Question

#include <cstdlib>
#include <iostream>
#include <string> 
#include <string.h> 

using namespace std;

class Portfolio{

public :

Portfolio(){}; 

private:   

Stock stocks[100]; 

void load_self(){}
void save_self(){}

};  


class Stock { 

public: 

Stock(int val , int amo , string db  , string symbol){

this->set_value(val); 
this->set_amount(amo); 
this->set_date(db); 
this->set_symbol(symbol); 

}

void set_value(int vlue){this->value=vlue;}
void set_amount(int am){this->amount=am;}
void set_date(string d){this->date_bought = d;}
void set_symbol(string symb){this->symbol = symb;} 

private: 

int value; 
int amount; 
string date_bought; 
string symbol; 

};

int main(int argc, char *argv[])
{

    return EXIT_SUCCESS;
}

Answer 1

You have two problems.

1. As others have said, reverse the order of the class declarations so the Stock is fully defined before Portfolio.
2. You need to define a suitable default constructor so that Stock can be instantiated by Stock stocks[100]

To solve the second you need something like this:

class Stock { 

    public: 

        Stock() {}; // Could add suitable defaults here.

    // Whatever else....

}