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/GCC 7

6.18 Structures with No Members

GCC permits a C structure to have no members:

struct empty {
};

The structure has size zero. In C++, empty structures are part of the language. G++ treats empty structures as if they had a single member of type char.

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https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/Empty-Structures.html