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copied!<p>Consider a variation of your class <code>A</code>.</p> <pre><code>class A { public: void Foo() const; void Moo(); private: int m_nState; // Could add mutable keyword if desired int GetState() const { return m_nState; } void SetState(int val) { m_nState = val; } }; const A *A1 = new A(); A *A2 = new A(); A1->Foo(); // OK A2->Foo(); // OK A1->Moo(); // Error - Not allowed to call non-const function on const object instance A2->Moo(); // OK </code></pre> <p>The <code>const</code> keyword on a function declaration indicates to the compiler that the function is contractually obligated not to modify the state of <code>A</code>. Thus you are unable to call non-<code>const</code> functions within <code>A::Foo</code> nor change the value of member variables.</p> <p>To illustrate, <em>Foo()</em> may not invoke <code>A::SetState</code> as it is declared non-<code>const</code>, <code>A::GetState</code> however is ok because it is explicitly declared <code>const</code>. The member <code>m_nState</code> may not be changed either unless declared with the keyword <code>mutable</code>.</p> <p>One example of this usage of <code>const</code> is for 'getter' functions to obtain the value of member variables.</p> <blockquote> <p>@1800 Information: I forgot about mutable!</p> </blockquote> <p>The <code>mutable</code> keyword instructs the compiler to accept modifications to the member variable which would otherwise cause a compiler error. It is used when the function needs to modify state but the object is considered logically consistent (constant) regardless of the modification.</p>

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