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C i C++ nisu ista stvar

Luka Mate Granić
C++ senior software developer @ Microblink

Zašto ovo predavanje?

  • "C/C++"
  • Predrasude:
    • Prvo treba naučiti C a onda C++
    • C++ je objektno orijentiran jezik
    • Potreban je programski jezik niske razine da se ostvare dobre performanse

C++

  • "C++ is a general-purpose programming language emphasizing the design and use of type-rich, lightweight abstractions."
    • The C++ Programming Language (4th Edition) - Bjarne Stroustrup
  • Zero-overhead principle
    • "What you don’t use, you don’t pay for. And further: What you do use, you couldn’t hand code any better." - Foundations of C++, Bjarne Stroustrup

Primjer 1

zapamtiti trenutak

spavanje 100 milisekundi

zapamtiti trenutak

prikazati provedeno vrijeme u sekundama

Usporedba: kôd

C

usleep( 100  * SECONDS_TO_MICROSECONDS / SECONDS_TO_MILLISECONDS );
// ...
float seconds = ( t2.tv_sec - t1.tv_sec ) + ( float )( t2.tv_nsec - t1.tv_nsec ) / SECONDS_TO_NANOSECONDS;

C++

std::this_thread::sleep_for( milliseconds( 100 ) );
// ...
float seconds = duration_cast< duration< float, seconds::period > >( end - start ).count();

Usporedba: kôd

C 1

typedef long long milliseconds;
typedef long long nanoseconds;
//...
milliseconds total_time = time1 + time2;

Usporedba: kôd

C 2

typedef struct { long long value; } Milliseconds;
Milliseconds add_milliseconds(Milliseconds a, Milliseconds b) {
    Milliseconds r = { .value = a.value + b.value };
    return r;
}
typedef struct { long long value; } Nanoseconds;
Nanoseconds add_nanoseconds(Nanoseconds a, Nanoseconds b) {
    Nanoseconds r = { .value = a.value + b.value };
    return r;
}
// ...
add_milliseconds(t1, t2);
// passing 'Nanoseconds' to parameter of incompatible type 'Milliseconds'

https://godbolt.org/z/EufDY-

Usporedba: kôd

C++

milliseconds time1;
nanoseconds time2;
// ...
milliseconds total_tome = time1 + time2;
// no viable conversion from 'duration<[...], ratio<[...], 1000000000>>'
// to
// 'duration<[...], ratio<[...], 1000>>

https://godbolt.org/z/ej-EXR

Usporedba: strojni kôd

#include <chrono>
using namespace std::chrono;
using TP = high_resolution_clock::time_point;
float convert( TP const a,TP const b ) {
    return duration_cast< duration< float, seconds::period > >( a - b ).count();
}
.LCPI0_0:
        .long   1315859240              # float 1.0E+9
convert(..., ...)
        sub     rdi, rsi
        cvtsi2ss        xmm0, rdi
        divss   xmm0, dword ptr [rip + .LCPI0_0]
        ret

https://godbolt.org/z/xnDcxg

Usporedba: strojni kôd

  • Veličina strukture
    • timespec: 128 bitova
    • std::duration: u prinicpu proizvnoljna veličina
    • std::chrono::high_resolution_clock::duration: 64 bita

Zaključak

  • Apstrakcija iznad samo jednog broja
    • Jasno izražavanje namjere
    • Izbjegavanje grešaka
  • Generirani strojni kôd je optimalan

Primjer 2

generiraj niz nasumičnih brojeva

izmjeri vrijeme

sortiraj niz

izmjeri vrijeme

provjeri da je niz sortiran

prikaži vrijeme

Usporedba: kôd

C

int intLess( void const * a, void const * b )
{
    int const ia = * ( int const * )( a );
    int const ib = * ( int const * )( b );
    return ia - ib;
}
// ...
qsort( arr, ARRAY_SIZE, sizeof( int ), intLess );

C++

std::sort( begin( v ), end( v ), []( int const a, int const b ){ return a < b; } );

Usporedba: performanse

Vrijeme (veliki raspon **) [ms] Vrijeme (mali raspon ***) [ms]
qsort 1199 628
std::sort 764 415
quick_sort* 799 412

Sistem: Intel Core i7 (I7-8850H), 16 GiB RAM

* vlastita implementacija ** veliki raspon: (-1e6, 1e6) *** mali raspon: (-1e3, 1e3)

Zaključak

  • Izbjegavanje indirekcije
    • Metoda usporedbe se ekpandira na mjestu poziva
  • Dupliciranje strojnog koda
  • Ograničenje jezika a ne algoritma
  • Jasan kôd bez degradacije performansi

