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POAccessing struct members with array subscript operator
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copied!<p>Let have a type T and a struct having ONLY uniform elements of T type.</p> <pre><code>struct Foo { T one, T two, T three }; </code></pre> <p>I'd like to access them in fallowing way:</p> <pre><code>struct Foo { T one, T two, T three T &operator [] (int i) { return *(T*)((size_t)this + i * cpp_offsetof(Foo, two)); } }; </code></pre> <p>where <code>cpp_offsetof</code> macro (it is considered to be correct) is:</p> <pre><code>#define cpp_offsetof(s, m) (((size_t)&reinterpret_cast<const volatile char&>((((s*)(char*)8)->m))) - 8) </code></pre> <p>The C++ standard doesn't guarantee it, but can we assume that members are distanced by a fixed offset and above is correct, cross-platform solution?</p> <p><br> 100% compatible solution would be:</p> <pre><code>struct Foo { T one, T two, T three T &operator [] (int i) { const size_t offsets[] = { cpp_offsetof(Foo, one), cpp_offsetof(Foo, two), cpp_offsetof(Foo, three) }; return *(T*)((size_t)this + offsets[i]); } }; </code></pre> <p><em>[edit]standard, compliant and faster version was presented by</em> <a href="https://stackoverflow.com/questions/3178347/accessing-struct-members-with-array-subscript-operator/3178461#3178461">snk_kid using pointers to data members</a><em>[/edit]</em><br> but it requires extra lookup table which I'm trying to avoid.</p> <p>//EDIT<br> And one more. I cannot use just an array and constants to index these fields, they have to be named fields of a struct (some macro requires that).</p> <p>//EDIT2<br> Why those have to be named fields of a struct? What is the macro? It is settings system of a bigger project. Simplifying it's sth like this:</p> <pre><code>struct Foo { int one; int two; } foo; struct Setting { void *obj, size_t filed_offset, const char *name, FieldType type } #define SETTING(CLASS, OBJ, FIELD, TYPE) { OBJ, cpp_offsetof(CLASS, FIELD), #OBJ #FIELD, TYPE } Setting settings[] = { SETTING(Foo, foo, one, INT_FIELD), SETTING(Foo, foo, two, INT_FIELD) }; </code></pre> <p>And once again: I'm not looking form 100% compatible solution but 99%. I'm asking if we can expect that some compilers will put non-uniform padding between uniform fields.</p>

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