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copied!<h1>Problem</h1> <p>You should not <code>#pragma once</code> in source files, and you shall not <code>#include</code> source files into each other. </p> <p>The problem in your case (probably, but your test-case is not exact): You <code>#include "Foo.cpp"</code>, but at that moment, the compiler has already triggered upon <code>#pragma once</code> long before when compiling <code>Foo.cpp</code>.</p> <p>In simple terms: <strong>The compiler has been confused because you mix up source-files with header-files.</strong></p> <h1>Canonical modus operandi</h1> <ul> <li><p>A header/source pair for each class</p> <ul> <li><p>the header generally only has declarations, e.g. <code>class Foo { void someMethod(); };</code></p></li> <li><p>the source includes the header and defines the class members, e.g. <code>void Foo::someMethod() {...}</code></p></li> </ul></li> <li><p>Other translation units ("source files") can then happily <code>#include</code> the header</p></li> </ul> <h1>Example</h1> <p><strong>foo.h</strong></p> <pre><code>#ifndef FOO_H #define FOO_H #pragma once // note: compiler extension, do as // you prefer, advantage of #pragma // once is near to nothing on today's // compilers, though class Foo { public: void someMethod(); }; #endif </code></pre> <p><strong>foo.cpp</strong></p> <pre><code>#include "foo.h" void Foo::someMethod() { // do something } </code></pre> <p>Then, you can use this in other translation units like this:</p> <p><strong>main.cpp</strong></p> <pre><code>#include "foo.h" int main () { Foo foo; foo.doSomething(); } </code></pre>

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