c++ - Packing structure results in return of reference to local? -

i've got following code, , i'm struggling understand why packing structure causes warning (and subsequently segfault when run.) firstly code:

#include <iostream>  using namespace std;  template <typename t, std::size_t d, std::size_t s> struct __attribute__((packed)) c {   union __attribute__((packed)) valuetype {     char fill[s];     t value;   };    valuetype data_;    const t& get() const {     return data_.value;   }    friend   std::ostream& operator<<(std::ostream& str, const c& f) {     return str << f.data_.value;   }      };  template <typename t, std::size_t d> struct __attribute__((packed)) c<t, d, d> {   t value;    const t& get() const {     return value;   }    friend   std::ostream& operator<<(std::ostream& str, const c& f) {     return str << f.value;   }     };  template <typename t, std::size_t s = 0> struct __attribute__((packed)) d {   c<t, sizeof(t), s> c;   const t& get() const {     return c.get();   }    friend   std::ostream& operator<<(std::ostream& str, const d& f) {     return str << f.get();   } };  template <typename t> struct __attribute__((packed)) d<t, 0> {   t v;   const t& get() const {     return v;   }    friend   std::ostream& operator<<(std::ostream& str, const d& f) {     return str << f.get();   } };  int main(void) {     d<int64_t> d1;     cout << d1 << endl;     d<int64_t, 8> d2;     cout << d2 << endl;     } 

with packing (it's gcc specific) following (gcc 5.2.1/5.3.1, c++14 -wall):

packed.cpp: in instantiation of ‘const t& d<t, 0ul>::get() const [with t = long int]’: packed.cpp:58:16:   required ‘std::ostream& operator<<(std::ostream&, const d<long int>&)’ packed.cpp:64:13:   required here packed.cpp:53:12: warning: returning reference temporary [-wreturn-local-addr]      return v;             ^ packed.cpp: in instantiation of ‘const t& c<t, d, d>::get() const [with t = long int; long unsigned int d = 8ul]’: packed.cpp:40:18:   required ‘const t& d<t, s>::get() const [with t = long int; long unsigned int s = 8ul]’ packed.cpp:45:16:   required ‘std::ostream& operator<<(std::ostream&, const d<long int, 8ul>&)’ packed.cpp:66:13:   required here packed.cpp:27:12: warning: returning reference temporary [-wreturn-local-addr]      return value; 

if structures not packed - code compiles , runs fine (prints rubbish value - that's expected.)

so question - why packing cause return of temporary?

what you're looking pragma pack. has same effect want, more common , cross-compiler.

note: adding modified version of code pragma pack

#include <iostream>  using namespace std;  #pragma pack(1)     template <typename t, std::size_t d, std::size_t s> struct c {   union valuetype {          char fill[s];     t value;   };    valuetype data_;    const t& get() const {     return data_.value;   }    }; #pragma pack()         template <typename t, std::size_t d> struct c<t, d, d> {   t value;    const t& get() const {     return value;   }  };  template <typename t, std::size_t d> struct c<t, d, 0> {   t value;    const t& get() const {     return value;   }  };  template <typename t, std::size_t s = 0> struct d : c<t, sizeof(t), s> {   friend   std::ostream& operator<<(std::ostream& str, const d& f) {     return str << f.get();   } };  #pragma pack(1)     struct e {     d<int16_t> a;     d<int64_t, 9> b; }; #pragma pack()      int main(void) {     d<int16_t> d1;     cout << d1.get() << ' ' << sizeof(d1) << endl;     d<int64_t, 9> d2;     cout << d2.get() << ' ' << sizeof(d2) << endl;       cout << sizeof(e) << endl; }