In many places in my algorithms, I am stacking small vectors.  I need semantics where I can call functions like the following:<div><br></div><div>template&lt;int N, int M&gt;</div><div>bounded_vector&lt;double, N+M&gt; my_func(const  bounded_vector&lt;double, N&gt;&amp; v1, const  bounded_vector&lt;double, N&gt;&amp; v2)</div>
<div>{</div><div><br></div><div>  bounded_vector&lt;double, 2&gt; v1;</div><div>  bounded_vector&lt;double, 3&gt; v2;</div><div>  //.... stuff</div><div>  return another_func(stack_vector(v1, v2));</div><div>}</div><div><br>
</div><div>My stack vector (specialized for the bounded_vector) is as you would expect.</div><div><div>template&lt;class T, int N, int M&gt;</div><div>ublas::bounded_vector&lt;T, N+M&gt; stack_vec(const ublas::bounded_vector&lt;T, N&gt;&amp; v1, const ublas::bounded_vector&lt;T, M&gt;&amp; v2)</div>
<div>{</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>ublas::bounded_vector&lt;T, N+M&gt; result;</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>subrange(result, 0, v1.size()) = v1;</div>
<div><span class="Apple-tab-span" style="white-space:pre">	</span>subrange(result, v1.size(), v1.size() + v2.size()) = v2;</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>return result;</div><div>}</div>
<div><br></div><div>And a more general version, but which requires a dynamically allocated temporary:</div></div><div><br></div><div><div>template&lt;class Vector_T1, class Vector_T2&gt;</div><div>ublas::vector&lt;double&gt; stack_vec2(const ublas::vector_expression&lt;Vector_T1&gt;&amp; v1, const ublas::vector_expression&lt;Vector_T2&gt;&amp; v2)</div>
<div>{</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>std::size_t s1 = v1().size();</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>std::size_t s2 = v2().size();</div><div><br>
</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>ublas::vector&lt;double&gt; result(s1 + s2);</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre">	</span>subrange(result, 0, s1) = v1;</div>
<div><span class="Apple-tab-span" style="white-space:pre">	</span>subrange(result, s1, s1 + s2) = v2;</div><div><span class="Apple-tab-span" style="white-space:pre">	</span>return result;</div><div>}</div></div><div><br></div>
<div><br></div><div>I am terrified that I will have 1 temporary caused inside of the stack_vector.  Another calculating the transformation within another_func (because it is unlikely to be able to use the return value optimization), and then a vector copy from  the caller of my_func.  Obviously I can&#39;t get rid of them all, but it sure would be nice to get rid of some of them.</div>
<div><br></div><div><br></div><div>I never really have figured out the ublas expression templates or there limitations, but I was wondering if it is possible to return an expression template to eliminate at least some of this.  Any idea of how it would work or if it is a good idea?  Is it hard?</div>
<div><br></div><div>(I am looking for something similar with matrix stacking, but that is another day).</div><div><br></div><div><div><div><div><br></div><div>Thanks,</div><div>Jesse</div></div></div></div>