static variable declaration and definition in C++ - c++

I have a sample code as shown below:
class myclass
{
public:
static int j;
myclass(){};
~myclass(){};
};
int main(int argc, char** argv) {
myclass obj;
return EXIT_SUCCESS;
}
Now I have declared a static int inside myclass and although I have not defined it, the compiler does not give me any errors until I began using the static variable. Why is that?

Because these are linker errors, not compiler errors. Linker errors never arise until you use an undefined symbol.

"the compiler does not give me any errors until I began using the static variable. Why is that?"
Because it didn't need to be linked with your code until that point (when you start to use it). Unused code is ignored / stripped off by the linker.

Related

g++ compiler not generating error/warning for undefined methods

I have a class that has a declared method but not defined/used anywhere. I expected this piece of code to generate linking error but it did not. Looks like compiler is smart enough to remove dead code. Which default optimization is doing this? How can I explicitly disable it to generate the error?
#include <iostream>
using namespace std;
class Base{
public:
int x;
string name;
void set(int val){ x = val;};
int get(){ return x;}
void Init();
};
int main() {
Base base;
base.set(10);
cout << base.get() << endl;
return 0;
}
EDIT1: Here Init() function is not defined and neither used anywhere. So, I expected compiler to complain about this not defined. But don't see any error/warning.
Thanks in advance.
Generally the linker will only produce errors for undefined symbols that are used. As you never call Init there is no error.
Looks like compiler is smart enough to remove dead code.
The compiler is not even "smart" here. There is no code using a function so that function is not needed to produce an executable program.
The function is not even "ODR used" so technically the compiler would be wrong to require a definition.

Why the singleton initialization failed (link error) [duplicate]

Very simply put:
I have a class that consists mostly of static public members, so I can group similar functions together that still have to be called from other classes/functions.
Anyway, I have defined two static unsigned char variables in my class public scope, when I try to modify these values in the same class' constructor, I am getting an "unresolved external symbol" error at compilation.
class test
{
public:
static unsigned char X;
static unsigned char Y;
...
test();
};
test::test()
{
X = 1;
Y = 2;
}
I'm new to C++ so go easy on me. Why can't I do this?
If you are using C++ 17 you can just use the inline specifier (see https://stackoverflow.com/a/11711082/55721)
If using older versions of the C++ standard, you must add the definitions to match your declarations of X and Y
unsigned char test::X;
unsigned char test::Y;
somewhere. You might want to also initialize a static member
unsigned char test::X = 4;
and again, you do that in the definition (usually in a CXX file) not in the declaration (which is often in a .H file)
Static data members declarations in the class declaration are not definition of them.
To define them you should do this in the .CPP file to avoid duplicated symbols.
The only data you can declare and define is integral static constants.
(Values of enums can be used as constant values as well)
You might want to rewrite your code as:
class test {
public:
const static unsigned char X = 1;
const static unsigned char Y = 2;
...
test();
};
test::test() {
}
If you want to have ability to modify you static variables (in other words when it is inappropriate to declare them as const), you can separate you code between .H and .CPP in the following way:
.H :
class test {
public:
static unsigned char X;
static unsigned char Y;
...
test();
};
.CPP :
unsigned char test::X = 1;
unsigned char test::Y = 2;
test::test()
{
// constructor is empty.
// We don't initialize static data member here,
// because static data initialization will happen on every constructor call.
}
in my case, I declared one static variable in .h file, like
//myClass.h
class myClass
{
static int m_nMyVar;
static void myFunc();
}
and in myClass.cpp, I tried to use this m_nMyVar. It got LINK error like:
error LNK2001: unresolved external symbol "public: static class...
The link error related cpp file looks like:
//myClass.cpp
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}
So I add below code on the top of myClass.cpp
//myClass.cpp
int myClass::m_nMyVar; //it seems redefine m_nMyVar, but it works well
void myClass::myFunc()
{
myClass::m_nMyVar = 123; //I tried to use this m_nMyVar here and got link error
}
then LNK2001 is gone.
Since this is the first SO thread that seemed to come up for me when searching for "unresolved externals with static const members" in general, I'll leave another hint to solve one problem with unresolved externals here:
For me, the thing that I forgot was to mark my class definition __declspec(dllexport), and when called from another class (outside that class's dll's boundaries), I of course got the my unresolved external error.
Still, easy to forget when you're changing an internal helper class to a one accessible from elsewhere, so if you're working in a dynamically linked project, you might as well check that, too.
When we declare a static variable in a class, it is shared by all the objects of that class. As static variables are initialized only once they are never initialized by a constructor. Instead, the static variable should be explicitly initialized outside the class only once using the scope resolution operator (::).
In the below example, static variable counter is a member of the class Demo. Note how it is initialized explicitly outside the class with the initial value = 0.
#include <iostream>
#include <string>
using namespace std;
class Demo{
int var;
static int counter;
public:
Demo(int var):var(var){
cout<<"Counter = "<<counter<<endl;
counter++;
}
};
int Demo::counter = 0; //static variable initialisation
int main()
{
Demo d(2), d1(10),d3(1);
}
Output:
Count = 0
Count = 1
Count = 2
In my case, I was using wrong linking.
It was managed c++ (cli) but with native exporting. I have added to linker -> input -> assembly link resource the dll of the library from which the function is exported. But native c++ linking requires .lib file to "see" implementations in cpp correctly, so for me helped to add the .lib file to linker -> input -> additional dependencies.
[Usually managed code does not use dll export and import, it uses references, but that was unique situation.]

