c++ - Can I have a const member function that returns *this and works with non-constant objects? -

-

if understand correctly, member function not supposed modify object should declared const let users know guarantee. now, happens when member function returns reference *this? example:

class c{ public:     c &f() const {return *this;} };

inside c::f(), this has type const c*, so, following not compile:

int main() {     c c; // non-constant object!     c.f();     return 0; }

of course, provide non-const version of c::f() work on non-constant objects:

class c{ public:     const c &f() const;     c &f(); };

however, not believe want. note non-constant version not change object, promise users not expressed... missing something?

edit: let me summarize question clarity: f() not modify object on called, declaring c &f(); without making const member misleading. on other hand, want able call f() on non-const objects. how resolve situation?

edit: comes out discussion took place in comments question based on incorrect understanding of constness of function member implies. correct understanding taking away myself is:

a member function returns non-const reference intended allow users change object through returned reference. therefore, though function not change object itself, there should no inclination declare const member!

your problem original function definition:

c &f() const {return *this;}

here return non-const reference const object, allow changing const object , dangerous, therefore it's forbidden.

if write 

const c &f() const {return *this;}

would callable both const , non-const objects , return const reference.

of course, provide non-const version of c::f() work on non-constant objects. however, not believe want.

probably want. it's way return non-const reference when called on non-const objects , keep const correctness when calling on const objects.


Comments

Post a Comment

Popular posts from this blog

c++ - Qt setGeometry: Unable to set geometry -

-
why? setgeometry: unable set geometry 22x22+320+145 on qwidgetwindow/'widgetclasswindow'. resulting geometry: 116x22+320+145 (frame: 8, 30, 8, 8, custom margin: 0, 0, 0, 0, minimum size: 22x22, maximum size: 16777215x16777215). the project is: project.pro qt += core gui greaterthan(qt_major_version, 4): qt += widgets target = untitled5 template = app sources += main.cpp\ widget.cpp headers += widget.h widget.h #ifndef widget_h #define widget_h #include <qwidget> class widget : public qwidget { q_object public: explicit widget(qwidget *parent = 0); ~widget(); private: }; #endif // widget_h widget.cpp #include "widget.h" #include <qvboxlayout> widget::widget(qwidget *parent) : qwidget(parent) { qvboxlayout *vlayout = new qvboxlayout(this); } widget::~widget() { } main.cpp #include "widget.h" #include <qapplication> int main(int argc, char *argv[]) { qapplication a...
Read more

python - argument must be rect style object - Pygame -

-
disclaimer: know other people on site have had error it's been in different contexts , fixes don't seem work here. hey stackoverflow, i've been making sideways space invaders game, ship follows mouse's y position stays on left, , clicking shoots bullet. enemies spawn randomly right , fly left, , objects removed once each side of screen. works fine, when tried implement collision detection see when shot hits enemy, error: typeerror: argument must rect style object. it points line "return hitbox.colliderect(target.rect)". try , work out causing made 'print' lines below see 'target.rect' (and whether proper rect object), , seemed same style shot's own rect, i'm lost why colliderect doesn't accept it. i've annotated pretty everything, sorry if it's insult intelligence should make things easier. class causes problems: class gameobject: def __init__(self, image, height, speed): self.speed = speed self.image...
Read more

How to resolve Delphi error: Incompatible types: 'PWideChar' and 'Pointer' -

-
i have piece of code delphi 7: var lprgndata: prgndata; pc: pchar; pr: prect; ... pc := @(lprgndata^.buffer[0]); in delphi xe4 gives following compile error: incompatible types: 'pwidechar' , 'pointer' how should code updated work correctly in xe4? thanks whether or not compiles depends upon setting of type-checked pointers option . have enabled option excellent decision. doing results in stricter type checking. with type-checked pointers disabled, code compile. type-checked pointers enabled, code not compile, want because code not valid. now, on types in question. defined in windows unit this: type prgndata = ^trgndata; {$externalsym _rgndata} _rgndata = record rdh: trgndataheader; buffer: array[0..0] of byte; reserved: array[0..2] of byte; end; trgndata = _rgndata; {$externalsym rgndata} rgndata = _rgndata; the benefit of using type-checked pointers compiler can tell doing not valid. knows lprgndat...
Read more