Are empty constructors always called in C++? -

i have general question, may little compiler-specific. i'm interested in conditions under constructor called. specifically, in release mode/builds optimised speed, compiler-generated or empty constructor called when instantiate object?

class noconstructor   {       int member;   };    class emptyconstructor   {       int member;   };  class initconstructor   {       initconstructor()           : member(3)        {}       int member;   };  int main(int argc, _tchar* argv[])   {       noconstructor* nc = new noconstructor(); //will call generated constructor?       emptyconstructor* ec = new emptyconstructor(); //will call empty constructor?       initconstructor* ic = new initconstructor(); //this call defined constructor        emptyconstructor* ecarray = new emptyconstructor[100]; //is different? } 

i've done lot of searching, , spent time looking through generated assembly code in visual studio. can difficult follow in release builds though.

in summary: constructor called? if so, why?

i understand depend on compiler, surely there's common stance. examples/sources can cite appreciated.

when in optimizing mode, if class or structure pod (has pod types) , constructor not specified, production quality c++ compiler not skip call constructor not generate it.

if class has non-pod members who's constructor(s) have called, compiler generate default constructor calls member's constructors. - not initialize pod types. i.e. if don't initialize member explicitly, may end garbage there.

the whole thing can fancies if compiler/linker has lto.

hope helps! , make program work first, use profiler detect slow places, optimize it. premature optimization may not make code unreadable , waste tons of time, not @ all. have know optimize first.

here disassembly code in example (x86_64, gcc 4.4.5):

main:     subq    $8, %rsp     movl    $4, %edi     call    _znwm     movl    $4, %edi     movl    $0, (%rax)     call    _znwm     movl    $4, %edi     movl    $0, (%rax)     call    _znwm     movl    $400, %edi     movl    $3, (%rax)     call    _znam     xorl    %eax, %eax     addq    $8, %rsp     ret 

as can see, there no constructors called @ all. there no classes @ all, every object 4 bytes integer.

with ms compiler, ymmv. have check disassembly yourself. result should similar.

good luck!