功能不明确

Question

//我的编译器给出者错误为：    // Main.cpp：21：6：错误：对“ greater”的引用不明确            // a = Greater（num1）;

#include<iostream>
using namespace std;
string greater(int num)
 {
    if(num>0)
    {
        return "Number is Positive";
    }
    else
    {
        return "Number is Negative";
    }
 }
int main()
 {


    string a;
        int num1;
        cout<<"Enter a number";cin>>num1;
        a = greater(num1);
        cout<<a;

 }