[問題]關於 static function

[ricky770927 (ricky)]

我寫:
#include<iostream>
using namespace std;
class ModInt
{
public:
    ModInt(const int& i){_number=i;};
        static void setMod(unsigned i){_modulus=i;}
private:
        unsigned _number;
    static  unsigned _modulus;
};
void main()
{
        int mod;
        cout<<"Please enter the modulus (> 0): "<<endl;
        cin>>mod;
        while(mod<=0)
        {
                cout<<"Please enter the modulus (> 0): "<<endl;
                cin>>mod;
        }
        ModInt a(5);
        ModInt::setMod(mod);
}
在build時出錯
HW9.OBJ : error LNK2001: unresolved external symbol "private: static unsigned int ModInt::_modulus" (?_modulus@ModInt@@0IA)
Debug/HW9.exe : fatal error LNK1120: 1 unresolved externals
到底是哪裡有問題?
請幫我解答一下   謝謝!!

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