c++ - Placement new overriding existing objects -

if placement new on objects created on stack:

struct s{ s() { std::cout << "c\n";} ~s() { std::cout << "d\n";} };  int main() {         s s1[3];         for(int i=0; i< 3; ++i)                 s* news = new (&s1[i]) s(); } 

i get:

c c c c c c d d d 

so not getting destructors first 3 objects, safe? if override memory objects allocated on heap/stack , objects not have free resources in destructors still safe?

wandbox example

your output not unexpected - placement new not invoke destructor of potential object stored in memory location you're accessing. need explictly invoke destructor of object occupies memory location you're trying use in order use placement new safely.

struct s {     s()     {         std::cout << "c\n";     }     ~s()     {         std::cout << "d\n";     } };  int main() {     // 3 's' instances constructed here.     s s1[3];      // (!) need explicitly destroy existing     // instances here.     for(int = 0; < 3; ++i)     {         s1[i].~s();     }      for(int = 0; < 3; ++i)     {         // 3 's' instances constructed here.         s* news = new(&s1[i]) s();     }      // 3 's' instances destroyed here. } 

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output:

c c c d d d c c c d d d