I tried to declare static array:
#include <iostream>
#include <string.h>
using namespace std;
struct tagData{
static string tags[];
};
int main(){
tagData::tags[3];
tagData::tags[0] = "default";
tagData::tags[1] = "player";
tagData::tags[2] = "enemy";
return 0;
}
But as the result an error occurred:
*Path*\cczPqyfY.o:main.cpp:(.text+0x1e): undefined reference to `tagData::tags[abi:cxx11]'
*Path*\cczPqyfY.o:main.cpp:(.text+0x32): undefined reference to `tagData::tags[abi:cxx11]'
*Path*\cczPqyfY.o:main.cpp:(.text+0x46): undefined reference to `tagData::tags[abi:cxx11]'
collect2.exe: error: ld returned 1 exit status
I was looking for some info about how to declare static arrays in structs, but couldn't.
Can anybody show me an example or help me in another way?
P.S.
I tried to compile this code with GCC version 8.1.0;
struct tagData{
static string tags[];
};
This static class member must be defined in global scope, and not as a local variable in a function like main:
string tagData::tags[3];
And now, once the formalities are taken care of, you can manually access its class members, in main or any other function:
int main(){
tagData::tags[0] = "default";
tagData::tags[1] = "player";
tagData::tags[2] = "enemy";
return 0;
}
However, if the goal is to initialize the array there's an even better way. Just initialize it as part of its definition:
string tagData::tags[3]={"default", "player", "enemy"};
Note: when defining static class member, you should always keep in mind the static initialization order fiasco.
Related
//this is main.cpp
#include <iostream>
#include "Cook.hpp"
using namespace std;
int main(void){
int l = Cook.get_life();
}
//this is cook.hpp
#ifndef HUNTER_H
#define HUNTER_H
class Cook
{
public:
int get_life(void);
private:
int life;
};
#endif
//this is cook.cpp
#include "Cook.hpp"
int Cook::get_life(void)
{
life=0;
return life;
}
They are all in same folder. And I get compile error when I run main.cpp. And Xcode recommended to use Cook().get_life() instead of Cook.get_life(). Can you explain why? I thought I should use Cook.get_life.
I use Xcode.
First you have to declare variable with type of you class (instance) and then you can use it, But classes has static functions too, that mean you can use function without declare instance of it first but in that you can't use member variable of class, Reading more about concept of classes and more ...
get_life is not a static function, you have to call it on an instance of your class Cook, and that's exactly what Cook() does. If you want to call get_life without an instance of Cook, you should declare your function this way :
static int get_life(void);
And then call it like that :
Cook::get_life();
The thing is you can't use class attributes from static member functions, so instead you need to instantiate your class Cook before calling your member function.
Cook c = Cook(); // Cook().get_life() works to, but you don't keep your newly created object
c.get_life();
Related to this. I'd like to avoid using global variables so I resorted to using structs with enum and std::string[] (see link) in order to build menus for a small application. I would also like to have these enums in a separate header file. The selected answer in the link implies using --std=c++17, which I'd like to avoid, at least for now, and decided to use a static const std::string[] -- no need to include extra array or vector since this is initialized once, never modified, only called, ALL is always known.
As other answers on this have made it clear, I need to either initialize A::names outside the struct, or use a static const std::string& setter (see this, for example). But all the answers so far dealt with a std::string, not an array, std::string[].
This is a simple example of what I tried. It simply tries to print the contents of A::names using a for() loop iterating through the enum in struct A:
a.h:
#ifndef A_H_INCLUDED
#define A_H_INCLUDED
#include <string>
struct A
{
enum E { ONE, TWO, ALL };
static const std::string names[ALL];
};
#endif // A_H_INCLUDED
a.cpp:
#include "a.h"
static const std::string A::names[A::ALL] { "one", "two" };
main.cpp:
#include "a.h"
#include <iostream>
int main()
{
for(int i=A::ONE; i<A::ALL; ++i)
std::cout << A::names[i] << '\n';
return 0;
}
The error after g++ main.cpp is:
main.cpp:(.text+0x24): undefined reference to `A::names[abi:cxx11]'
collect2: error: ld returned 1 exit status
Seeing the cxx11, I thought g++ --std=c++11 main.cpp would solve it, but it doesn't.
So, what am I doing wrong, or, how could I adapt the version with the setter to return an array, std::string[]? My goal is to have an alternative to a global variable, that has only one instance in memory no matter how many calls.
