How to master C++

How to master C++
Florian Richoux
March 13, 2014
Florian Richoux How to master C++ 1/57

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Why C++?

Why C++?
CC BY 2.0

Why C++?
CC BY 2.0
http://www.lextrait.com/vincent/implementations.html

Outline
Quick recalls about virtual
Object copy
Memory management
Extra
Randall Munroe, CC BY-NC 2.0
http://xkcd.com/138/

Some (virtual) recalls

Some quick recalls
#i n c l u d e i o s t r e a m
using namespace s t d ;
struct A {
void f ( ) { c o u t Class A e n d l ; }
} ;
struct B : A {
void f ( ) { c o u t Class B e n d l ; }
} ;
i n t main ( ) {
A ∗a = new B ;
a−f ( ) ;
delete a ; // ?
}

Some quick recalls
struct A {
void f ( ) { c o u t Class A e n d l ; }
} ;
struct B : A {
} ;
i n t main ( ) {
A ∗a = new B ;
a−f ( ) ;
delete a ; // ?
}
Output
Class A

Some quick recalls
struct A {
v i r t u a l void f ( ) { c o u t Class A e n d l ; }
} ;
struct B : A {
} ;
i n t main ( ) {
A ∗a = new B ;
a−f ( ) ;
delete a ; // ?
}

Some quick recalls
struct A {
v i r t u a l void f ( ) { c o u t Class A e n d l ; }
} ;
struct B : A {
} ;
i n t main ( ) {
A ∗a = new B ;
a−f ( ) ;
delete a ; // ?
}
Output
Class B

Some quick recalls
c l a s s Base
{
. . .
} ;
c l a s s D e r i v e d : public Base
{
~ D e r i v e d ( )
{
// Do some i m p o r t a n t c l e a n u p
}
}
Base ∗b = new D e r i v e d ( ) ;
// u s e b
delete b ; // Here ' s t h e p r o b l e m : ( u s u a l l y ) c a l l ~Base ( )
http:
//stackoverflow.com/questions/461203/when-to-use-virtual-destructors

Some quick recalls
c l a s s Base
{
public : v i r t u a l ~Base ( ) { }
} ;
c l a s s D e r i v e d : public Base
{
~ D e r i v e d ( )
{
// Do some i m p o r t a n t c l e a n u p
}
}
Base ∗b = new D e r i v e d ( ) ;
// u s e b
delete b ; // c a l l ~ D e r i v e d ( )
http:
//stackoverflow.com/questions/461203/when-to-use-virtual-destructors

Some quick recalls
struct A { v i r t u a l ~A ( ) { } } ;
struct B : A { } ;
struct C { } ;
struct D : C { } ;
i n t main ( ) {
B b ;
A∗ ap = b ;
A a r = b ;
c o u t ap: typeid ( ∗ ap ) . name ( ) e n d l ;
c o u t ar: typeid ( a r ) . name ( ) e n d l ;
D d ;
C∗ cp = d ;
C c r = d ;
c o u t cp: typeid ( ∗ cp ) . name ( ) e n d l ;
c o u t cr: typeid ( c r ) . name ( ) e n d l ;
}

Some quick recalls
struct B : A { } ;
struct C { } ;
struct D : C { } ;
i n t main ( ) {
B b ;
A∗ ap = b ;
A a r = b ;
D d ;
C∗ cp = d ;
C c r = d ;
}
Output
ap: B
ar: B
cp: C
cr: C

Some quick recalls
struct B : A { } ;
struct C { } ;
struct D : C { } ;
i n t main ( ) {
B b ;
A∗ ap = b ;
A a r = b ;
D d ;
C∗ cp = d ;
C c r = d ;
}
Output
ap: B
ar: B
cp: C
cr: C
http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?
topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fthe_typeid_operator.htm

Object copy

Copy constructor and copy assignment operator
c l a s s P e r s o n
{
s t d : : s t r i n g name_ ;
i n t age_ ;
public :
P e r s o n ( s t d : : s t r i n g name , i n t age )
: name_ ( name ) , age_ ( age ) { }
} ;
i n t main ( )
{
P e r s o n a ( Bjarne Stroustrup , 6 3 ) ;
P e r s o n b ( a ) ; // What h a p p e n s h e r e ?
b = a ; // And h e r e ?
}

