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POC++ comma operator overloading and vector of references
 

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  1. POC++ comma operator overloading and vector of references
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    <p>There are many posts (questions) about comma operator overloading in c++. Most of the answers advice to DO NOT use comma operator overloading. I want to write a c++ library with a syntax very similar to Matlab language. The basic object is a Matrix MX. I want to be able to make the end-user of the library to write expressions like :</p> <pre><code>MX a = b(i);// get b elements at indices i b(i,j)= a; // set elements of b at indices i,j. </code></pre> <p>I have an idea about how to do make the setter &amp; the getter work as written above using a Proxy Class that save a pointer to the MX object and save also the indices i,j objects. For example b(i,j) will create a Proxy object ProxMX(b,i,j). Then we define a method to assign ProxMX to MX and visversa (using operator =) that do the hard job of getting &amp; setting the elements of b.</p> <p>I need help to make function calls like :</p> <pre><code>(x,y,z)= ff(a,b,c) </code></pre> <p>Where a, b, c are inputs argments (MX objects) and x, y , z are output argments . if the above syntaxe is not not possible i can think about a syntaxe like :</p> <pre><code>ff((a,b,c), (x,y,z) ) </code></pre> <p>I started writing this test code:</p> <pre><code>#include "stdafx.h" #include &lt;iostream&gt; #include &lt;fstream&gt; #include &lt;vector&gt; using namespace std; class MX {// Matrix public: MX(double va) { elem=va;// only one double for the moment to test the syntaxe } MX &amp;operator ()(MX idx){ // get &amp; set MX(i) return *this;// }; MX &amp;operator ()(MX idx1,MX idx2) { // get set MX(i,j) return *this; } ; friend ostream &amp;operator&lt;&lt;(ostream &amp;stream, MX a); double elem; }; ostream &amp;operator&lt;&lt;(ostream &amp;stream, MX a) { stream &lt;&lt; a.elem ; return stream; } typedef vector&lt;const MX &gt; MXR; class ArgList { // Proxy public: //ArgList(const MX&amp; a){ // data.push_back(a); //} ArgList() {}; ArgList&amp; operator , (const MX &amp;a){ data.push_back(a); return *this; } ArgList&amp; operator =(ArgList&amp; ins){ for (int i=0 ;i &lt;ins.data.size();i++) (this-&gt;data[i]).elem=ins.data[i].elem; return *this; }; MXR data; }; ArgList operator , (const MX&amp; a, const MX&amp; b){ ArgList out; out.data.push_back(a); out.data.push_back(b); return out; } ArgList ff(ArgList argins) { int n = argins.data.size(); MX a= argins.data[0]; MX b= argins.data[1]; MX x(a.elem+1.0); MX y(b.elem+10.0); MX z(a.elem+b.elem); return ( x, y , z); } void gg(ArgList argins, ArgList &amp;argout) { int n = argins.data.size(); MX a= argins.data[0]; MX b= argins.data[1]; MX x(a.elem+1.0); MX y(b.elem+10.0); MX z(a.elem+b.elem); argout = ( x, y , z); } int _tmain(int argc, _TCHAR* argv[]) { MX a(1.0);MX b(2.0);MX c(3.0); MX aa = a(MX(3.0)); aa(MX(2.0),MX(3.0))=MX(5.0); cout &lt;&lt; "a=" &lt;&lt; a &lt;&lt; ",b=" &lt;&lt; b &lt;&lt; ",c=" &lt;&lt; c &lt;&lt; endl; MX x(0.0);MX y(0.0);MX z(0.0); cout &lt;&lt; "x=" &lt;&lt; x &lt;&lt; ",y=" &lt;&lt; y &lt;&lt; ",z=" &lt;&lt; z &lt;&lt; endl; (x,y,z)= ff((a , b, c )); cout &lt;&lt; "x=" &lt;&lt; x &lt;&lt; ",y=" &lt;&lt; y &lt;&lt; ",z=" &lt;&lt; z &lt;&lt; endl; gg((a,b,c) , (x,y,z)); cout &lt;&lt; "x=" &lt;&lt; x &lt;&lt; ",y=" &lt;&lt; y &lt;&lt; ",z=" &lt;&lt; z &lt;&lt; endl; return 0; } </code></pre> <p>This code compiles &amp; runs without errors using VS2010 Express :). But as expected it does not give the expected results because I need to save references to variables in the ArgList instead of making a copy of objects to vector. I know that we cannot use std::vector as a container of references to objects.</p> <p>Any help in order to make theses expressions writables and working in c++ code. Thanks.</p>
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    1. COCan you please elaborate more what your actual problem? That it doesn't behave as you expect it to? That you need some other container than `std::vector`? Something else?
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    2. COI need help to make expressions like (x,y,z)=ff((a,b,c)) work to call a function in c++. My first test code let me think that a container that accepte reference to ojects could be the right direction. But I'm not sur.
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    3. COThere are a number of existing open-source linear algebra libraries for C++ that aim to provide MATLAB-like syntax, such as [Armadillo](http://arma.sourceforge.net/) and [Blitz++](http://www.oonumerics.org/blitz/). You could have a look at their source.
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