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POWhy c++ puts same-named variables defined in separate modules into same address in memory?
 

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  1. POWhy c++ puts same-named variables defined in separate modules into same address in memory?
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    <p>Let's take header file <code>var.h</code> </p> <pre><code>#include &lt;iostream&gt; class var {public: var () {std::cout &lt;&lt; "Creating var at " &lt;&lt; this &lt;&lt; std::endl; } ~var () {std::cout &lt;&lt; "Deleting var at " &lt;&lt; this &lt;&lt; std::endl; } }; </code></pre> <p>and two source files, first <code>lib.cpp</code></p> <pre><code>#include "var.h" var A; </code></pre> <p>and second <code>app.cpp</code></p> <pre><code>#include "var.h" var A; int main () {return 0; } </code></pre> <p>then, if I attempt to compile them</p> <pre><code>g++ -c app.cpp g++ -c lib.cpp g++ -o app app.o lib.o </code></pre> <p>linker return multiply defined variable error. But, if I compile it onto shared library + main app</p> <pre><code>g++ -fPIC -c lib.cpp g++ --shared -o liblib.so lib.o g++ -fPIC -c app.cpp g++ -o app -llib -L . app.o </code></pre> <p>it links without error. However program doesn't work properly:</p> <pre><code>./app Creating var at 0x6013c0 Creating var at 0x6013c0 Deleting var at 0x6013c0 Deleting var at 0x6013c0 </code></pre> <p>so different variables was created at the same memory address! It might put into serious trouble, for example, in a case when library and application expect them to have different values (values of object fields in this case).</p> <p>if <code>class var</code> do memory allocation/deleting valgrind warns about accessing memory in recently deleted block.</p> <p>Yes, I know I could put <code>static var A;</code> instead of <code>var A;</code> and both ways to compile will work properly. My question is: why one can't use same-named variables (or even functions?) in different libraries? Library creators might know nothing about names each other use and not to be warned to use <code>static</code>. Why GNU linked doesn't warn about this conflict?</p> <p>And, BTW, could <code>dlload</code> put into same trouble?</p> <p>UPD. Thank you all for explaining about namespaces and extern, I see why same symbols are placed into same memory address, but still I can't get why no linking error or even warning about doubly defined variable is shown but wrong code produced in second case.</p>
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