I am trying to write some namespaces statics methods and variables in order to have a set of functions i can use from anywhere in the code. This is what I have:
Header:
namespace CProfileIO
{
static void setAppDir(std::string appDir);
static int reloadProfiles();
static std::string directory;
static std::list<std::string> profilesList;
}
Source:
namespace CProfileIO
{
void setAppDir(std::string appDir)
{
directory = appDir;
}
int reloadProfiles()
{
// ...
}
} // namespace CProfileIO
Then somewhere in the code I have:
#include "CProfileIO.h"
int main(int argc, char * argv[])
{
string appDir = string(dirname(*argv));
CProfileIO::setAppDir(appDir);
.
.
.
}
When I try to compile, i get error at the line I am using the function:
... undefined reference to `CProfileIO::setAppDir(std::string)'
I cant figure out what is wrong. I would aprichiate all help!
You should not use static functions here, as they are visible only in the current translation unit. Thus, you declare a static function which you define (statically) in the cpp, then it won't be visible from other translation units.
You should simply not use the static keyword here, but declare variables (but not functions) as extern.
Also, I recommend to pass the string argument as const reference (void setAppDir(const std::string& appDir);)
That is because static methods are only visible in the current module, (source file). They are not linked.
Hence you other sourcefile doesn't find the function. That is supposed to happen if you use static. I don't know why you would declared naked functions as static, maybe you meant to put them into a class?
Related
I need to set a variable in the main function and access it from a different function in the same file. I can not pass it to the function because it means changing the entire code structure, which is not an option. To avoid declaring a global variable I crated a namespace and I want to check if this is a good programming practice or is there a cleaner way to do it.
This is the code:
namespace mylocalnamespace{
int myglobalvar;
}
static void myFunc()
{
..... some code
operationX(mylocalnamespace::myglobalvar);
..... some code
}
int main(int argc, char **argv)
{
..... some code
mylocalnamespace::myglobalvar = atoi(argv[0]);
..... some code
}
Alternatives To Global Variables in C++
In the example, function argument is a good alternative to avoid a global variable:
static void myFunc(int mylocalvar)
{
..... some code
operationX(mylocalvar);
..... some code
}
int main(int argc, char **argv)
{
..... some code
mylocalvar = atoi(argv[0]);
..... some code
myFunc(mylocalvar);
}
I can not pass it to the function
Oh well, then you have to use a global variable.
Since you apparently use it in a function with internal linkage, you could improve slightly by using global with internal linkage as well. This way the global won't leak to other translation units. A handy way to achieve that is an anonymous namespace:
namespace {
int myglobalvar;
void myFunc() {
// ...
To avoid declaring a global variable I crated a namespace
Global variables are still global variables even if not in the global namespace.
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 am attempting to compile a .cpp file that includes a .h file and then implements the class outlined.
In String.h:
using namespace std;
class String
{
private:
int _length;
char *data;
int getCharArraySize(char arr[]);
In String.cpp:
#include "String.h"
using namespace std;
/*
* Private vars and methods
*/
int String::_length;
char String::*data;
int String::getCharArraySize(char arr[])
{
//method body
return 0;
}
When I attempt to compile with g++ I get this error:
% g++ String.cpp -c
String.cpp:14:17: error: ‘int String::_length’ is not a static data member of ‘class String’
int String::_length;
I am only having issues with String::_length. I was initially thinking that it was due to _length being private but all the other private methods/vars, compile just fine. I also need to leave this .h file as is so I cannot just make it public. Any help would be appreciated!
Well, it's exactly as the compiler says: _length is not a static member variable. Yet, you are treating it as one by providing it with its own definition. Simply do not do that. Only static member variables should be defined like that.
The same goes for data.
By the way, if you do ever have to define a char* variable, then this is wrong:
char String::*data;
and this is right:
char* String::data;
Lexical grammar production oddities (inherited from C) notwithstanding, the * is part of the type, not the name.
If I have a class similar to this :
//body.h
class Body
{
static int i;//line 1
};
int Body::i=2;/line 2
and a class like this:
//ball.h
#include <body.h>
//line 3
class Ball:public Body
{
int f();
};
and in ball.cpp :
int Ball::f()
{
return 1;
}
this results in multiple definition of i.
I tried putting extern in line1,line2 and defining it in line 3 and still having the same error, I also searched the web, most of the results I find talks about a variable declared solely(not in a class) while I have a static class variable.
I understood that extern is opposite to static but making i extern in line1 didn't help, also I saw many questions in SO , this talks about namespaces which I don't want, this doesn't address my issue.
As A note, there is no body.cpp, there are classes other than Ball that inherits body and there is main.cpp which will access all the children classes.
So what to do to be able to use Body::i outside body.h ?
PS
all classes are surrounded in header guards .
Create another translation unit Body.cpp and move the definition there
int Body::i=2;
Even with header guards, as you mention to have them, the definition appears in multiple translation units, hence the multiple definition error.
In your particular case the static class member is a primitive, and can be initialized at the point of declaration alternatively:
class Body {
static int i = 2;
};
You should not define the static member in the header. You need to make a source (.cpp) file for it:
body.h (declare statics but don't define)
class Body
{
static int i; // only declare
};
// int Body::i=2; // do not define it here
body.cpp (define statics here)
#include "body.h"
int Body::i = 2; // now we define it
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.]