share variables between methods within a class

* a special map instance field to hold all such data. Map of Objects, so casting needed everywhere.

* (laziest) instance field? reserved for instance attributes. Don’t abuse.
* (laziest) static field? reserved for class attributes. Don’t contaminate.
* return a collection? A bit clumsy but recommended by many.
* best option: avoid sharing. Stick to local vars if possible.
* 2nd best: sometimes the shared variable really belongs to another class. That class/object perhaps is already shared between your methods. Essentially use that object as a courier. More work than the laziest.

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hibernate — essential techniques to SELECT query

* Technique: standard associations (m:1,m:n etc) — You need not write the query to SELECT the “associated” objects. If you load Students and want the associated Course loaded, Hibernate automatically constructs the Course query based on the Course.hbm.xml.

* Technique: HQL — You may need to write HQL to select students.

* Technique: views — Completely outside and unknown to hibernate, you can implement complex SELECT in a view and mention its name in a hbm.xml file, as if it’s a table.

* technique: native sql —

* technique: proc — existing logic in proc? This would be the ultimate. most powerful and customized.

2 FX interbank broker ^ 2 interdealer treasury broker

Treasury Inter-dealer brokers are the backbones of treasury market. BrokerTec and Espeed…

FX interbank brokers are the backbones of currency market. In Spot FX, EBS and Reuters (see separate blog) are the only 2 big brokers. See http://en.wikipedia.org/wiki/Interbank_market and the investorpedia article. Big banks handle very large transactions often in billions of dollars. These transactions cause the primary movement of currency prices (in the short term?) In the long term, fx is influenced not by the big bank’s actions, but economies.

Reuters’ system for Spot is a the electronic version of traditional voice execution. A screen-based “conversational” system so both sides know each other. Trades execute in the conversation, much like voice execution.

In contrast, EBS is anonymous. Trades execute when market-takers hit a button on screen.

For FX Forward, Reuters (see the other post) is dominant, but Tullet Prebon is popular too.

top 3 key nlg pearls on thread methods

A: wait(), lock()

#1. Each of the Thread object’s static/non-static methods must execute in a call stack — ie a real thread, and this real thread can be unrelated to the Thread. You should always ask “Is this Thread method affecting the call stack (ie the real thread) ?”

#2 Q: Many other methods (sometimes designed to) affect their call stack, but are not defined in Thread.java. Example?

v-table and v-ptr in pseudo code

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.4 is a one-pager with pseudo code. Here are some of my comments
tanbin – one v-table per class in the hierachy, shared by all instances. Parent’s v-table, child’s v-table…
tanbin – one v-ptr per instance. Since a child instance wraps a parent, the entire “onion” has a single v-ptr
tanbin – the v-ptr is reseated once when each “onion” layer is added during construction. Each child constructor in the hierarchy can reseat the v-ptr to point to the child’s own v-table

————
Let’s work an example. Suppose class Base has 5 virtual functions: virt0() through virt4().

// Your original C++ source code

class Base {
public:
virtual
arbitrary_return_type virt0(…arbitrary params…);
virtual
arbitrary_return_type virt1(…arbitrary params…);

};

Step #1: the compiler builds a static table containing 5 function-pointers, burying that table into static memory somewhere. Many (not all) compilers define this table while compiling the .cpp that defines Base‘s first non-inline virtual function. We call that table the v-table; let’s pretend its technical name is Base::__vtable. If a function pointer fits into one machine word on the target hardware platform, Base::__vtable will end up consuming 5 hidden words of memory. Not 5 per instance, not 5 per function; just 5 for the class. It might look something like the following pseudo-code:

// Pseudo-code (not C++, not C) for a static table defined within file Base.cpp

// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Base::__vtable[5] = {
&Base::virt0, &Base::virt1, &Base::virt2, &Base::virt3, &Base::virt4
};

Step #2: the compiler adds a hidden pointer (typically also a machine-word) to each object of class Base. This is called the v-pointer. Think of this hidden pointer as a hidden data member, as if the compiler rewrites your class to something like this:

// Your original C++ source code

class Base {
public:

FunctionPtr* __vptr;
supplied by the compiler, hidden from the programmer

};

Step #3: the compiler initializes this->__vptr within each constructor. The idea is to cause each object’s v-pointer to point at its class’s static v-table, as if it adds the following instruction in each constructor’s init-list:

Base::Base(…arbitrary params…)
: __vptr(&Base::__vtable[0])
supplied by the compiler, hidden from the programmer

{

}

Now let’s work out a derived class. Suppose your C++ code defines class Der that inherits from class Base. The compiler repeats steps #1 and #3 (but not #2). In step #1, the compiler creates a new hidden v-table for class Der, keeping the same function-pointers as in Base::__vtable but replacing those slots that correspond to overrides. For instance, if Der overrides virt0() through virt2() and inherits the others as-is, Der‘s v-table might look something like this (pretend Der doesn’t add any new virtuals):