// 1 . c o py c o n s t r u c t o r
P e r s o n ( const P e r s o n t h a t )
: name_ ( t h a t . name_ ) , age_ ( t h a t . age_ ) { }
// 2 . c o py a s s i g n m e n t o p e r a t o r
P e r s o n operator=( const P e r s o n t h a t )
{
name_ = t h a t . name_ ;
age_ = t h a t . age_ ;
return ∗ t h i s ;
}
Signature
C l a s s n a m e ( const C l a s s n a m e ) // c o py c t o r
C l a s s n a m e operator=( const C l a s s n a m e ) // a s s i g n m e n t op

i n t main ( )
{
P e r s o n b ( a ) ; // C a l l t h e co p y c t o r
b = a ; // C a l l t h e co p y a s s i g n m e n t o p e r a t o r
P e r s o n c = a ; // ?
}

i n t main ( )
{
P e r s o n b ( a ) ; // C a l l t h e co p y c t o r
b = a ; // C a l l t h e co p y a s s i g n m e n t o p e r a t o r
P e r s o n c = a ; // C a l l t h e co p y c t o r
// ( a l m o s t e q u i v a l e n t t o P e r s o n c ( a ) )
}
More about initializations
http://herbsutter.com/2013/05/09/gotw-1-solution/

In which situations is the C++ copy constructor called?
MyClass a ;
MyClass b ( a ) ; // co py c o n s t r u c t o r
// //////////////
void f o o ( MyClass x ) ;
f o o ( a ) ; // co py c o n s t r u c t o r
// ( b u t can be moved i n C++11)
// A s i m p l e t h i n g t o a v o i d t h i s ?
// //////////////
MyClass f o o ( )
{
MyClass temp ;
. . .
return temp ; // co py c o n s t r u c t o r
// ( b u t u s u a l l y RVO a p p l i e s )
}
http://stackoverflow.com/questions/21206359/
in-which-situations-is-the-c-copy-constructor-called

In which situations is the C++ copy constructor called?
MyClass a ; // c o n s t r u c t o r
MyClass b ; // c o n s t r u c t o r
a = b ; // co py a s s i g n m e n t op
b = MyClass ( a ) ; // co py c t o r + co py a s s i g n m e n t op
MyClass ∗a = new MyClass ( ) ; // c o n s t r u c t o r
MyClass ∗b ; // n o t h i n g i s c a l l e d
b = a ; // s t i l l n o t h i n g i s c a l l e d
b = new MyClass ( ∗ a ) ; // co py c o n s t r u c t o r
in-which-situations-is-the-c-copy-constructor-called

Why writing a copy ctor and a copy assignment operator?
Question
Why do we need to (sometimes) write them?

Question
Reformulated question
When do we need to write them?

Question
Reformulated question
When do we need to write them?
Answer
Each time you have a class managing resources (like manipulating
memory, pointers)!

c l a s s A {
public :
A ( ) { i = new i n t ; }
i n t ∗ i ;
} ;
A a ;
A b = a ;
// same s t o r y w i t h j u s t b = a
s t d : : c o u t a . i s t d : : e n d l b . i s t d : : e n d l ;

c l a s s A {
public :
A ( ) { i = new i n t ; }
i n t ∗ i ;
} ;
A a ;
A b = a ;
Output
0x3A28213A
0x3A28213A

c l a s s A {
public :
A ( ) { i = new i n t ; } // c t o r
A( const A o t h e r ) { // co py c t o r
i = new i n t ;
∗ i = ∗( o t h e r . i ) ;
}
i n t ∗ i ;
} ;
A a ;
A b = a ;

c l a s s A {
public :
A ( ) { i = new i n t ; } // c t o r
A( const A o t h e r ) { // co py c t o r
i = new i n t ;
∗ i = ∗( o t h e r . i ) ;
}
i n t ∗ i ;
} ;
A a ;
A b = a ;
Output
0x3A28213A
0x6339392C

The copy-and-swap idiom
I explained you:
What copy ctor and copy assignment operator are.
When they are called.
Why it is important to (sometimes) write them.
But I did not explain yet how to implement them properly.
Good implementation
Apply the copy-and-swap idiom.

