c++ - What is an undefined reference/unresolved external symbol error and how do I fix it? -


what undefined reference/unresolved external symbol errors? common causes , how fix/prevent them?

feel free edit/add own.

compiling c++ program takes place in several steps, specified 2.2 (credits keith thompson reference):

the precedence among syntax rules of translation specified following phases [see footnote].

  1. physical source file characters mapped, in implementation-defined manner, basic source character set (introducing new-line characters end-of-line indicators) if necessary. [snip]
  2. each instance of backslash character (\) followed new-line character deleted, splicing physical source lines form logical source lines. [snip]
  3. the source file decomposed preprocessing tokens (2.5) , sequences of white-space characters (including comments). [snip]
  4. preprocessing directives executed, macro invocations expanded, , _pragma unary operator expressions executed. [snip]
  5. each source character set member in character literal or string literal, each escape sequence , universal-character-name in character literal or non-raw string literal, converted corresponding member of execution character set; [snip]
  6. adjacent string literal tokens concatenated.
  7. white-space characters separating tokens no longer significant. each preprocessing token converted token. (2.7). resulting tokens syntactically , semantically analyzed , translated translation unit. [snip]
  8. translated translation units , instantiation units combined follows: [snip]
  9. all external entity references resolved. library components linked satisfy external references entities not defined in current translation. such translator output collected program image contains information needed execution in execution environment. (emphasis mine)

[footnote] implementations must behave if these separate phases occur, although in practice different phases might folded together.

the specified errors occur during last stage of compilation, commonly referred linking. means compiled bunch of implementation files object files or libraries , want them work together.

say defined symbol a in a.cpp. now, b.cpp declared symbol , used it. before linking, assumes that symbol defined somewhere, doesn't yet care where. linking phase responsible finding symbol , correctly linking b.cpp (well, object or library uses it).

if you're using msvs, you'll see projects generate .lib files. these contain table of exported symbols, , table of imported symbols. imported symbols resolved against libraries link against, , exported symbols provided libraries use .lib (if any).

similar mechanism exist other compilers/platforms.

common error messages error lnk2001, error lnk1120, error lnk2019 msvs , undefined reference symbolname gcc.

the code:

struct x {    virtual void foo(); }; struct y : x {    void foo() {} }; struct {    virtual ~a() = 0; }; struct b: {    virtual ~b(){} }; extern int x; void foo(); int main() {    x = 0;    foo();    y y;    b b; }

would generate following errors gcc:

/home/abisfw/ccvvuhox.o: in function `main': prog.cpp:(.text+0x10): undefined reference `x' prog.cpp:(.text+0x19): undefined reference `foo()' prog.cpp:(.text+0x2d): undefined reference `a::~a()' /home/abisfw/ccvvuhox.o: in function `b::~b()': prog.cpp:(.text._zn1bd1ev[b::~b()]+0xb): undefined reference `a::~a()' /home/abisfw/ccvvuhox.o: in function `b::~b()': prog.cpp:(.text._zn1bd0ev[b::~b()]+0x12): undefined reference `a::~a()' /home/abisfw/ccvvuhox.o:(.rodata._zti1y[typeinfo y]+0x8): undefined reference `typeinfo x' /home/abisfw/ccvvuhox.o:(.rodata._zti1b[typeinfo b]+0x8): undefined reference `typeinfo a' collect2: ld returned 1 exit status

and similar errors msvs:

1>test2.obj : error lnk2001: unresolved external symbol "void __cdecl foo(void)" (?foo@@yaxxz) 1>test2.obj : error lnk2001: unresolved external symbol "int x" (?x@@3ha) 1>test2.obj : error lnk2001: unresolved external symbol "public: virtual __thiscall a::~a(void)" (??1a@@uae@xz) 1>test2.obj : error lnk2001: unresolved external symbol "public: virtual void __thiscall x::foo(void)" (?foo@x@@uaexxz) 1>...\test2.exe : fatal error lnk1120: 4 unresolved externals

common causes include:


Comments

Post a Comment

Popular posts from this blog

php - failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request -

this code i'm using find latitude , longitude of place: function coords($address){ echo $url="http://maps.googleapis.com/maps/api/geocode/json?address=".$address; $json = file_get_contents(urlencode($url)); $data = json_decode($json, true); print_r($data['results'][0]['geometry']['location']['lat']); print_r($data['results'][0]['geometry']['location']['lng']); } however returns warning: failed open stream: http request failed! http/1.0 400 bad request if use above code in simple procedure works fine not in function. note: have tried curl_init()... method produced same result? instead of encoding whole url encode address part. following code work fine $add='jamshoro phase 2'; $url="http://maps.googleapis.com/maps/api/geocode/json?address=".urlencode($add); $json = file_get_contents($url); $data = json_decode($json, true); print_r($data['r
Read more

java - How to filter a backspace keyboard input -

i have code gives message when key number pressed: txtvalueofclause.addeventfilter(keyevent.key_typed, new eventhandler<keyevent>() { @override public void handle(keyevent t) { char ar[] = t.getcharacter().tochararray(); char ch = ar[t.getcharacter().tochararray().length - 1]; if (!(ch >= '0' && ch <= '9')) { system.out.println("the char entered not number"); t.consume(); } } }); now if press on wrong button , press backspace remove it, error message. how can add backspace input if statement? i have found answer: txtvalueofclause.addeventfilter(keyevent.key_typed, new eventhandler<keyevent>() { @override public void handle(keyevent t) { char ar[] = t.getcharacter().tochararray(); char ch = ar[t.getcharacter().tochar
Read more

java - Show Soft Keyboard when EditText Appears -

i have code in android app, alert dialog , keyboard, , when dialog shown want keyboard appear. not , not sure why? here code: final alertdialog.builder alert = new alertdialog.builder(this); alert.settitle("title"); alert.setmessage("message"); final edittext input = new edittext(this); input.setinputtype(inputtype.type_class_number); alert.setview(input); input.setonfocuschangelistener(new view.onfocuschangelistener() { @override public void onfocuschange(view v, boolean hasfocus) { if (hasfocus) { log.i(tag, "in focus"); inputmethodmanager inputmethodmanager = (inputmethodmanager) getsystemservice(context.input_method_service); inputmethodmanager.showsoftinput(input, inputmethodmanager.show_implicit); } } }); but keyboard not appear though edit text in focus, , tell appear why, doesn't come up? thanks in advance.
Read more