Error to use of static and static function in C++

Question

i am beginner in programming, start C++ one weak ago. i have problem to use of my static variable. i read about use of static variable in various same question but i understand just this Car::countOfInput;. from below post:

this is my code:

#include <iostream>
#include <conio.h>
#include <string.h>

using namespace std;

class Car{
    private:
        static int countOfInput;
        char   *carName;
        double carNumber;
    public:
        Car() {
            static int countOfInput = 0;
            char carName = {'X'};
            double carNumber = 0;
        }

        void setVal(){
            double number;

            cout << "Car Name: ";
            char* str = new char[strlen(str) + 1];
            cin>>str;
            strcpy(carName, str);

            cout << endl << "Car Number: ";
            cin >> number; cout << endl;
            carNumber = number;

            Car::countOfInput += 1;
       }

       friend void print(){
            if(Car::countOfInput == 0){
                cout << "Error: empty!";
                return;
            }
            cout << "LIST OF CarS" << endl;
            cout << "Car Name: " << carName << "\t";
            cout << "Car Number: " << carNumber << endl;
      } const

      void setCarNumber(int x){carNumber = x;}
      int  getCarNumber(){return carNumber;}

      void setcarName(char x[]){strcpy(carName, x);}
      char getcarName(){return *carName;}

      int getCountOfInput(){return countOfInput;}
      void setCountOfInput(int x){countOfInput = x;}
};

int main(){
    Car product[3];
    product[0].setVal();
    product[0].print();

    getch();
    return 0;
}

when i run this:

F:\CLion\practise\main.cpp: In function 'void print()':

F:\CLion\practise\main.cpp:10:13: error: invalid use of non-static data member 'Car::carName' char *carName; ^

F:\CLion\practise\main.cpp:40:33: error: from this location cout << "Car Name: " << carName << "\t"; ^

F:\CLion\practise\main.cpp:11:12: error: invalid use of non-static data member 'Car::carNumber' double carNumber; ^

F:\CLion\practise\main.cpp:41:35: error: from this location cout << "Car Number: " << carNumber << endl; ^

F:\CLion\practise\main.cpp: In function 'int main()':

F:\CLion\practise\main.cpp:57:16: error: 'class Car' has no member named 'print' product[0].print();

I use CLion, Thanks in advance.

If a class variable is marked static, it isn't associated with any instance of its class. Are you sure that's what you want? — erip
– erip, Commented May 15, 2016 at 14:16
Roughly, i want increase count of my static variable every time setVal() function call. — Death Programmer
– Death Programmer, Commented May 15, 2016 at 14:18
I think you need to take much smaller steps. You have built up a mass of at least four separate errors here - writing less code and testing earlier would have simplified this. — Martin Bonner supports Monica
– Martin Bonner supports Monica, Commented May 15, 2016 at 14:33

Community · Accepted Answer · 2017-05-23 12:16:04Z

4

The variable declaration inside Car() are local variables, not initializations of the member variables. To initialize the member variables, just do this (it's a member initializer list):

Car() : carName("X"), carNumber(0) {}

and put the definition of countOfInput outside the class in a .cpp file (in global scope):

int Car::countOfInput = 0;

(If you want to reset countOfInput to 0 every time the Car() constructor is called, you can do that in the constructor body: countOfInput = 0;)

edited May 23, 2017 at 12:16

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answered May 15, 2016 at 14:24

flogram_dev

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3 Comments

erip Over a year ago

Ooooo, nice catch with locals. :) Seems like a quiz question from school.

Death Programmer Over a year ago

What can i do if i want to have my print function be friend?

flogram_dev Over a year ago

@DeathProgrammer Declare it friend inside the class and define it outside, like normally.

Martin Bonner supports Monica · Accepted Answer · 2016-05-15 14:27:22Z

Oh dear. You have a number of problems. Firstly, your constructor doesn't initialize any of the member variables.

   Car() {
        static int countOfInput = 0;
        char carName = {'X'};
        double carNumber = 0;
    }

Instead it is declaring three local variables and setting them to values. What you want is:

   Car() 
       : carName(nullptr)
       , carNumber(0.0) 
   {
   }

Then setValue is copying the string into the memory pointed at by this->carName, but that is uninitialized, so could be anywhere. Also, strlen(str) before you initialize str is doomed to failure. Make carName a std::string, then you don't need to construct it, and the code becomes:

   Car() 
       : carNumber(0.0) 
   {
   }

   void setVal(){

        cout << "Car Name: ";
        cin >> carName;

        cout << endl << "Car Number: ";
        cin >> carNumber; cout << endl;

        Car::countOfInput += 1;
   }

Next, you need to make print not be a friend (and make it const).

   void print() const {
   ...

Finally you need to define coutOfInput. You have declared it, but you also need a definition. Outside any function do:

int Car::countOfInput = 0;

songyuanyao · Accepted Answer · 2016-05-15 14:29:47Z

3

The error messages have nothing to do with static variables or static methods.

The keyword friend is in front of print(), make it a non-member function (note the last error message) and then can't access the member variables directly. According to the usage it should be a member function, so just remove the keyword friend.

And as @tuple_cat suggested, const should be put before the beginning {.

void print() const {
     if(Car::countOfInput == 0){
         cout << "Error: empty!";
         return;
     }
     cout << "LIST OF CarS" << endl;
     cout << "Car Name: " << carName << "\t";
     cout << "Car Number: " << carNumber << endl;
 }

edited May 15, 2016 at 14:29

answered May 15, 2016 at 14:18

songyuanyao

174k16 gold badges327 silver badges432 bronze badges

4 Comments

flogram_dev Over a year ago

And put const before the opening {.

Death Programmer Over a year ago

without friend and even const and even put const before the beginninng, not working yet.

Death Programmer Over a year ago

Give me this error: F:/CLion/practise/main.cpp:31: undefined reference to `Car::countOfInput'.

songyuanyao Over a year ago

@DeathProgrammer It's another issue. See tuple_cat's answer.

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