c l a s s MyClass {
public :
MyClass ( s t d : : s i z e _ t s i z e = 0 ) // c t o r
: s i z e ( s i z e ) ,
a r r a y ( s i z e ? new i n t [ s i z e ] : n u l l p t r )
{}
MyClass ( const MyClass o t h e r ) // co py c t o r
: s i z e ( o t h e r . s i z e ) ,
a r r a y ( s i z e ? new i n t [ s i z e ] : n u l l p t r )
{ s t d : : co py ( o t h e r . a r r a y , o t h e r . a r r a y + s i z e , a r r a y ) ; }
private :
s t d : : s i z e _ t s i z e ;
i n t ∗ a r r a y ;
} ;
what-is-the-copy-and-swap-idiom/

c l a s s MyClass {
. . .
public :
MyClass operator=( const MyClass o t h e r ) // co py asgmt op
{
i f ( t h i s != o t h e r )
{
// p u t i n t h e new d a t a . . .
s t d : : s i z e _ t n e w S i z e = o t h e r . s i z e ;
i n t ∗ n e w A r r a y = n e w S i z e ? new i n t [ n e w S i z e ] : n u l l p t r ;
s t d : : co p y ( o t h e r . a r r a y , o t h e r . a r r a y + s i z e , n e w A r r a y ) ;
// . . . and g e t r i d o f t h e o l d d a t a
delete [ ] a r r a y ;
s i z e = n e w S i z e ;
a r r a y = n e w A r r a y ;
}
}
} ;

c l a s s MyClass {
. . .
public :
MyClass operator=( const MyClass o t h e r ) // co py asgmt op
{
i f ( t h i s != o t h e r ) // o f t e n u s e l e s s
{
// p u t i n t h e new d a t a . . .
s t d : : s i z e _ t n e w S i z e = o t h e r . s i z e ;
i n t ∗ n e w A r r a y = n e w S i z e ? new i n t [ n e w S i z e ] : n u l l p t r ;
s t d : : co p y ( o t h e r . a r r a y , o t h e r . a r r a y + s i z e , n e w A r r a y ) ;
// ( t h e s e 3 l i n e s a r e c o d e d u p l i c a t i o n )
// . . . and g e t r i d o f t h e o l d d a t a
delete [ ] a r r a y ;
s i z e = n e w S i z e ;
a r r a y = n e w A r r a y ;
}
}
} ;

c l a s s MyClass {
. . .
public :
void swap ( MyClass o t h e r )
{
s t d : : swap ( this −s i z e , o t h e r . s i z e ) ;
s t d : : swap ( this −a r r a y , o t h e r . a r r a y ) ;
}
MyClass operator=( MyClass o t h e r ) // no r e f e r e n c e !
{
swap ( o t h e r ) ;
}
} ;
what-is-the-copy-and-swap-idiom/

Memory management

The Rule of Three
Rule of 3
If your class needs any of
a destructor,
or a copy constructor,
or a copy assignment operator.
dened explicitly, then it is likely to need all three of them.
Put in other words
If your class manages resources, you need to explicitly dene:
a destructor,
a copy constructor,
and a copy assignment operator.

The Rule of Three
These three are linked
What does a copy assignment operator do?
It copies a new state (copy ctor),
and it deletes the old state (destructor).
what-is-the-rule-of-three

The Rule of Three
The rule a delete for each new is not sucient!
c l a s s A
{
public :
A( i n t i ) : a r r a y _ ( i ? new i n t [ i ] : n u l l p t r ) { }
~A ( ) { delete [ ] a r r a y _ ; }
private :
i n t ∗ a r r a y _ ;
} ;
A ∗a1 = new A ( 4 2 ) ;
A ∗a2 = new A ( 2 4 ) ;
. . .
( ∗ a1 ) = ( ∗ a2 ) ;
. . .
delete a1 ;
delete a2 ;

The Rule of Three
The rule A delete for each new is not sucient!
c l a s s A
{
public :
A( i n t i ) : a r r a y _ ( i ? new i n t [ i ] : n u l l p t r ) { }
~A ( ) { delete [ ] a r r a y _ ; }
private :
i n t ∗ a r r a y _ ;
} ;
A ∗a1 = new A ( 4 2 ) ;
A ∗a2 = new A ( 2 4 ) ;
. . .
( ∗ a1 ) = ( ∗ a2 ) ; // Memory l e a k !
. . . // We h a v e l o s t o r i g i n a l a1 ' s a r r a y _
delete a1 ;
delete a2 ; // U n d e f i n e d b e h a v i o r !

