I now know this is referred to as the "struct hack." This really is undefined behavior in the C++11 standard... there was a proposal to make the struct hack valid for C++11:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n791.htm <https://www.google.com/url?sa=D&q=http%3A%2F%2Fwww.open-std.org%2Fjtc1%2Fsc22%2Fwg14%2Fwww%2Fdocs%2Fn791.htm> >From another google engineer: 5.2.1 tells you that x[y] is identical to *(x + y). 5.7.5 tells you that x + y has undefined behavior if the result points outside of the array. >From the C++11 standard: 5.2.1 Subscripting [expr.sub] 1 A postfix expression followed by an expression in square brackets is a postfix expression. One of the expressions shall have the type “pointer to T” and the other shall have unscoped enumeration or integral type. The result is an lvalue of type “T.” The type “T” shall be a completely-defined object type.62 The expression E1[E2] is identical (by definition) to *((E1)+(E2)) [ Note: see 5.3 and 5.7 for details of * and + and 8.3.4 for details of arrays. — end note ] 5.7 Additive operators ... 5.7.5 When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integral expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i + n-th and i − n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. On Tue, May 16, 2017 at 7:35 AM, Even Rouault <[email protected]> wrote: > On mardi 16 mai 2017 07:01:53 CEST Kurt Schwehr wrote: > > > w.r.t. https://trac.osgeo.org/gdal/changeset/38405 > > > > > > Exactly why is it okay to resize this fixed size structure? > > > > Well, this is a good old trick of grey-beard C programmers, isn't it ? Not > sure what the C/C++ standards says about that, but it works pretty well in > practice. > > > > Some explanations at: > > http://stackoverflow.com/a/3123792 > > > > -- > > Spatialys - Geospatial professional services > > http://www.spatialys.com >
_______________________________________________ gdal-dev mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/gdal-dev