C++ const circular declaration

Why does the following code compile with a circular const variable referencing itself?
#include <iostream>
extern int main(int argc, char* argv[])
{
const int foo = foo * 60;
std::cout << foo << std::endl;
return 0;
}
I'm compiling on a Solaris 5.10 x86 host with the SUNWspro compiler:
/opt/SUNWspro/bin/CC test.cpp
For completeness, this is what it prints:
$ ./a.out
-519270512
In C++, variables are in scope and can be used as part of their own initializers. For example, the following code is also legal:
int x = x;
Doing this results in undefined behavior, since you're referring to the value of x before it has been initialized. However, it's perfectly legal code. I think that the rationale behind this is that you might in some cases want an object to refer to itself during its construction. For example, you could conceivably do something like this:
MyObject x(137, &x); // Pass a pointer to x into its own constructor
I'm honestly not sure why you'd ever want to do this, though.
Hope this helps!

"static const int" causes linking error (undefined-reference)

I am baffled by the linker error when using the following code:
// static_const.cpp -- complete code
#include <vector>
struct Elem {
static const int value = 0;
};
int main(int argc, char *argv[]) {
std::vector<Elem> v(1);
std::vector<Elem>::iterator it;
it = v.begin();
return it->value;
}
However, this fails when linking -- somehow it needs to have a symbol for the static const "value."
$ g++ static_const.cpp
/tmp/ccZTyfe7.o: In function `main':
static_const.cpp:(.text+0x8e): undefined reference to `Elem::value'
collect2: ld returned 1 exit status
BTW, this compiles fine with -O1 or better; but it still fails for more complicated cases. I am using gcc version 4.4.4 20100726 (Red Hat 4.4.4-13).
Any ideas what might be wrong with my code?
If you want to initialize it inside the struct, you can do it too:
struct Elem {
static const int value = 0;
};
const int Elem::value;
Try writing it as
struct Elem {
static const int value;
};
const int Elem::value = 0;
etc
.
static class members are generally supposed to be defined outside the class (declared inside, defined outside) in one compilation unit.
I don't remember how that interacts with inline initialization of const static integral members.
Also see this post: essentially, the problem is that somehow compiler ends up expanding your code into taking the address of Elem::value.
Why not just do this?
return Elem::value;
But the answer is that you are assigning a value in the declaration. This is supposed to work for basic types such as int, and is only required for complex types (i.e. classes, such as if you had a string instead of int). What I have found in practice is that this is hit or miss depending on what version of what compiler you are using. And, as you found out, which optimization level.
In most of the compilers defining in-class static const <integral type> works fine. But some compilers like Android NDK, such in-class definitions results in linker errors. For such case, we may use the typed enums:
struct X
{
enum : int64_t { VALUE = 100; }; // == static const int64_t VALUE = 100;
};

static main from static class?

I'm trying to figure out why this is not working. I want to do like in Java where main is a static function in a class but this is producing unresolved external symbol error:
static class MainClass
{
public:
static int _tmain(int argc, char* argv[])
{
return 0;
}
};
Why doesn't this work?
Thanks
C++ does not work like that.
You need main as a function:
int main(int argc,char* argv[])
{
//STUFF
}
Because _tmain is mangled in the binary with the class name MainClass in it, so multiple classes could have a function _tmain in them and not conflict. It's not got the same mangled name as ::_tmain is going to have.
I remember that with an earlier version of MSVC, it accepted the following without a linker error which ended up accidentally as a result of macro expansion in my code base once
namespace bar {
int main() { }
}
It apparently treated the name main specially and didn't mangle it or mangle it the same as ::main. But such a thing is not Standard conformant. Like in the class case, such a function is completely unrelated to the main function.
_tmain is called from CRT. You need to set in your linker an entry point to another function that will call MainClass::_tmain instead.
Because, in C++, you cannot put an entry point inside a class. This answer might help.
Why doesn't that work? Because it's not C++.
struct MainClass {
static int main(int argc, char** argv) {
return 0;
}
};
int main(int argc, char** argv) {
return MainClass::main(argc, argv);
}
It's not really C++, so much as the standard linker process which is looking for an export of a specific name (with C-style linkage). The specific export varies based on the compiler and executable type, but the format is specific for each type. For example, Windows GUI exe's have different entry points than console, and specific for ASCII or UNICODE. But the linker is always looking for a function with a specific link name, which is why a static member won't work.
You always need main() defined as a global function. This is where the program always starts in C++. You could simple call your static class function from main and pass on the variables if you really want to:
class MainClass
{
public:
static int _tmain(int argc, char* argv[])
{
return 0;
}
};
int main(int argc, char* argv[]) {
return MainClass::_tmain(argc, argv);
}