C++11 and move semantics
c l a s s V e c t o r {
i n t ∗ s t o r a g e _ ;
s i z e _ t s i z e _ ;
public :
// c t o r
V e c t o r ( s i z e _ t numElements )
: s t o r a g e _ ( new i n t [ numElements ] ) ,
s i z e _ ( numElements )
{ }
// d t o r
~ V e c t o r ( ) { delete [ ] s t o r a g e _ ; }
} ;
http://kholdstare.github.io/technical/2013/11/23/
moves-demystified.html

V e c t o r c = a + b ;
? ? ? operator+ ( V e c t o r const a , V e c t o r const b ) ;

V e c t o r c = a + b ;
Return by value seems bad.

V e c t o r c = a + b ;
Return a pointer is bad too: you must make disallocation
somewhere, and can't chained + operations (like a+b+c).

V e c t o r c = a + b ;
Return a pointer is bad too: you must make disallocation
somewhere, and can't chained + operations (like a+b+c).
Return a reference seems not a good idea either.

You need to
move!

Is returning by value really bad?
V e c t o r operator+ ( V e c t o r const a , V e c t o r const b )
{
// c r e a t e r e s u l t o f same s i z e
a s s e r t ( a . s i z e ( ) == b . s i z e ( ) ) ;
V e c t o r r e s u l t ( a . s i z e ( ) ) ;
// compute a d d i t i o n
s t d : : t r a n s f o r m (
a . b e g i n ( ) , a . end ( ) , // i n p u t 1
b . b e g i n ( ) , // i n p u t 2
r e s u l t . b e g i n ( ) , // r e s u l t
s t d : : p l u s int () // b i n a r y o p e r a t i o n
) ;
return r e s u l t ; // RVO u s u a l l y a p p l i e s
}

Yes, but...
Reason #1
s t d : : s t r i n g f ( bool cond = f a l s e ) {
s t d : : s t r i n g f i r s t ( first ) ;
s t d : : s t r i n g s e c o n d ( second ) ;
return cond ? f i r s t : s e c o n d ; // r e t u r n u n d e r c o n d i t i o n :
// u s u a l l y no RVO
}
Reason #2
RVO applies when one transfers a value out of a scope.
What if we need to transfer into a scope?
http://en.wikipedia.org/wiki/Return_value_optimization

Transferring value into a scope
Ray computeRay ( )
{
V e c t o r o r i g i n ;
V e c t o r d i r e c t i o n ;
. . .
return Ray (
o r i g i n , // COPY!
d i r e c t i o n // COPY!
) ; // c e r t a i n l y RVO
}

lvalue Vs rvalue
lvalue
c = a + b;
rvalue
c = a + b;
Must be a temporary, non-named value.
what-are-rvalues-lvalues-xvalues-glvalues-and-prvalues

V e c t o r : : V e c t o r ( V e c t o r o t h e r )
// s h a l l o w co p y
: s t o r a g e _ ( o t h e r . s t o r a g e _ ) ,
s i z e _ ( o t h e r . s i z e _ )
{
// n u l l i f y s o u r c e
o t h e r . s t o r a g e _ = n u l l p t r ;
o t h e r . s i z e _ = 0 ;
}
http://kholdstare.github.io/technical/2013/11/23/moves-demystified.html

Transferring value into a scope
Ray computeRay ( )
{
V e c t o r o r i g i n ;
V e c t o r d i r e c t i o n ;
. . .
return Ray (
s t d : : move ( o r i g i n ) , // moved !
s t d : : move ( d i r e c t i o n ) // moved !
) ; // c e r t a i n l y RVO
}

The Rule of Four and a Half
When a class manipulates resources
Rule of 4.5
=
Rule of 3
+
dene the move ctor
(+ dene a move assignment operator?)
rule-of-three-becomes-rule-of-five-with-c11
http:
//stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom

RAII and Smart pointers
RAII
Resource Acquisition Is Initialization: release resource automatically.
Some applications
Files,
Network sockets,
Mutex,
Memory.
Smart pointers std::unique_ptr and std::shared_ptr.

Rule of Zero
Rule of 0
Using smart pointers (and RAII principle) to manage resources, no need
to explicitly declare dtor, copy ctor, etc.
http://flamingdangerzone.com/cxx11/2012/08/15/rule-of-zero.html

Rule of Zero
Rule of 0
Using smart pointers (and RAII principle) to manage resources, no need
to explicitly declare dtor, copy ctor, etc.
You can
rest!
http://flamingdangerzone.com/cxx11/2012/08/15/rule-of-zero.html

Extra

I did not talk about
const (http://duramecho.com/ComputerInformation/WhyHowCppConst.html)
Exceptions (and C++11 noexecpt)
operator+= and operator+ (and operator++ and stu)
C++ cast
Functors
Most of C++11/14/17 features
(https://github.com/AnthonyCalandra/modern-cpp-features)

Safety
Asserts
Use assert. Disable them with the -DNDEBUG compile option.
Valgrind
valgrind leak-check=full show-reachable=yes ./your_program
Wonderful GUI: kcachegrind
Warnings
Try to solve them!

