I am making a toy programming language in c++, but i have run into a problem. I have noticed that in c++ a stack can only store one type of data. I was wondering if there was an easy way to fix this problem, such as by storing in the stack a byte array of each object. I was wondering if anyone knows how the jvm overcomes this issue. The types i would need to store on the stack would be char, short, int, float, double, strings, arrays, and references to objects. I understand that the jvm stack might be more of an abstraction, but if it is i would still like to know how they have accomplished it. If it makes any difference, i am only planning to target windows computers.
3 Answers
You know C++ has support for inheritance and polymorphism, right? A far easier way to do this is to derive all your tokens from a common base class, and make a stack of Base * objects, for instance:
#include <iostream>
#include <string>
#include <stack>
#include <memory>
class base {
public:
virtual void print_token() = 0;
virtual ~base() {}
};
class token_a : public base {
public:
token_a(int n) : n(n) {}
virtual void print_token() { std::cout << n << std::endl; }
private:
int n;
};
class token_b : public base {
public:
token_b(std::string s) : s(s) {}
virtual void print_token() { std::cout << s << std::endl; }
private:
std::string s;
};
int main(void) {
std::stack<std::shared_ptr<base> > my_stack;
my_stack.push(std::shared_ptr<base>(new token_a(5)));
my_stack.push(std::shared_ptr<base>(new token_b("a word")));
for ( int i = 0; i < 2; ++i ) {
std::shared_ptr<base> pb = my_stack.top();
pb->print_token();
my_stack.pop();
}
return 0;
}
outputs:
paul@local:~/src/cpp/scratch$ ./stack
a word
5
paul@local:~/src/cpp/scratch$
Comments
The way I have solved this problem (in C, for a lisp interpretr, about 25 years ago, but same idea applies today) is to have a struct with a type and a union inside it:
struct Data // or class
{
enum kind { floatkind, intkind, stringkind, refkind };
Kind kind;
union
{
double f;
int i;
std::string s;
Data* r; // reference, can't use Data &r without heavy trickery.
} u;
Data(double d) { kind = floatkind; u.f = d; }
Data(int i) { kind = intkind; u.i = i; }
...
}
std::stack<Data> st;
st.push(Data(42));
st.push(Data(3.14));
Comments
Just a guess, but the jvm probably treats everything as an object, so the stack is simply a collection of objects.
You can do the same, if you create a base data object class and derive all your supported data types from it.
Comments
std::stack<boost::variant<...>>.