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POstruct vs class as STL functor when using not2
 

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  1. POstruct vs class as STL functor when using not2
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    <p>Studing STL I have written a a simple program to test functors and modifiers. My question is about the difference aon using CLASS or STRUCT to write a functor and try to operate on it with function adaptors. As far as I understand in C++ the difference beetween CLASS and STRUCT is that in the last case the members are public by default. This is also what I read many times in the answers in this site. So please explain me why this short piece of code will fail to compile even if I declared all members ( just a function overloading () ) public when I try to use the not2 modifier. (I have not tried other modifiers e.g. binders yet)</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;functional&gt; #include &lt;algorithm&gt; using namespace std; template &lt;class T&gt; void print (T i) { cout &lt;&lt; " " &lt;&lt; i; } // In the manual I read: // "In C++, a structure is the same as a class except that its members are public by default." // So if I declare all members public it should work.... template &lt;class T&gt; class mystruct : binary_function&lt;T ,T ,bool&gt; { public : bool operator() (T i,T j) const { return i&lt;j; } }; template &lt;class T&gt; class generatore { public: generatore (T start = 0, T stp = 1) : current(start), step(stp) { } T operator() () { return current+=step; } private: T current; T step; }; int main () { vector&lt;int&gt; first(10); generate(first.begin(), first.end(), generatore&lt;int&gt;(10,10) ); first.resize(first.size()*2); generate(first.begin()+first.size()/2, first.end(), generatore&lt;int&gt;(1,17) ); cout &lt;&lt; "\nfirst :"; for_each (first.begin(), first.end(), print&lt;int&gt;); cout &lt;&lt; "\nFORWARD SORT :"; sort(first.begin(),first.end(),mystruct&lt;int&gt;()); // OK ! even with CLASS for_each (first.begin(), first.end(), print&lt;int&gt;); sort(first.begin(),first.end(),not2(mystruct&lt;int&gt;())); // &lt;--- THIS LINE WILL NOT COMPILE IF I USE CLASS INSTEAD OF STRUCT cout &lt;&lt; "\nBACKWARD SORT :"; for_each (first.begin(), first.end(), print&lt;int&gt;); cout &lt;&lt; endl; } </code></pre> <p>Everithing runs as expected if I use:</p> <pre><code>struct mystruct : binary_function&lt;T ,T ,bool&gt; { public : bool operator() (T i,T j) const { return i&lt;j; } }; </code></pre> <p>Part of the error message I obtain is:</p> <blockquote> <p>g++ struct.cpp<br> /usr/include/c++/4.2.1/bits/stl_function.h:<br> In instantiation of ‘std::binary_negate >’:<br> struct.cpp:52: instantiated from here<br> /usr/include/c++/4.2.1/bits/stl_function.h:116:<br> error: ‘typedef int std::binary_function::first_argument_type’ is inaccessible<br> /usr/include/c++/4.2.1/bits/stl_function.h:338:<br> error: within this context<br> /usr/include/c++/4.2.1/bits/stl_function.h:119:<br> error: ‘typedef int std::binary_function::second_argument_type’ is inaccessible .... </p> </blockquote> <p>Seems that at least in this case a struct is not equivalent to a class with public members, but why ?</p>
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