Read!
Books
Ecient C++ by Scott Meyers (C++11/14 update soon!)
Exceptionnal C++ by Herb Sutter (C++11/14 update soon!)
Blog
Herb Sutter's Guru of the Week
http://herbsutter.com/category/c/gotw/
Twitter
@isocpp
@cppstack

Use!
Boost library
http://www.boost.org/
algorithm
Gotta use 'em all!
http://www.cplusplus.com/reference/algorithm/

Fonctional C++
John Carmack's blog
http://www.altdevblogaday.com/2012/04/26/
functional-programming-in-c/
Modern Functional Programming in C++
http://zao.se/~zao/boostcon/10/2010_presentations/thu/
funccpp.pdf
C++17: I See a Monad in Your Future!
http://bartoszmilewski.com/2014/02/26/
c17-i-see-a-monad-in-your-future/

Template Metaprogramming
Books
Modern C++ Design by Andrei Alexandrescu.
C++ Template Metaprogramming by Dave Abrahams and
Aleksey Gurtovoy.
C++ Templates: The Complete Guide by David Vandevoorde
and Nicolai Josuttis (second edition planned for 2015).
A nice intro
http://www.codeproject.com/Articles/3743/
A-gentle-introduction-to-Template-Metaprogramming
A nice and modern intro
http://www.oreilly.com/programming/free/
practical-c-plus-plus-metaprogramming.csp (Free e-book!)

SOLID
Single Responsibility: One reason to exist, one reason to change
Open Closed Principle: Open for extension, closed for modication
Liskov Substitution Principle: An object should be semantically
replaceable for it's base class/interface
Interface Segregation Principle: Don't force a client to depend on an
interface it doesn't need to know about
Dependency Inversion Principle: Depend on abstractions, not
concrete detail or implementations
http://stackoverflow.com/questions/1423597/solid-principles
http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29

Repository and comments
svn, git, mercurial, ...
Ultimate combo
GitHub + Travis
(http://docs.travis-ci.com/user/getting-started/)
Comments
Comment your code with doxygen

Repository and comments
svn, git, mercurial, ...
Ultimate combo
GitHub + Travis
(http://docs.travis-ci.com/user/getting-started/)
Comments
Comment your code with doxygen in English!

To make progress
Code!

To make progress
Code!
Teach!

C++ hiring questions 1/3
How many ways are there to initialize a primitive data type in C++
and what are they?
Why should you declare a destructor as virtual?
What does it mean that C++ supports overloading?
What are examples of overloading in C++?
What is name mangling in C++ and why is it used?
What is an abstract base class?
What is RTTI?
How can you access a variable that is hidden by another variable of
the same name?
What is a namespace and how is it used.
What are the dierences between a class and a struct in C++, and
how does this compare to C?
What are templates? How are they used?
What is a copy constructor and when is it used, especially in
comparison to the equal operator.
What is the dierence between a shallow and a deep copy?
What is the const operator and how is it used?

What are the dierences between passing by reference, passing by
value, and passing by pointer in C++?
When is it and when is it not a good idea to return a value by
reference in C++?
What is the dierence between a variable created on the stack and
one created on the heap?
How do you free memory allocated dynamically for an array? What
are the implications of just using delete?
What is multiple inheritance? When should it be used?
What is a pure virtual function?
What does the keyword mutable do?
What does the keyword volatile do?
What is the STL?
What is a Vector?
What is contained in the algorithms header?

What is the dierence between #include iostream.h and
#include iostream?
What's the dierence between ++i and i++?
What is short circuit evaluation? How can it be used? Why can is
be dangerous?
What is the `,' operator?
What is the only ternary operator? How is it used?
What is the use of a const member function and how can it be used?
How is try/catch used in C++?
Why should you never throw an exception in a destructor?
What is the explicit keyword?
What is the proper way to perform a cast in C++?
What does inline do?

Thanks!

How to master C++

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