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POPointers to members of members or an alternative?
 

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  1. POPointers to members of members or an alternative?
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    <p>Say I have a template function which sets data in a struct via pointers-to-member:</p> <pre><code>struct S { double d; unsigned int u; }; struct A { S s, s2; float f; int i; }; template&lt;typename Stream, typename Class, typename Type&gt; void read(Stream&amp; s, Class&amp; out, Type Class::*member) { s &gt;&gt; out.*member; } </code></pre> <p>Now for regular members like <code>f</code> and <code>a</code> I can use this function on an arbitrary instance of <code>struct A</code>:</p> <pre><code>#include &lt;sstream&gt; int main(int, char**) { std::stringstream stream; A a; stream.clear(); stream.str("1"); read(stream, a, &amp;A::i); stream.clear(); stream.str("1.5"); read(stream, a, &amp;A::f); stream.clear(); stream.str("2"); printf("%i\n", a.i); printf("%f\n", a.f); } </code></pre> <p>But is there any way to access members of <code>A::s</code> or <code>A::s2</code> in a similar fashion <em>without</em> writing a new, effectively identical function for every composite member (<code>s</code>, <code>s2</code>, etc.)? Pointers to members of members do not exist to my knowledge, but is there an alternative solution? (It has to work on arbitrary instances of a <code>struct</code> or <code>class</code>, similarly to how the pointer-to-member solution above works and should not require additional methods in <code>A</code> or <code>S</code>).</p> <p>To clarify the question somewhat (see also the comments in <a href="https://stackoverflow.com/a/16928366/2361316">this answer</a>): In the real code (a config file reader), the caller does a lookup in a map with a first string and doesn't actually know what is currently being read (using the second string), as the pointer-to-member parameter is bound. Also the map is initialized once and used for multiple instances of <code>A</code>, therefore the pointers-to-members.</p> <p>Please note the c++03 tag. Therefore c++11 lambdas are not an option.</p>
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    1. COThere is no such member pointer. [What are you trying to do](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)? This is an awfully weird way to populate a class.
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    2. CO@GManNickG added some clarification (and I already stated in the question itself that I know that there is no such pointer...)
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