I tried to look this up, but did not find it anywhere. So here's the question:
Static functions in C/C++ can be used to "make them invisible to the outer world". Great, when having two same-named static functions in two different compiled units (.c files), it makes me sure that I call the right one. But can I also be sure that I call my local static function when there exists a same-named non-static function somewhere in the project or libraries? That is, does the static function locally hide the non-static one?
Sure I can test it (and I did) but I want to know whether this behaviour has fixed definition in C/C++. Thanks.
Edit: Simplified example code which caused unexpected behaviour to me. The question is about the fix of the problem (suppose I cannot change the library).
In mylib.c:
#include "mylib.h"
int send(void * data, int size);
...
int send(void * data, int size) {
return send_message(queueA, data, size);
}
void libfunc(void) {
send(str, strlen(str));
}
In mylib.h:
// only libfunc is declared here
void libfunc(void);
In myprog.c:
#include "mylib.h"
int send(void * data, int size);
...
int send(void * data, int size) {
return send_message(queueB, data, size);
}
void progfunc(void) {
// expected to send a message to queueB
// !!! but it was sent to queueA instead !!!
send(str, strlen(str));
}
Compiled mylib.c + further files -> mylib.a
Compiled myprog.c -> myprog.o
Linked myprog.o + mylib.a -> myprog
You'd get a compilation error because functions have default external linkage, thus the new static function would result in a conflict of linkage specifiers.
If the declaration of the non-static function isn't visible, the static one will be called:
void foo(); //external linkage
static void foo() {}; //internal linkage and error
It does not hide functions with same name declared in the same scope. However you may not have a function with the same signature declared as having internal and external linkage.
Related
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.]
I read some answers about this topic, but I am still not sure:
In C++ a global const variable definition is automatically static. However I can access it from another cpp-file through extern:
// module.cpp
const int i = 0;
and
// main.cpp
extern const int i;
int main ()
{
if (i > 10)
return 0;
else
return 1;
}
Why is this possible (accessing an object with internal linkage from another module)? Normally I should be forced to define i as extern const int i = 0 in module.cpp, to have an explicitely global but non-static const, or?
In comparison, this is not possible:
// module.cpp
static int i = 0;
and
// main.cpp
extern int i;
int main ()
{
i = 10; // but read-only access like (i > 10) would be possible!
return 0;
}
So is the answer that: yes, you can access internal linked objects from other modules, but only for read (so always with const)?
Edit:
Sorry, but I made a mistake: in my original code I just tried an expression without effect (in both examples):
extern const int i; // or extern int i for second example
int main ()
{
i>10;
return 0;
}
I thought, that it behaves the same, as if program flow or data was dependent of this expression, but in fact it does not! The compiler seems to just cut out this effectless expression, so that the linker does not see it! So all is alright: in the first example i must be indeed defined extern const int i = 0 in module.cpp, and in the second example i cannot be accessed at all (unless in effectless expression). Compiler is VC++2010.
Edit2:
However, now I don't understand why this is possible:
// module.cpp
extern const int i = 0;
and
// main.cpp
int i = 99;
int main ()
{
bool b = i>10;
return 0;
}
i have both external linkage. But no error. When in module.cpp I define int i = 0, then error (multiple symbols). Why?
As for me it looks like a compiler bug. Constant variable i is not defined It is only declared in main.cpp. As for variable i in module.cpp then it has internal linkage and shall not be accessible outside the module.
Relative to your addition to the original post then the compiler has nothing common with this situation. It is linker that checks whether there are duplicate external symbols. I think it decided that if one variable has qualifier const and other has no it then there are two different variables. I think that it is implementation defined whether the linker will issue an error or not. Moreover it can have some options that could control the behaviour of the linker in such situations.
(i have edited my original question to make it more understandable)
here is the prototype for the problem ....
//Txn.h ---this has a static variable, usable by the pgms including it.
class Txn
{
public:
static int i;
static void incr_int();
};
Txn::i=0;
//Txn.cpp
void Txn::incr_int() {i++;}
->produce LibTxn.so
//class1.cpp -> one of the pgm using the static var from Txn.h
#include Txn.h
Txn::incr_int()
-> produce class1.o, with LibTxn.so.
// class2.cpp ->another pgm using the static var from Txn.h
#include Txn.h
cout<<"Txn::i;
-> produce class2.o, by including LibTxn.so
-> .produce class3 (an exe) by using class1.o,class2.o. Since, both class1 and 2 has the statement "Txn::i=0" from "Txn.h", multiple declaration issue happens.
-> .If I remove the statement "Txn::i=0" from Txn.h, then "undefined reference" error appears.
