Effective Code Transformations in C++

Effective Code Transformations in C++
Marco Arena

Paolo Polce

marco@italiancpp.org

paolo@webshell.it

Reggio Emilia, 30 Novembre 2013

Italian C++ Community

www.italiancpp.org
@italiancpp

2

When I hear they’re adding features to C++
This PLT Life - http://this-plt-life.tumblr.com/
4

Outline
• C++11, benvenuto nel team!
– C++98  C++11 & snippet C#

• Il team cambia stile & coding standards
– Design patterns
– Multi-threaded C++

• Alcune linee guida
5

Outline
– C++98  C++11 & snippet C#

– Design patterns

6

C++ o C#?!

var dati = new List<int>(){1,2,3,4,5};

7

C++ o C#?!

auto dati = vector<int>{1,2,3,4,5};

8

auto
// C++98
vector<int> vec (10);
vector<int>::iterator it = vec.begin();

auto
// C++98
it

10
1

2

3

4
12

6

it

5

7

11

13
10

auto
// C++98

// C++11

auto it = vec.begin();

auto
// C++98
vector< pair<string, int> > cards = ... ;
vector< pair<string, int> >::iterator it = cards.begin();

12

auto
// C++98
vector< pair<string, int> > cards = ... ;
vector< pair<string, int> >::iterator it = cards.begin();

// C++11

auto it = cards.begin();
13

range-based iteration
// C++98
for (vector<int>::iterator it = vec.begin();
it != vec.end();
++it)
{
cout << *it << " ";
}

14

// C++11
for (auto i : vec)
{
cout << i << " ";
}

15

// C++11

for (auto i : vec)
{
cout << i << " ";
}
for (const auto& i : vec)
{
cout << i << " ";
}
16

C++98

C++11

for ( vector<int>::iterator it =
vec.begin();
it != vec.end();
++it)
{
cout << *it << " ";
}

for (auto i : vec)
{
cout << i << " ";
}

C#
foreach (var i in arrayList)
{
System.Console.Write(i);
}
17

lambdas & algorithms
// C++98
vector<int>::const_iterator find_in_range(const vector<int>& vec)

{
vector<int>::const_iterator i = vec.begin();

for( ; i != vec.end(); ++i )
{
if( *i > 0 && *i < 10 ) break;
}
return i;

}
18

// C++98
bool elem_in_range(int elem)
{
return elem > 0 && elem < 10;
}
vector<int>::const_iterator it =
find_if (vec.begin(), vec.end(), elem_in_range);

19

// C++11
auto it = find_if (begin(vec), end(vec),
[](int i) {
return i > 0 && i < 10;
});

20

// C++14
auto it = find_if (begin(vec), end(vec),
[](auto i) {
return i > 0 && i < 10;
});

21

C++14

[](auto i) { return i > 0 && i < 10; }

C#
(int i) => i > 0 && i < 10;

22

C++ o C#?!

auto dati = vector<int>{1,2,3,4,5};

23

initializer lists
// C++98
int arr[] = {1,2,3,4,5};
vector<int> data;
data.push_back(1);
data.push_back(2);
...
vector<int> data (arr, arr + 5);
24

initializer lists
// C++98
int arr[] = {1,2,3,4,5};
vector<int> data (arr, arr + 5);

// C++11
vector<int> data = {1,2,3,4,5};
// oppure (auto-everything syntax):

auto data = vector<int>{1,2,3,4,5};

25

initializer lists
C++98

C++11

vector<int> data;
data.push_back(1);
data.push_back(2);
data.push_back(3);
...

auto data = vector<int>{1,2,3,4,5};

C#

var dati = new List<int>() {1, 2, 3, 4, 5};

26

smart pointers
// C++98
{
Car* car = new Car(...);
...
car->Move();
...
delete car;
}
27

smart pointers
// C++98
{
Car* car = new Car(...);
...
car->Move();
throw exception(…)
...
delete car;
}
28

smart pointers
// C++98

Car *car = NULL;
try
{
car = new Car(...);
car->Move();
delete car;
}
catch(...)
{
delete car;
ASSERT(FALSE);
LOG(ERROR) << "critical";
throw;
}
29

smart pointers
// C++98

catch(...)
{
delete car;
ASSERT(FALSE);
LOG(ERROR) << "critical";

throw;
}

30

smart pointers
{ // C++11
unique_ptr<Car> car {new Car{arg1}};
...
car->Move();
...

} // delete automatico
// C++14
auto car = make_unique<Car>(arg1)
31

Outline
– C++98  C++11 & snippet C#

– Design patterns

32

Modern C++: Factory
// C++98
IWriter* Create(...)
{
...
return new CoutWriter(...);
}
// chiamante:
IWriter* writer = Create (...);
...
delete writer;

33

Modern C++: Factory
// C++11
unique_ptr<IWriter> Create (...)
{
...
return unique_ptr<IWriter>(new CoutWriter(...));
}
// chiamante:

auto writer = Create(...);

34

Modern C++: Factory
// C++98
class WriterFactory
{
public:
IWriter* Create(const string& where)
{
if (where == "cout")
return new CoutWriter();
if (where == "dbgview")
return new DbgViewWriter();
if (where == " null")
return new NullWriter();
throw exception("where to write?");
}
};

