String not returning to main function

I have just started with c++ and this is for a project that I'm working on. The problem is that I am not able to return a string from the SearchFunction to main. The search function itself is working perfectly and easily finding and displaying the row its supposed to find but the string temp remains empty despite me returning a string from Search Function. As a result, the DeleteFunction is not working because its not being passed the string that it's supposed to delete.

I have tried using pointers instead of returning value but still the result is same. Please help me understand where I'm going wrong.

#include<iostream>
#include<stdio.h>
#include<string>
#include<fstream>
using namespace std;

string data,temp;

string SearchFunction(string);                  
void DeleteFunction(string);                    

int main()
{
    int choice=0,choice3=0;
    char yn1;
    string search;

    cout<<"1. Press 1 to delete."<<endl;
    cin>>choice;
    cin.clear();                                        
    cin.ignore(1000,'\n');                              

    if(choice==1)
    {   
        cout<<"Enter RegNo. of record to be deleted: ";
        getline(cin,search);                        
        search="RegNo.: "+ search;                  //Concatenate with "RegNo: " to ensure that the search is done "by RegNo".
        temp=SearchFunction(search);                

        cout<<"1. "<<temp<<"\n\n";
        cout<<temp.length()<<endl;

        cout<<"Are you sure you want to delete the above record of"<<search<<"? Y/N";
        yn1=getchar();
        cin.clear();                                        
        cin.ignore(1000,'\n');                          

        if(!(yn1=='y' || yn1=='Y' || yn1=='n' || yn1=='N'))
        {
            do
            {
                cout<<"Enter 'Y' or 'N': ";
                yn1=getchar();
            }while(!(yn1=='y' || yn1=='Y' || yn1=='n' || yn1=='N'));
        }

        if(yn1=='y' || yn1=='Y')
        {
            DeleteFunction(temp);                   //Call delete function to delete record.
        }
    }
    return 0;
}


string SearchFunction(string search)
{

    int found=0, check=0;                               //Declare and initialize both variables to 0.
    ifstream outfile;                                   //Create object for reading file.

    outfile.open("student.txt");                        //Open file.

    while(!outfile.eof())                               //Continue loop until the end of file.
    {
        found=0, check=0;                               //Initialize both variables to 0 again in anticipation of repititions.

        getline(outfile, data);                         //Input one row from file to string variable data.
        found=data.find(search, found);                 //Search for the search term in string data.
        if(found!=string::npos)                         //If search term found.
        {
            cout<<data<<endl;                           //Display row.
        }
    }
    outfile.close();

    return data;
}

void DeleteFunction(string temp)
{
    string line;
    ifstream in("student.txt");
    if( !in.is_open())
    {
        cout << "Input file failed to open\n";
    }

    ofstream out("temp.txt");

    while( getline(in,line) )
    {
        if(line != temp )
            out << line << "\n";
    }
    in.close();
    out.close();    

    remove("student.txt");
    rename("temp.txt","student.txt");
}