Compiler: How is class instantiation code compiled?

Question

Assume we have (in C++): MyClass* x = new MyClass(10)

Can someone please explain what 'exactly' happens when compiler parses this statement? (I tried taking a look at the Red Dragon book but couldn't find anything useful).

I want to know what happens in the stack/heap or compiler's symbol table. How compiler keeps track of the type of x variable? How later calls to x->method1(1,2) will be resolved to appropriate methods in MyClass (for simplicity assume there is no inheritance and MyClass is the only class that we have).

Answer 1

MyClass* x is a definition of pointer to object (instance) of type MyClass. Memory for that variable is allocated according to the place of its definition: if it is defined in the method, and is a local variable, stack is used. And it is memory to store the address.

Then expression new MyClass(10) is a command to allocate memory in heap for an object (instance) itself and return address to be stored in x. To fill the memory of new object MyClass (set up its initial state) special method (at least one) is executed automatically - constructor (or several in some cases) - that receives value 10 in your example.

Because C++ allows inheritance (this is also the reason for the execution of several constructors when an instance created) there are some mechanism to determine which method should be exactly called. You should read somewhere about Virtual method table.

In the simplest case (without inheritance), type of variable x (pointer to object of type MyClass) provide all necessary information about object structure. So, x->method1(1,2) or (*x).method1(1,2) provide call of member method1 to execute it with parameters 1 and 2 (stored in stack) as well as with data that form the state of object (stored in heap) and available by this pointer inside any non-static member of class. The method itself, of course, not stored in the heap.

UPDATE:

You can make example to make same experiments, like:

#include <iostream>
#include <string>
using namespace std;

class MyClass
{
private:
    int innerData;
    long long int lastValue;
public:
    MyClass() // default constructor
    {
        cout << "default constructor" << endl;
        innerData = 42;
        lastValue = 0;
    }
    MyClass(int data) // constructor with parameter
    {
        cout << "constructor with parameter" << endl;
        innerData = data;
        lastValue = 0;
    }
    int method1(int factor, int coefficient)
    {
        cout << "Address in this-pinter " << this << endl;
        cout << "Address of innerData " << &innerData << endl;
        cout << "Address of lastValue " << &lastValue << endl;
        cout << "Address of factor " << &factor << endl;
        lastValue = factor * innerData + coefficient;
        return lastValue;
    }
};

int main(void)
{
    MyClass* x = new MyClass(10);
    cout << "addres of object " << x << endl;
    cout << "addres of pointer " << &x << endl;
    cout << "size of object " << sizeof(MyClass) << endl;
    cout << "size of pointer " << sizeof(x) << endl;
    x->method1(1, 2);
}