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    <p>Put yourself in the compiler's position: when you forward declare a type, all the compiler knows is that this type exists; it knows nothing about its size, members, or methods. This is why it's called an <em>incomplete type</em>. Therefore, you cannot use the type to declare a member, or a base class, since the compiler would need to know the layout of the type.</p> <p>Assuming the following forward declaration.</p> <pre><code>class X; </code></pre> <p>Here's what you can and cannot do.</p> <p><strong>What you can do with an incomplete type:</strong></p> <ul> <li><p>Declare a member to be a pointer or a reference to the incomplete type:</p> <pre><code>class Foo { X *pt; X &amp;pt; }; </code></pre></li> <li><p><em>Declare</em> functions or methods which accept/return incomplete types:</p> <pre><code>void f1(X); X f2(); </code></pre></li> <li><p><em>Define</em> functions or methods which accept/return pointers/references to the incomplete type (but without using its members):</p> <pre><code>void f3(X*, X&amp;) {} X&amp; f4() {} X* f5() {} </code></pre></li> </ul> <p><strong>What you cannot do with an incomplete type:</strong></p> <ul> <li><p>Use it as a base class</p> <pre><code>class Foo : X {} // compiler error! </code></pre></li> <li><p>Use it to declare a member:</p> <pre><code>class Foo { X m; // compiler error! }; </code></pre></li> <li><p><em>Define</em> functions or methods using this type</p> <pre><code>void f1(X x) {} // compiler error! X f2() {} // compiler error! </code></pre></li> <li><p>Use its methods or fields, in fact trying to dereference a variable with incomplete type</p> <pre><code>class Foo { X *m; void method() { m-&gt;someMethod(); // compiler error! int i = m-&gt;someField; // compiler error! } }; </code></pre></li> </ul> <hr> <p>When it comes to templates, there is no absolute rule: whether you can use an incomplete type as a template parameter is dependent on the way the type is used in the template.</p> <p>For instance, <code>std::vector&lt;T&gt;</code> requires its parameter to be a complete type, while <code>boost::container::vector&lt;T&gt;</code> does not. Sometimes, a complete type is required only if you use certain member functions; <a href="https://stackoverflow.com/questions/6012157/is-stdunique-ptrt-required-to-know-the-full-definition-of-t">this is the case for <code>std::unique_ptr&lt;T&gt;</code></a>, for example.</p> <p>A well-documented template should indicate in its documentation all the requirements of its parameters, including whether they need to be complete types or not.</p>
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    1. POWhen can I use a forward declaration?
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    1. USLuc Touraille
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    1. POWhen can I use a forward declaration?
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    1. COif we include the class's header file , will we be able to use it as members type and etc right? If we include only the class header file will it be enough ? and thus would then the forward declaration be unnecessary ?
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    2. COGreat answer but please see mine below for the engineering point on which I disagree. In short, if you don't include headers for incomplete types you accept or return, you force an invisible dependency on the consumer of your header having to know which others they need.
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      1. USAndy Dent
    3. CO@AndyDent: True, but the consumer of the header only needs to include the dependencies (s)he actually uses, so this follows the C++ principle of "you only pay for what you use". But indeed, it can be inconvenient for the user who would expect the header to be standalone.
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