c++ - Writing a Default Constructor Forces Zero-Initialization? -

these class definitions:

class foo{     int _ent; public:     void printent() const{cout << _ent << ' ';} };  class bar{     foo _foo; public:     void printent() const{_foo.printent();} }; 

and test code:

char* buf = new char[sizeof(foo) + sizeof(foo) + sizeof(bar)];  fill(buf, buf + sizeof(foo) + sizeof(foo) + sizeof(bar), 'j');  cout << ((int*)buf)[0] << ' ' << ((int*)buf)[1] << ' ' << ((int*)buf)[2] << endl;  foo* first = new (buf) foo; foo* second = new (buf + sizeof(foo)) foo(); bar* third = new (buf + sizeof(foo) * 2) bar;  first->printent(); second->printent(); third->printent(); 

my output is:

1246382666 1246382666 1246382666
1246382666 0 1246382666

but if add public default ctor foo: foo() : _ent(0) {}

my output becomes:

1246382666 1246382666 1246382666
0 0 0

is correct behavior? should adding own default ctor remove possibility of default initialization?

i'm running code on gcc 4.8.1 if matters. results should dependable because i'm running in debug , asserting: assert(sizeof(foo) == sizeof(int) && sizeof(bar) == sizeof(int));

once provide constructor type, invoked, both default initialization , value initialization. it's fundamental principle of language. once define foo::foo(), called time construct foo; if there default constructor, invoked, in case of default initialization. behavior seeing correct.

edit:

default initialization explained §8.5/7, in particular:

to default-initialize object of type t means:

— if t (possibly cv-qualified) class type (clause 9), default constructor (12.1) t called [...]

in case, you'll want @ how compiler generates default constructor if none provided, §12.1/4; in particular, generated default constructor invokes default constructor of base classes or members.

value initialization in §8.5/8. default initialization preceded 0 initialization, default initialization doesn't still finds 0 initialized.

more fundamentally, however: in case, fundamental principle of c++ involved, dating long before first standard: if provide constructor object, will used. without doing sorts of strange pointer casts, impossible object without being constructed. standard describes how occurs, , covers lot of other special cases, basic principle has been there start (and proposal cause not respected in standard bound fail).