问C++编译后的头收到大量错误

EN

#include <iostream>
#include <limits>
#include "add.h"


int main()
{
    std::cout << "this will call function which will have you type in numbers which when done will call the second function as well as giving the second function answers to what it needs. " << '\n';

    int numOne{ function() };
    int numTwo{ function() };
    std::cout << "With those two numbers we can add them! and our answer is!" << functionTwo(numOne, numTwo) << '\n';
    std::cout << "we now sent functionTwo the numbers we made using function for simplness. Now functionTwo can have the numbers it needs to do what it needs to do." << '\n';


    // This code will make it not close automatically.
    std::cin.clear();
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    std::cin.get();

    return 0;
}

#include <iostream>
#include "add.h"

int functionTwo(int a, int b)

{
    std::cout << " fdsaf dasf dsa" << '\n';


    std::cout << 2 * a << '\n';
    std::cout << 2 + b << '\n';
    int c{ 2 + b };
    int d{ 2 * a };



    return c + d;

}

#include <iostream>
#include "add.h"
int function()

{
    int a{ 0 };
    std::cin >> a;
    return a;

}

#ifndef _IOSTREAM_
#define _IOSTREAM_

int function();



int functionTwo(int a, int b);



#endif