unresolved externals of creating a static instance in C++
<p>I have produced this C++ code:</p>

<pre><code>class TestInstance
{
    public:
        TestInstance();
        ~TestInstance();
        static TestInstance&amp; GetInstance();

    private:
        static TestInstance* testInstance;
};
</code></pre>

<p>But I got this error when compiling:</p>

<blockquote>
  <p>error LNK2001: unresolved external symbol "private: static class
  TestInstance* TestInstance::testInstance"
  (?testInstance@TestInstance@@0PAV1@A)</p>
  
  <p>fatal error LNK1120: 1 unresolved externals</p>
</blockquote>

<p>Any idea?</p>

<p>Thanks in advance. </p>
