some Technicalities for QnA IV

We often call these “obscure details”. At the same level, these are a small subset of a large amount of details, so we can’t possibly remember them all 😦

Surprisingly, interviewers show certain patterns when picking which technicality to ask. Perhaps these “special” items aren’t at the same level as the thousands of other obscure details??

These topics are typical of QQ i.e. tough topics for IV only, not tough topics for GTD.

• archetypical: which socket syscalls are blocking and when
• $LD_LIBRARY_PATH
• hash table theoretical details? too theoretical to be part of this discussion
• select() syscall details
• vptr, vtable

 

c++method default arg isn’t saved]vtable

1. Google c++ style guide forbids default args in virtual functions. It’s error-prone.
2. [[c++primer]] P607 has a slightly more liberal advice — on a virtual function, subclass/baseclass always use same default arg.

Ashish said he was asked a common question in multiple c++ interviews. The default arg value is “declared” in base class and (or) derived class but never saved in the vtable. It’s basically decided statically (i.e. at compile time).

If you use a pointer to Base to invoke the virtual method, then the Base version’s declared default arg applies unconditionally, even though subclass override is chosen at run time, via the vtable. Highly confusing.

c++ stream — IKM findings

——tellg and tellp?
http://answers.yahoo.com/question/index?qid=20110507033952AAXZBz2 is a short explanation.

tellg(void) and tellp(void) both return their pointer’s position
——
ostream& endl (ostream& os);
——
stream::rdbuf() changes the filebuf… http://www.cplusplus.com/reference/ios/ios/rdbuf/
——
endl – flush() implicitly
——
If your variable to “populate” is an int, then extraction operator stops when hitting ….”any character that couldn’t be part of an int” which is not limited to white space.
—-which stream classes can be used for writing to a file
ofstream?
fstream?
ostream?
——which ios modes are used for reading a file

c++ template technicality – IKM findings

Never quizzed when I declare I’m no template wizard…

to specialize a func template,

template <typename T> void fn(T a){}
template<nothing here> void fn(char c){…} //empty param to specialize an existing func template

template<class T> void f(T arg){ cout<<arg<<endl;}
template<> void f<char>(char arg){ cout<<"char: "<<arg<<endl;}
// the <char> after f is optional !
int main() {
  f(1.3);
  f('a');
}
///////////////// Some<int> is distinct from Some<char> -- which is the basis of my friend caching factory in 
https://github.com/tiger40490/repo1/blob/cpp1/cpp/template/friendFactory.cpp 
template <typename T> class Some{
  public: static int stat;
};

template<typename T> int Some<T>::stat = 10;
int main(){
  Some<int>::stat = 5;
  cout<<Some<int>::stat<<endl;
  cout<<Some<char>::stat<<endl;
  cout<<Some<short>::stat<<endl;
  cout<<Some<long>::stat<<endl;
}

select2perform/brain-bench c++ Q&A

Q: vector1.erase(remove(vector1.rbegin(),vector.rend(),someValue).base())

Q: To put 5 identical values into a vector,

std::fill_n() ?

std::fill() ?

Q: If class E virtually extends C and extends D, where C/D both virtually extend B, what’s the construction sequence?
AA: P1000 [[primer]]

Q: find intersection of 2 compatible vectors?
%%A: std::find_first_of()
Q: can you pass a float into a func(int) or func(char)?
AA: yes but if you cout in func(), it interprets and prints the object differently.
Q: can a non-static reference field be initialized upon declaration?
%%A: i don’t think so, reference field or nonref. The java-style quick initi is illegal for non-static.
AA: gcc allows java-style init only for a static AND const field.

Q: can you “extern” a prototype and then declare the same prototype again?
AA: legal

Q[u]:difference between “new Account” and “new Account()” with the parentheses?
A: subtle difference. See other posts on this blog such as http://bigblog.tanbin.com/2012/07/default-initializevalue-initialize-new.html

Q[u]: declare a pointer to a method accepting an int and returning void. Need to memorize the exact syntax
AA: void (Dog::*ptr2method) (int);

Q: my ex spec mentions std::runtime_error. Can I throw a range_error, or an std::exception, or std::invalid_argument?
AA:

exception

runtime_error

range_error

exception logic_error invalid_argument

Q: reverse print a string using copy to cout
AA: copy(str.rbegin(), str.rend(), ostream_iterator< char >(cout, ” “));
* The ” ” 2nd argument is not necessary, but I’d suggest we memorize just a single form of the ostream_iterator ctor, the 2-arg form.
* The type is char not  ptr-to-char

Q: using N1::f; f(); using N2::f; f();
AA: GCC complains 2nd all to f() is ambiguous. http://www.cplusplus.com/doc/tutorial/namespaces/ explains 

Q[u]: multiple inheritance dreaded diamond – what if one of the 2 “virtual” keywords is omitted? How many times will you call the base constructor?
AA: http://bigblog.tanbin.com/2010/07/virtual-inheritance-diamond-malformed.html

Q[u]: anything wrong with
Base & var = static_cast(*(new Derived));
AA: legal in gcc.
—
[u = unimportant/trivial/unlikely to be issue in practice. Often compiler will inform you clearly.]