// Pseudo-code (not C++, not C) for a static table defined within file Der.cpp

// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Der::__vtable[5] = {
&Der::virt0, &Der::virt1, &Der::virt2, &Base::virt3, &Base::virt4
}; ^^^^----------^^^^---inherited as-is

In step #3, the compiler adds a similar pointer-assignment at the beginning of each of Der‘s constructors. The idea is to reseat each Der object’s v-pointer so it points at Der class’s v-table. (This is not a second v-pointer; it’s the same v-pointer that was defined in the base class, Base; remember, the compiler does not repeat step #2 in class Der.)
Finally, let’s see how the compiler implements a call to a virtual function. Your code might look like this:

// Your original C++ code

void mycode(Base* p)
{
p->virt3();
}

The compiler has no idea whether this is going to call Base::virt3() or Der::virt3() or perhaps the virt3() method of another derived class that doesn’t even exist yet. It only knows for sure that you are calling virt3() which happens to be the function in slot #3 of the v-table. It rewrites that call into something like this:

// Pseudo-code that the compiler generates from your C++

void mycode(Base* p)
{
p->__vptr[3](p);
}

On typical hardware, the machine-code is two ‘load’s plus a call:

  1. The first load gets the v-pointer, storing it into a register, say r1.
  2. The second load gets the word at r1 + 3*4 (pretend function-pointers are 4-bytes long, so r1+12 is the pointer to the right class’s virt3() function). Pretend it puts that word into register r2 (or r1 for that matter).
  3. The third instruction calls the code at location r2.

Conclusions:

  • Objects of classes with virtual functions have only a small space-overhead compared to those that don’t have virtual functions.
  • Calling a virtual function is fast — almost as fast as calling a non-virtual function.
  • You don’t get any additional per-call overhead no matter how deep the inheritance gets. You could have 10 levels of inheritance, but there is no “chaining” — it’s always the same — fetch, fetch, call.

local variable declaration IS allocation@@

– local nonref (i.e. stackVar) declaration always allocate-create the object — the C tradition. (java made a bold departure.) This is the big deal in this post.

MyClass c; // allocates without initializing. calls noArg ctor? I doubt it?
MyClass c= …. // ditto

– local ptr variable declaration — always allocates the 32 bits for the pointer. [1]
– local ref variable declaration — always allocates and initializes the reference. Compare to ptr.
– function param declaration (nonref) always allocate-create the object, on stack.
– A field declaration doesn’t allocate-create the field object — not until host class construction-time. Same as in java.
^ obviously, if you see new … then you know it calls constructor, new-expression is fundamentally different from nonref variable declarations because
^^ heap
^^ returns pointer
^^ object created is nameless. In contrast, nonref variable is a name-plate carved on a memory location.

[1] It may initialize it to 0 or seat it to a real pointee object

std::string field and other non-ptr fields in a c++ class

Background — I feel this is a fundamental but overlooked design consideration.

In java, any non-primitive field is a ptr. In c++, std::string is a common field type. No ptr needed. But What other non-ptr fields are common? Let’s exclude primitive types like double or “bool” (shorter spelling than java “boolean”)

I feel any class like Address can be used as the nonref type of a field myAddr in a new class Customer. Customer ctor needs to allocate this field, but is it allocated on stack or heap? It depends on the context.

– If you allocate the entire Customer object on heap, then myAddr field is also on heap
– ditto for stack. You can trace the steps of myAddr’s allocation and there’s no malloc().

Either way, upon Customer de-allocation, myAddr is automatically /bulldozed/ since myAddr is on the real estate of the Customer object. sizeof(Customer) includes sizeof(Address), which is not the case for pimpl.

Now we know nonref field like qq{ Address myAddr; } is the c++ default whereas qq{ Address * myAddrPtr; } is the pimpl/java/c# version.

c++ casts usually work with pointers and refences

dynamic_cast — target type must be ref or ptr
dynamic_cast — source must be ref or ptr
———
const_cast can operate on nonref, but usually operates on ptr and references. Specifically, const-ref is a common func param, and const_cast abounds here.
Q: const_cast — target/source type can be nonref?
A: less common. I believe LHS is a distinct object. See post on casting-creates-new-pointer. In this case const_cast is like a return-by-clone function.

See also post on const nonref
——–
static_cast is _less_ common, but …
Q: static_cast — target and source type can be nonref?
Q: static_cast — source can be nonref?
Q: static_cast — LHS can be nonref?
A: similar to const_cast.
A: yes effC++ says copy ctor may be invoked.
A: yes http://www.cplusplus.com/doc/tutorial/typecasting/