Question

my program is simple. a class of a bank account. Each account has a balance and an ower. each account has the same interest rate. I got the error when compile it. What’s wrong? Thanks in advance.

  2 #include <iostream>
  3 #include <string>
  4 using namespace std;
  5 class bank
  6 {
  7 private:
  8         double balance;
  9         string owner;
 10         static double InterestRate;
 11 public:
 12         static void AccountInfo(const bank& ac)
 13         {
 14                 cout << "name: " << ac.owner << endl << "balance: " << ac.balance;
 15         }
 16         static void SetAccount(bank& ac)
 17         {
 18                 cout << "enter a name: " << flush;
 19                 cin >> ac.owner;
 20                 cout << "enter a balance: " << flush;
 21                 cin >> ac.balance;
 22         }
 23         static void SetRate(const double& n)
 24         {
 25                 InterestRate = n;
 26         }
 27         static void Balance(bank& ac)
 28         {
 29                 ac.balance += ac.balance * InterestRate;
 30         }
 31 };
 32 int main ()
 33 {
 34         bank NewAccount;
 35         bank::SetAccount(NewAccount);
 36         bank::SetRate(0.15);
 37         bank::Balance(NewAccount);
 38         bank::AccountInfo(NewAccount);
 39 return 0;
 40 }

and the output is:

  /tmp/ccUh8Sd9.o: In function `bank::SetRate(double const&)':
    e1237.cpp:(.text._ZN4bank7SetRateERKd[bank::SetRate(double const&)]+0x12): undefined reference to `bank::InterestRate'
    /tmp/ccUh8Sd9.o: In function `bank::Balance(bank&)':
    e1237.cpp:(.text._ZN4bank7BalanceERS_[bank::Balance(bank&)]+0x1c): undefined reference to `bank::InterestRate'
    collect2: ld returned 1 exit status

Answer 1

You just declared the static member InterestRate in your class definition but,
You also need to define your static member in one of your cpp file:

double bank::InterestRate = 0.0;

Without this the linker cannot find its definition and gives the linking error.