-> .At high lvl, this problem is a kind of having a session variable, which should be assessible from any func in a exe. Those func can be in any obj files used to form the exe. I am fine for any sol, even without static. But I can't change the creation of different .o files (which are using this session var) and combining the .o to produce the exe.
It is hard to figure out exactly what the problem is if you cannot provide the real code, or at least an example which has the same problem as the real code.
However, most likely the root cause of the problem is that you are not only declaring, but also defining your class's static variable in the header file that contains the class definition.
This means that all the translation units (i.e. .cpp files) which include that header will contain a definition for the static variable, and when merging all the corresponding object files, the linker will eventually complain about that symbol being defined multiple times.
If this is the case, what you should do is to take the initialization of the static variable out of the header file which contains your class's definition and put it in one (and only one) .cpp file.
I tried to recreate the problem as you described, but it compiled just fine on my computer, and it is difficult to go further without seeing your code.
In the code below, the header tells (declares) every .cpp file that includes it about Foo::x, but Foo::x lives in (is defined in) Foo.cpp (and Foo.o)
foo.h:
class Foo {
public:
static int x;
};
Foo.cpp:
#include "foo.h"
int Foo::x;
main.cpp:
#include <iostream>
#include "foo.h"
int main(int argc, char *argv[]) {
Foo::x = 42;
std::cout << "Foo::x is " << Foo::x;
}
Yes. it worked by defining the static variable in .cpp.
Special thanks to Andy Prowl and iWerner.
Is there any chance that a function defined with static can be accessed outside the file scope?
It depends upon what you mean by "access". Of course, the function cannot be called by name in any other file since it's static in a different file, but you have have a function pointer to it.
$ cat f1.c
/* static */
static int number(void)
{
return 42;
}
/* "global" pointer */
int (*pf)(void);
void initialize(void)
{
pf = number;
}
$ cat f2.c
#include <stdio.h>
extern int (*pf)(void);
extern void initialize(void);
int main(void)
{
initialize();
printf("%d\n", pf());
return 0;
}
$ gcc -ansi -pedantic -W -Wall f1.c f2.c
$ ./a.out
42
It could be called from outside the scope via function pointer.
For example, if you had:
static int transform(int x)
{
return x * 2;
}
typedef int (*FUNC_PTR)(int);
FUNC_PTR get_pointer(void)
{
return transform;
}
then a function outside the scope can call get_pointer() and use the returned function pointer to call transform.
No, unless there's a bug in the compiler. Normally the static function code is not tagged with a name used for exporting the function in the object file, so it is not presented to the linker and it just can't link to it.
This of course only applies to calling the function by name. Other code within the same file can get the function address and pass it into a non-static function in another file and then the function from another file can call your static function.
It cannot be accessed outside a file by it's name. But, you can as well assign it to function pointer and use it wherever you want.
"Accessed"? It depends on what you mean by this term. I assume when you say "static function" you are talking about standalone function declared static (i.e. declared with internal linkage) as opposed to static class member functions in C++, since the latter are obviosly and easily accessible from anywhere.
Now, a standalone function declared static has internal linkage. It cannot be linked to from any other translation unit. Or, putting it differently, it cannot be referred to by name from any other translation unit. If that's what you meant by "access from outside the file scope", then no, it can't be done.
However, if the other translation units somehow get a pointer to that function (i.e. if you somehow allow that pointer to "leak" into the ouside world), then anybody can still call that function by making an idirect call and thus "access" it. For example, if you declare
static void foo_static(void) {
}
extern void (*foo_ptr)(void) = foo_static;
then in any other translation unit the user will be able to do
extern void (*foo_ptr)(void);
foo_ptr();
and the call will go to your foo_static function. I don't know if that kind of access qualifies as "access" in your question.
Following the standard, a static function cannot be accessed outside of the scope of the file by name because it is subject to internal linkage. It's name is not exported, and not provided to the linker. However, it can still be accessed and called by function pointer, like any other function.
Only with trickery. The function is generally not visible to the linker so it won't let you do it.
But, if you provide a function inside the same compilation unit (as the static function) which returns the address of that function:
In main.c:
#inclde <stdio.h>
int (*getGet7(void))(void);
int main (void) {
int (*fn)(void) = getGet7();
printf ("Result is: %d\n", fn());
return 0;
}
In hidden.c:
static int get7 (void) {
return 7;
}
int (*getGet7(void)) (void) {
return get7;
}
This will result in the static function get7 being called.
pax> gcc -o demo main.c hidden.c ; ./demo
Result is: 7
No, the purpose of the keyword static is to limit the scope of the function name to the file.
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.]