02 C++: C++ Advance Notes (TBC)

02 C++: C++ Advance Notes (TBC)

1. Const

const int i=0;

const int * const p = &i; // read from right

const_cast<int&>(i)=1; // ok, 1

int j=0;

const_cast<const int&>(j)=1; // error assign read only location

Why const?

Self documented intention. Avoid inadvertent write. Compiler optimize variable when it is defined with const by storing to program executable at compiler time. 

2. Const in function

class C { void f(int); void f(const int); };

cannot compiler overload f by const param

class C { const string & getName(); }; is good to return by const string ref.

const member function overloading

class C { void f() {}void f() const {} };

const reference argument overloading

class C { void f(string); void f(const string&); };

3. logic const with mutable vs bitwise const by compiler

class C { volatile int counter; } ;

const_cast<C*>(this)->count++; // also ok

4. Compiler generated functions

copy constructor, copy assignment operator, default constructor, destructor

All public, inline, generated only if have caller

class C{};

class C {

C(const C&){} // member initialization

C& operator=(const C&){} // member copy, not generated if const member or reference member

C(); // base default, member default

~C(); // Member destroy, base destroy

class C { C()=default; }; // force generate

5. Disallow function

class C { public: C(C&)=delete; }; // no call

Use =delete or privatize the function

private destructor does not compile when used as stack object, forcing store at heap

6. Virtual destructor

factory pattern is a centralized place to produce objects.

Use virtual destructor.

Use shared_ptr without virtual destructor also works, but unique_ptr will not work.

All C++ standard library has no virtual destructor. 

7. Exception in destructor

Crash if exception thrown in destructor

Solution is to swallow all exceptions in destructor, or move exception-safe code away from desctructor

What is rvalue?

R value cannot be addressed

2 = i; so 2 is r value

&(i+1); so (i+1) is r value

(i+1)=2; so (i+1) is r value

&C(); so C() is r value

&sum(1,2); so sum(1,2) is r value

What is reference?

A variable reference a lvalue

So int& i=5; compiles error

const int& i=5; is 2 step. Step 1: temp l value assigned with 5 2. Reference a temp l-value

int f(int& i) {}; f(1); //compile error

int i=1; int j=i; l-value i is implicitly transformed to r-value before assignment

r-value can be used to create l-value:

int v[3]; *(v+2)=0; // r-val become l-val

Misconcept: Operator can return l-value

int g; int& f(){return g;};

f()=1;

(a+b)=1;

v[0]=1;

Misconcept: l-value always modifiable

In C yes. In C++ no, because of const int i;

Misconcept: r-value not modifiable

No. c().f(); the r value c() state is modified.

Expression is either l-value or r-value where l-value has identifiable memory address.