请教C++的 union struct 应该如何理解

请问C++的 union struct 应该怎么理解?

C/C++ code

struct _zend_class_entry {.....    union {        struct {            const char *filename;            zend_uint line_start;            zend_uint line_end;            const char *doc_comment;            zend_uint doc_comment_len;        } user;        struct {            const struct _zend_function_entry *builtin_functions;            struct _zend_module_entry *module;        } internal;    } info;