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POWhy can I not define a member function in a class if that function is to be linked from another translation unit?
 

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  1. POWhy can I not define a member function in a class if that function is to be linked from another translation unit?
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    <p>I am a bit of a newbie in C++, but I just stumbled on the following. </p> <p>If I have these files:</p> <p>myclass.hpp:</p> <pre><code>class myclass { public: myclass(); void barf(); }; </code></pre> <p>mymain.cpp:</p> <pre><code>#include "myclass.hpp" int main() { myclass m; m.barf(); return 0; } </code></pre> <p>And I use this implementation of myclass:</p> <p>myclassA.cpp:</p> <pre><code>#include &lt;iostream&gt; using namespace std; class myclass { // or include myclass.hpp, it both works fine public: myclass(); void barf(); }; myclass::myclass() { } //empty void myclass::barf() { cout &lt;&lt; "barfing\n"; } </code></pre> <p>then everything is OK. But if I use this implementation of myclass, which is exactly the same except the members are defined inside the class definition, I get a linking error:</p> <p>myclassB.cpp:</p> <pre><code>#include &lt;iostream&gt; using namespace std; class myclass { public: myclass() { } void barf() { cout &lt;&lt; "barfing\n"; } }; </code></pre> <p>The error I get:</p> <pre><code>$ g++ myclassB.cpp mymain.cpp /tmp/cc4DTnDl.o: In function `main': mymain.cpp:(.text+0xd): undefined reference to `myclass::myclass()' mymain.cpp:(.text+0x16): undefined reference to `myclass::barf()' collect2: ld returned 1 exit status </code></pre> <p>Apparently the object file built from myclassB.cpp doesn't export the member functions. Why aren't both of these implementations behaving te same? Is there some gotcha rule in C++ that says member definitions inside class definitions are not globally visible, but they are if they are defined outside of the class? </p>
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