title: Unresolved external symbol in object files
body: <p>During coding in Visual Studio I got an unresolved external symbol error
  and I've got no idea what to do. I don't know what's wrong.
  Could you please decipher me? Where should I be looking for what kind of errors?</p>
  
  <pre><code>1&gt;Form.obj : error LNK2019: unresolved external symbol "public: class Field * __thiscall Field::addField(class Field *)" (?addField@Field@@QAEPAV1@PAV1@@Z) referenced in function "public: void __thiscall Form::parse(class std::basic_stringstream&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt; &amp;)" (?parse@Form@@QAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
  1&gt;Form.obj : error LNK2019: unresolved external symbol "public: virtual void __thiscall Field::parse(class std::basic_stringstream&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt; &amp;)" (?parse@Field@@UAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: __thiscall InputField::InputField(class std::basic_stringstream&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt; &amp;)" (??0InputField@@QAE@AAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
  1&gt;Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::prompt(void)" (?prompt@Field@@UAEXXZ)
  1&gt;Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt; __thiscall Field::getName(void)" (?getName@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)
  1&gt;Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string&lt;char,struct std::char_traits&lt;char&gt;,class std::allocator&lt;char&gt; &gt; __thiscall Field::getType(void)" (?getType@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)
  1&gt;Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::describe(void)" (?describe@Field@@UAEXXZ)
  1&gt;C:\Users\tomy\Documents\Visual Studio 2010\Projects\zapoctovkac++\Debug\zapoctovkac++.exe : fatal error LNK1120: 6 unresolved externals
  </code></pre>