Xcode C++ extern variable linker error - c++

I am learning to code in C++ and am working in Xcode9.1 on OS X 10.13.1. While trying to understand the use of keyword extern, I encountered the problem that the following code:
extern int foo;
#include <iostream>
int main() {
foo = 7;
std::cout << foo << std::endl;
return 0;
}
results in a linker error when run:
Undefined symbols for architecture x86_64:
"_foo", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I am not sure why the linker cannot find foo despite the definition being the first line in main.
Thanks very much for looking into my problem!

The linker cannot find foo, because it's not defined anywhere. By declaring extern int foo', you're telling the linker that the definition is somewhere else. Remove extern, or define foo somewhere where the linker can find it.
Have a look at this example on Wikipedia.

Related

C++: Undefined symbols for architecture arm64 ... linker command failed [duplicate]

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed last month.
I have the following files,
my.h
extern int foo;
void print_foo();
void print(int);
my.cpp
#include <iostream>
#include "./headers/my.h"
void print_foo() {
std::cout << foo << '\n';
}
void print(int i) {
std::cout << i << '\n';
}
use.cpp
#include "./headers/my.h"
int main() {
foo = 7;
print_foo();
print(99);
}
building gives the following error,
/> g++ -W -std=c++11 -o output *.cpp
Undefined symbols for architecture arm64:
"_foo", referenced from:
print_foo() in my-e8b938.o
_main in use-318772.o
(maybe you meant: print_foo())
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Tried removing each definition and its references one by one and testing. It compiles fine when
void print(int)
and its references are alone. The other two defintions give a similar error shown above.
My environment: Mac M1 (Monterey), VSCode, g++ v.14
New to C++. What's going on? How do I correct?
extern int foo;
That does not define foo, it just declares it. You need exactly 1 .cpp file with int foo defined somewhere, and optionally initialized.

Usage of undefined extern variable with in the library is giving linker error in macOS

I am creating a dynamic library.
foo.h
extern unsigned int myoperator;
int operate(int a, int b);
foo.cpp
#include "foo.h"
int operate(int a, int b){
switch(myoperator){
case 0:
return a+b;
case 1:
return a-b;
default:
return a*b;
}
}
libfoo is built exceptionally well on linux gcc C++14, however its throwing a linker error in macOS clang C++14. The error is
Undefined symbols for architecture x86_64:
"_myoperator", referenced from:
operate(int, int) in foo.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
The closest link to the issue I got on google is https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/DynamicLibraryDesignGuidelines.html
I am not sure if the answer is in it.
If the "real" definition of 'myoperator' variable is in another file which is part of the same project foo belongs to, I think it should be better if you put the line code
extern unsigned int myoperator;
in your cpp file

Accessing a static atomic data member causes a linker error

How should a static atomic variable be accessed without causing a linker error?
I've reduced my code example to the following:
#include <iostream>
#include <atomic>
class MyClass
{
public:
static std::atomic_bool onOrOff;
};
std::atomic_bool onOrOff(false);
int main(int argc, const char * argv[]) {
std::cout << "It is: " << (MyClass::onOrOff? "on": "off") << "\n";
return 0;
}
This results in the following linker error (MaxOS X 10.11, XCode 7.2):
Undefined symbols for architecture x86_64:
"MyClass::onOrOff", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
NB: There are no build errors if I remove the line in main() accessing
MyClass::onOrOff, so I think that rules out problems with compiler options.
This:
std::atomic_bool onOrOff(false);
Should be:
std::atomic_bool MyClass::onOrOff(false);
As it stands you have declared two different onOrOff variables, one inside the class and one outside. And only allocated storage for one.
You forgot that onOrOff is a scoped name to MyClass. You need
std::atomic_bool MyClass::onOrOff(false);
Live Example

Undefined symbols for architecture x86_64:

interface:
class rmKeyControl {
static map<char, function<char(char)>> sm_function_list;
public:
static bool addKeyAction(char, function<char(char)>);
};
implementation:
bool rmKeyControl::addKeyAction(char key, function<char(char)> func) {
if (!sm_function_list.count(key)) {
sm_function_list.insert(pair<char, function<char(char)>>(key, func));
return true;
} return false;
}
The full error message is:
Undefined symbols for architecture x86_64:
"control::rmKeyControl::sm_function_list", referenced from:
control::rmKeyControl::addKeyAction(char, std::__1::function) in rm_KeyControl.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
This seems to be a standard linker error for Xcode 4, but it seems to occur for all sorts of reasons, and it never elaborates. This error seems to indicate the presence of binary instructions that don't work on the x86_64 architecture, but that doesn't make sense in this context. Why am I getting this error?
Edit: I forgot to mention that rmKeyControl is in namespace control. I am using namespace control; in the implementation, although you cannot see it.
Static member is just declaration. Define it in the implementation/source file like-
// include interface header and then do -
map<char, function<char(char)>> rmKeyControl::sm_function_list;

C++ does not compile (link?) in XCode 4.2

I have a very small program in Xcode only displaying a label and changing the text of the label in the viewWillAppear method. The label.text should come from a C++ library with a function like this:
int getNumber(){
return 42;
}
The problem is, that including the class with #import "TestLibMain.h" in my *.mm(!) class and using the function with
TestLibMain *tlb = new TestLibMain();
int myInt = tlb->getNumber();
NSString *myString = [NSString stringWithFormat:#"%d",myInt];
doesn't invoke a compiler error, but a linker error:
Undefined symbols for architecture i386:
"TestLibMain::getNumber()", referenced from:
-[tbViewController buttonPressed:] in tbViewController.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)
My impression is, that the cpp-class has not been compiled.
I tried a lot of things around, but somewhere it's hanging. It's all in the same directory, I use the .mm extension, everything fine, but always this linker error. Getting crazy :-)
Mac OS X Lion, XCode 4.2
Any ideas?
int getNumber() {
return 42;
}
in a .cpp or .mm defines getNumber() as a free function. This:
int TestLibMain::getNumber() {
return 42;
}
defines getNumber() as a member of TestLibMain.