String not recognized in function definition

I am trying to code the number of occurences of 'e' in a particular string but it is giving me error while defining the function as it says string is not defined in this scope and occurence_e(which is my user-def function) to be not declared in this scope as well. Following is the code:

#include<iostream>
#include<string.h>
void occurence_e(string);
using namespace std;
int main(){
 string input; 
cout<<"Enter any name"<<endl;
cin>>input;
occurence_e(input);
    return 0;
}

void occurence_e(string input){
    int count=0;
    for (int i = 0; i < input.length(); i++)
    {
        if(input.at(i)=='e'){
            count++;
        }
    }
    cout<<"No. of times e comes in this name is "<<count;
}