STL algorithmi

all_of, any_of, none_of, for_each, for_each_n, count, count_if, mismatch, find, find_if, find_if_not, find_end, find_first_of, adjacent_find, search, search_n, copy, copy_if, copy_n, copy_backward, move, move_backward, fill, fill_n, transform, generate, generate_n, remove, remove_if, remove_copy, remove_copy_if, replace, replace_if, replace_copy, replace_copy_if, swap, swap_ranges, iter_swap, reverse, reverse_copy, rotate, rotate_copy, shift_left, shift_right, shuffle, sample, unique, unique_copy, is_partitioned, partition, partition_copy, stable_partition, partition_point, is_sorted, is_sorted_until, sort, partial_sort, partial_sort_copy, stable_sort, nth_element, lower_bound, upper_bound, binary_search, equal_range, merge, inplace_merge, includes, set_difference, set_intersection, set_symmetric_difference, set_union, is_heap, is_heap_until, make_heap, push_heap, pop_heap, sort_heap, max, max_element, min, min_element, minmax, minmax_element, clamp, equal, lexicographical_compare, lexicographical_compare_three_way, is_permutation, next_permutation, prev_permutation, iota, accumulate, inner_product, adjacent_difference, partial_sum, reduce, exclusive_scan, inclusive_scan, transform_reduce, transform_exclusive_scan, transform_inclusive_scan

Pauza

Primjer 3

izračunati magnetsko polje idealnog Hertzovog dipola

prikazati to polje s ASCII znakovima

$$ H = \frac{ I_0 \Delta z }{ 4 \pi } \left( 1 + \frac{ 1 }{ j k r } \right) \frac{ e^{ - j k r } }{ r } sin( \Theta ) \pmb{ \hat{ \phi } } $$

Postava

#define TABLE_SIZE 50
typedef struct
{
    float data[ TABLE_SIZE ][ TABLE_SIZE ];
} FieldImage;
  • Velika struktura ( 1kB )
  • Pri računanju, možemo birati kut

Ispis

void printTable( FieldImage const * image )  // C
{
    if ( !image ) return;
    // ...
}
void printTable( FieldImage const & image )  // C++
{
    // ...
}
  • Nije moguće prikazati "prazni" parametar

Povratna vrijednost: 1

FieldImage const * generateImage_01()  // C
{
    // ...
}
FieldImage const & generateImage_01()  // C++
{
    // ...
}
const T * const T &
nullptr ili vrijednost vrijednost

Digresija

Tony Hoare (QCon London 2009): "I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years."

Povratna vrijednost: 2

FieldImage * generateImage_02( float const rot )  // C
{
    // ...
}
// ...
FieldImage * image = generateImage_02( rot );
//...
free( image );
std::unique_ptr< FieldImage > generateImage_02( float const rot ) {  // C++
    // ...
}
// ...
std::unique_ptr< FieldImage > = generateImage_02( rot );
  • Jasna poruka da pozivatelju funkcije dajemo vlasništvo objekta.

Povratna vrijednost: 3 [ C ]

void generateImage_03( FieldImage * image, float const rot ) {
    // ...
}

void main_03() {
    FieldImage image;
    generateImage_03( &image, M_PI * 0.75f );
    printTable( &image );
}

void printAndClean( FieldImage * image ) {
    printTable( image );
    free( image );
}

void main_04() {
    FieldImage * image = malloc( sizeof( FieldImage ) );
    generateImage_03( image, M_PI );
    printAndClean( image );
}

Povratna vrijednost: 3 [ C++ ]

void generateImage_03( FieldImage & image, float const rot ) {
    // ...
}

void main_03() {
    FieldImage image;
    generateImage_03( image, M_PI * 0.75f );
    printTable( image );
}

void printAndClean( std::unique_ptr< FieldImage > image ) {
    printTable( *image );
}

void main_04() {
    std::unique_ptr< FieldImage > image = std::make_unique< FieldImage >();
    generateImage_03( *image, M_PI );
    printAndClean( std::move ( image ) );
}

Zaključak

  • Jasna komunikacija
    • Postoji li vrijednost?
    • Tko je vlasnik?
  • Sigurno korištenje
    • Dvostruko oslobađanje memorije
    • Memory leak
  • unique_ptr ima istu veličinu kao i normalni pokazivač
  • shared_ptr i weak_ptr: Brojanje referenci

Za kraj

  • Apstraktni kôd bez gubitka performansi
  • Velika standardna biblioteka
  • Sigurnije upravljanje memorijom

Hvala!

Izvori

Zajednica u Zagrebu