Here's an adapted code, from a small program, on how I would build a menu using struct with enum and string (menu_design = new QMenu... and menuDesignAction() is the function that updates):
for(unsigned char i=0; i<A::ALL; ++i) // needs initializing
{
QAction *tmpAction {new QAction(tr(A::names[i].c_str()))};
tmpAction->setObjectName(QString("%1").arg(i));
connect(tmpAction, SIGNAL(triggered(bool)), this, SLOT(menuDesignAction()));
menu_design->addAction(tmpAction);
}
As a side-note, in the snippet above, I have to use .c_str(), but I am using a std::string in the enum. If I could make it *char[] instead of std::string[], would I avoid extra calls? If I am not wrong, how could the answers to my problem (assuming there are) be adapted so as to be able to fit somehow in the Qt snippet?
I need to make a program that gets a fraction from the user and then simplifies it.
I know how to do it and have done most of the code but I keep getting this error "error: expected unqualified-id before ‘.’ token".
I have declared a struct called ReducedForm which holds the simplified numerator and denominator, now what Im trying to do is send the simplified values to this struct.
Here is my code;
In Rational.h;
#ifndef RATIONAL_H
#define RATIONAL_H
using namespace std;
struct ReducedForm
{
int iSimplifiedNumerator;
int iSimplifiedDenominator;
};
//I have a class here for the other stuff in the program
#endif
In Rational.cpp;
#include <iostream>
#include "rational.h"
using namespace std;
void Rational :: SetToReducedForm(int iNumerator, int iDenominator)
{
int iGreatCommDivisor = 0;
iGreatCommDivisor = GCD(iNumerator, iDenominator);
//The next 2 lines is where i get the error
ReducedForm.iSimplifiedNumerator = iNumerator/iGreatCommDivisor;
ReducedForm.iSimplifiedDenominator = iDenominator/iGreatCommDivisor;
};
You are trying to access the struct statically with a . instead of ::, nor are its members static. Either instantiate ReducedForm:
ReducedForm rf;
rf.iSimplifiedNumerator = 5;
or change the members to static like this:
struct ReducedForm
{
static int iSimplifiedNumerator;
static int iSimplifiedDenominator;
};
In the latter case, you must access the members with :: instead of . I highly doubt however that the latter is what you are going for ;)
The struct's name is ReducedForm; you need to make an object (instance of the struct or class) and use that. Do this:
ReducedForm MyReducedForm;
MyReducedForm.iSimplifiedNumerator = iNumerator/iGreatCommDivisor;
MyReducedForm.iSimplifiedDenominator = iDenominator/iGreatCommDivisor;
ReducedForm is a type, so you cannot say
ReducedForm.iSimplifiedNumerator = iNumerator/iGreatCommDivisor;
You can only use the . operator on an instance:
ReducedForm rf;
rf.iSimplifiedNumerator = iNumerator/iGreatCommDivisor;
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
undefined reference to static member variable
What is an undefined reference/unresolved external symbol error and how do I fix it?
#include<iostream>
using namespace std;
class abc {
private:
static int a ;
public:
abc(int x) {
a = x;
}
void showData() {
cout<<"A = "<<a<<endl;
}
};
int main() {
abc a1(4);
abc a2(5);
a1.showData();
a2.showData();
return 0;
}
When I try to compile this function on Ubuntu with GCC compiler. I get the following error.
/tmp/ccCKK2YN.o: In function `main':
static1.cpp:(.text+0xb): undefined reference to `Something::s_nValue'
static1.cpp:(.text+0x14): undefined reference to `Something::s_nValue'
collect2: ld returned 1 exit status
Compilation failed.
Where as the following code runs fine
#include<iostream>
using namespace std;
class Something
{
public:
static int s_nValue;
};
int Something::s_nValue = 1;
int main()
{
Something cFirst;
cFirst.s_nValue = 2;
Something cSecond;
std::cout << cSecond.s_nValue;
return 0;
}
Is this because Static member variables needs to initialized explicitly before accessing them via objects.Why so ?
static int s_nValue; doesn't allocate any storage to store the int, it just declares it.
You allocate somewhere in memory to store the variable with:
int Something::a=0;
The declaration of a static data member in the member list of a class is not a definition. You must define the static member outside of the class declaration, in namespace scope.
See this thread.
In short, the static member needs to be initialized somewhere in a .cpp file so that the compiler allocates space for it. The declaration would look like this:
int abc::a = 0;
That happens because since static members are shared between all instances of a class, they need to be declared in one single place.
If you define the static variable inside the class declaration then each include to that file would have a definition to that variable (which is against to the static meaning).
Because of that you have to define the static members in the .cpp.
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;
};