defaults):</p> <pre><code>Foo::Foo() </code></pre> <p>...and the convert destructor is basically anything else. For example:</p> <pre><code>Foo::Foo(const Bar&) </code></pre> <hr> <blockquote> <p>why is the copy constructor being called?</p> </blockquote> <p>In your case, you're calling a <code>RailwayLineLink</code> constructor but passing in a const reference to a <code>RailwayLine</code>, so you're actually calling a convert constructor, not a copy constructor.</p> <p>The convert constructor is called because you called it:</p> <pre><code>new RailwayLineLink(data); </code></pre> <p>This constructs a new <code>RailwayLineLink</code> object, passing <code>data</code> as the one and only parameter to the constructor.</p> <p>If you have an empty convert constructor, that looks something like this:</p> <pre><code>RailwayLineLink::RailwayLineLink(const RailwayLine&) { } </code></pre> <p>...then all of the members of the new copy will be default constructed, and probably contain garbage.</p> <p>Typically when you implement a copy constructor, you would want to do some kind of memberwise copy from the original to the copy, like this:</p> <pre><code>RailwayLineLink::RailwayLineLink(const RailwayLine& rhs) : value_(rhs.value_) { } </code></pre>

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