creating object beforehand in c++ - c++

Question

Here is my concrete example of what I'm trying to do. I need to have a type of object created by the end of this function so I can store it in something else(not created yet), that is either a string, Id, or expression parameter. The problem I am facing is that I don't know what type of object to create if I haven't done logic yet, and If I create the object inside the if statement while I am doing logic, it will exist no more there. I want to store the objects into a vector of type parameter in another class not listed here. at the end of the code
bool parameter(Predicate& pred)//look for the following: STRING | ID | expression
{
//store a parameter in this predicate
Parameter // <- I don't know which type of object to create yet!!!!
//create parameter
get_grammar_type(token, sfile);
token_type = token.Get_type();
if(token_type == STRING)
{
//would create object of type string here
//can't create object here. It won't exist after.
}
else if(token_type == ID)
{
//would create object of ID string here
//can't create object here. It won't exist after.
}
if(expression(pred))
{
//would create object of Expression here.can't create object here. It won't exist after.
}
//store object in object pred here. Pred has a private member of a vector of type parameters within it.
return true;
}
#ifndef PARAMETER_H
#define PARAMETER_H
#include <string>
#include <vector>
#include "Predicate.h"
using namespace std;
class Parameter
{
public:
private:
}
class String : public Parameter
{
public:
insert_string(string in_string);
private:
string my_string;
}
class ID : public Parameter
{
public:
insert_id(string in_ID);
private:
string my_ID;
};
class Expression : public Parameter
{
private:
Parameter left_parameter;
Parameter right_parameter;
string op;
public:
};
#endif
//also I want to know how I will be able to create left and right parameters in the expression class if i don't know what type they are yet

Answer 1

You can use pointers in this case. Create a Parameter* variable further up, and dynamically allocate the data structure inside one of your if-statements and assign to that pointer. For example:
Parameter *parameter = NULL;
if(token_type == STRING)
{
parameter = (Parameter*) new String();
}
Do something fancier with smart pointers if you don't want to worry about deallocation.