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POCompilation error when using pointer to member function
 

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  1. POCompilation error when using pointer to member function
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    <p>This is an example of State machine design pattern .. i am facing some issue in it please explain and give solution for it .</p> <p>This is the code:</p> <pre><code>#include &lt;iostream&gt; using namespace std; class Machine { class State *current; public: Machine(); void setCurrent(State *s) { current = s; } void on(); void off(); }; class State { public: virtual void on(Machine *m) { cout &lt;&lt; " already ON\n"; } virtual void off(Machine *m) { cout &lt;&lt; " already OFF\n"; } }; void Machine::on() { current-&gt;on(this); } void Machine::off() { current-&gt;off(this); } class ON: public State { public: ON() { cout &lt;&lt; " ON-ctor "; }; ~ON() { cout &lt;&lt; " dtor-ON\n"; }; void off(Machine *m); }; class OFF: public State { public: OFF() { cout &lt;&lt; " OFF-ctor "; }; ~OFF() { cout &lt;&lt; " dtor-OFF\n"; }; void on(Machine *m) { cout &lt;&lt; " going from OFF to ON"; m-&gt;setCurrent(new ON()); delete this; } }; void ON::off(Machine *m) { cout &lt;&lt; " going from ON to OFF"; m-&gt;setCurrent(new OFF()); delete this; } Machine::Machine() { current = new OFF(); cout &lt;&lt; '\n'; } int main() { void(Machine:: *ptrs[])() = { Machine::off, Machine::on }; Machine fsm; int num; while (1) { cout &lt;&lt; "Enter 0/1: "; cin &gt;&gt; num; (fsm. *ptrs[num])(); } } </code></pre> <p>This is the error:</p> <blockquote> <p>prog.cpp: In function ‘int main()’:<br> prog.cpp:89: error: invalid use of non-static member function ‘void Machine::off()’<br> prog.cpp:89: error: invalid use of non-static member function ‘void Machine::on()’<br> prog.cpp:97: error: expected unqualified-id before ‘*’ token</p> </blockquote>
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    1. COThere are so many bizarre things here that I don't know where to start. For one, listen to your compiler... You're trying to use the off and on methods like they're static. You need an instance of a Machine object in order to use them.
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      1. USScoPi
    2. CO@ScoPi He has one: `fsm`. Its the initializer list and invoker that is broken (the latter literally by a single space).
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    3. COMy recommendation is just not to write C++ that looks like this.
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