35

Modern C++: Factory
// C++11-14
class WriterFactory
{
public:
unique_ptr<IWriter> Create(const string& where)
{
static map<string, function<unique_ptr<IWriter>()>> m =
{
{"cout", []{return make_unique<CoutWriter>()}},
{"dbgview", []{return make_unique<DbgViewWriter>()}},
{"null", []{return make_unique<NullWriter>()}}
};
return m.at(where)(); // può sollevare eccezione
// oppure una policy diversa
}
36
};

Modern C++: RAII & DEFER
bool FileExists(const char* path)
{
ifstream file{path}; // file.open()
return file.is_open();
} // file.close() automatico

37

void SomeFuntion()
{
... Anything ...
Log("SomeFunction executed");
}

38

void SomeFuntion()
{
... Anything ...

throw exception(…)

}

39

// go style
func SomeFuntion()
{
defer func() {
Log("SomeFunction executed")
}
... Anything ...
}

40

void SomeFuntion()
{
defer d{ []{
}};
... Anything ...
}

41

class defer
{
function<void()> m_toExec;
public:
defer(function<void()> f) : m_toExec{f} { }

~defer()
{
m_toExec();
}
};
Qui una versione migliore e completa: http://www.italiancpp.org/2013/07/16/defer-con-raii-lambda/
42


Live Demo
git clone https://ilpropheta@bitbucket.org/ilpropheta/iad2013.git

main.cpp: DeferTest

43

Outline
– C++98  C++11 & snippet C#

– Design patterns

44

Multi-Threaded C++
int main()
{
vector<int> vec(100000);
iota(begin(vec), end(vec), 0); // 0 1 2 3 4 ...
int avg = 0;
thread t1 { [&]{
avg = accumulate(begin(vec), end(vec), 0)/vec.size();
}};
auto maxEl = max_element(begin(vec), end(vec));
t1.join();

cout << "max: " << *maxEl << endl;
cout << "avg: " << avg << endl;
}
45

Multi-Threaded C++

Live Demo

main.cpp: ThreadTest

46

Multi-Threaded C++
int main()
{
vector<int> vec(100000);
iota(begin(vec), end(vec), 0);
auto willbeAvg = async ( [&]{
return accumulate(begin(vec), end(vec), 0)/vec.size();
}};
cout << "max: " << *max_element(begin(vec), end(vec)) << endl;
cout << "avg: " << willbeAvg.get() << endl;
}

47

Multi-Threaded C++

Live Demo

main.cpp: FutureTest

48

Multi-Threaded C++

future

promise

get()

v

set_value(v)

Thread 1

Thread 2
49

Multi-Threaded C++

Live Demo

main.cpp: PromiseTest

50

Multi-Threaded C++: PPL

void vector_mult(vector<double>& v, double c)
{
for_each( begin(v), end(v),
[&](double& n) {
n *= c;
}
);
}

51

#include <ppl.h>
using namespace Concurrency;
void vector_mult(vector<double>& v, double c)
{
parallel_for_each( begin(v), end(v),
[&](double& n) {
n *= c;
}
);
}

52


Live Demo

PPLSesame.cpp

53

Outline
– C++98  C++11 & snippet C#

– Design patterns

54

Alcune linee guida
• Devo migrare tutto subito?
– No, intanto assicurati di cosa supporta il tuo
compilatore.
– Inizia migrando le cose deprecate (e.g. auto_ptr).
– Aggiungi qualche test su quello che stai
modificando.
– Vale sempre il principio del boyscout…
55

Alcune linee guida
• Il mio compilatore non supporta alcune
feature del C++11. Che devo fare?
– Se puoi, pianifica una migrazione del compilatore.
– Individua cosa potrà beneficiare del C++11 e come
lo riscriveresti.
– Metti dei reminder per quando migrerai, tipo:
//TO-DO-CPP11: …
56

Alcune linee guida
• Il miglior consiglio per usare C++11/C++14?
– Provalo!
– Il materiale di qualità sul C++ è aumentato.
– Frequenta isocpp.org
– Frequenta italiancpp.org 

57

Marco & Paolo
Marco Arena
marco@italiancpp.org
http://marcoarena.wordpress.com
Paolo Polce
paolo@webshell.it
@paolopolce

webshell.it

agileworks.it
58

~talk() noexcept
{
answer_questions();
}

59

~talk() noexcept
{
answer_questions();
}
Quanto è nerd questa slide?

60

Bonus: non-member begin/end
// C++98
int arr[] = {1,2,3,4,5};
arr.begin(); // eh ???
// C++11
auto it = begin(vec);
auto it = begin(arr); // ok!
62

Bonus: make_unique (1)
// C++14
auto car = make_unique<Car>(arg1)
• auto-everything syntax
• Exception-safety:
void func(unique_ptr<Car> c1, unique_ptr<Car> c2);
func(unique_ptr<Car>{new Car{}}, unique_ptr<Car>{new Car{}});

• Le istruzioni per creare i due Car* possono essere riordinate
dal compilatore.
63

Bonus: make_unique (2)
func(unique_ptr<Car>{new Car{}}, unique_ptr<Car>{new Car{}});
Esempio di possibile esecuzione:

1. Allocazione memoria per la prima Car
2. Allocazione memoria per la seconda Car
3. Costruzione prima Car
4. Costruzione seconda Car
5. Costruzione primo unique_ptr
6. Costruzione secondo unique_ptr
Che succede se 4 tira un’eccezione? Memory leak della prima Car!

Se chiamiamo due funzioni (come make_unique), il compilatore è costretto
a fare 1,2,5 insieme e poi 3,4,6! Più dettagli a questo link (Sutter)
64

Effective Code Transformations in C++

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