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copied!<p>The problem is calling a non-<code>const</code> function <code>test2.test()</code> on a <code>const</code> object <code>test2</code> from <code>testing::test1</code>.</p> <p><code>testing::test1</code> gets <code>test2</code> as a parameter <code>const testing &test2</code>. So within <code>testing::test1</code>, <code>test2const</code>. Then in the first line of the function:</p> <pre><code>test2.test() </code></pre> <p>The <code>testing::test</code> function is called on <code>test2</code>. That function is not declared with <code>const</code> at the signature end, so it may modify the object it is called on (the <code>this</code> pointer implicitly passed to it), and even though it does not, the compiler assumes so. By letting you call it there, the compiler would let you modify a <code>const</code> variable without an explicit cast, which C++ is not supposed to allow. <strong>Therefore to explain the error message</strong>:</p> <pre><code>test.cpp:15: error: passing ‘const testing’ as ‘this’ argument of ‘int testing::test()’ discards qualifiers </code></pre> <p><code>this</code> refers to the object the member function (<code>testing::test</code>) operates on, and in this case it is not <code>const</code>, because <code>testing::test</code> was not declared with <code>const</code>, and thus the mismatch is detected when trying to make a non-<code>const</code> pointer (<code>this</code>) refer to a <code>const</code> object (<code>testing</code>), ignoring the <code>const</code> <em>qualifier</em>.</p> <p><strong>To solve this</strong>, decide whether the <code>testing::test</code> function should ever need to modify the object it is called on (the way it is written now it does not, as all it does is <code>return 1</code>, however that may change, so you need to think at what its intended functionality is). If it should, then obviously calling it on a <code>const</code> object is bad, although you can use <code>const_cast</code> to ask the compiler to override that, but this is dangerous. If it should not, then mark it <code>const</code>, so that it can be called on <code>const</code> objects as well:</p> <pre><code>class testing{ int test1() const; // ... } int testing::test() const { // ... } </code></pre>

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