Namespace
Varianti

std::common_type

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C++
 
Utilità libreria
Tipo di supporto (basic types, RTTI, type traits)
Gestione della memoria dinamica
La gestione degli errori
Programma di utilità
Funzioni variadic
Data e ora
Funzione oggetti
initializer_list(C++11)
bitset
hash(C++11)
Gli operatori relazionali
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Relational operators
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rel_ops::operator!=
rel_ops::operator>
rel_ops::operator<=
rel_ops::operator>=
Coppie e tuple
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Pairs and tuples
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pair
tuple(C++11)
piecewise_construct_t(C++11)
piecewise_construct(C++11)
Swap, in avanti e spostare
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Swap, forward and move
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swap
forward(C++11)
move(C++11)
move_if_noexcept(C++11)
declval(C++11)
 
Tipo di supporto
Tipi base
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Basic types
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Tipi fondamentali
Larghezza fissa interi tipi (C++11)
Limiti numerici
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Numeric limits
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numeric_limits
C numerico limita interfaccia
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Runtime type information
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Tipo tratti
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Type traits
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Categorie di tipo primario
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is_void(C++11)
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is_arithmetic(C++11)
is_reference(C++11)
is_lvalue_reference(C++11)
is_rvalue_reference(C++11)
is_member_pointer(C++11)
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is_member_function_pointer(C++11)
Proprietà del tipo
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Type properties
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is_const(C++11)
is_volatile(C++11)
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is_empty(C++11)
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is_abstract(C++11)
Operazioni supportate
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Supported operations
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is_constructible
is_trivially_constructible
is_nothrow_constructible		(C++11)
(C++11)
(C++11)
is_default_constructible
is_trivially_default_constructible
is_nothrow_default_constructible		(C++11)
(C++11)
(C++11)
is_copy_constructible
is_trivially_copy_constructible
is_nothrow_copy_constructible		(C++11)
(C++11)
(C++11)
is_move_constructible
is_trivially_move_constructible
is_nothrow_move_constructible		(C++11)
(C++11)
(C++11)
is_assignable
is_trivially_assignable
is_nothrow_assignable		(C++11)
(C++11)
(C++11)
is_copy_assignable
is_trivially_copy_assignable
is_nothrow_copy_assignable		(C++11)
(C++11)
(C++11)
is_move_assignable
is_trivially_move_assignable
is_nothrow_move_assignable		(C++11)
(C++11)
(C++11)
is_destructible
is_trivially_destructible
is_nothrow_destructible		(C++11)
(C++11)
(C++11)
has_virtual_destructor(C++11)
I rapporti e le query di proprietà
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Relationships and property queries
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is_same(C++11)
is_base_of(C++11)
is_convertible(C++11)
alignment_of(C++11)
rank(C++11)
extent(C++11)
Tipo modifiche
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Type modifications
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remove_cv
remove_const
remove_volatile		(C++11)
(C++11)
(C++11)
add_cv
add_const
add_volatile		(C++11)
(C++11)
(C++11)
make_signed(C++11)
make_unsigned(C++11)
Tipo di trasformazioni
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Type transformations
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aligned_storage(C++11)
aligned_union(C++11)
decay(C++11)
enable_if(C++11)
conditional(C++11)
common_type(C++11)
underlying_type(C++11)
result_of(C++11)
Digitare costanti tratto
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Type trait constants
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integral_constant(C++11)
 
Elemento definito nell'header <type_traits>
template< class... T >
struct common_type;
 (dal C++11)
Determina il tipo di comune tra tutti i tipi di T..., che è il tipo di tutte le T... può essere convertito in modo implicito.
Original:
Determines the common type among all types T..., that is the type all T... can be implicitly converted to.
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  • std::common_type possono essere specializzati per tipi personalizzati, ad es std::chrono::time_point.
    Original:
    std::common_type may be specialized for custom types, e.g. std::chrono::time_point.
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  • Per non specializzato std::common_type, le regole per la determinazione del tipo comune tra ogni coppia T1, T2 sono esattamente le regole per determinare il tipo di ritorno del operatore condizionale ternario dove T1 e T2 sono i tipi del suo secondo e del terzo operandi.
    Original:
    For non-specialized std::common_type, the rules for determining the common type between every pair T1, T2 are exactly the rules for determining the return type of the operatore condizionale ternario where T1 and T2 are the types of its second and the third operands.
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  • Per i tipi aritmetici, il tipo comune può anche essere visto come il tipo di (eventualmente a modalità mista) espressione aritmetica come T0() + T1() + ... + Tn().
    Original:
    For arithmetic types, the common type may also be viewed as the type of the (possibly mixed-mode) arithmetic expression such as T0() + T1() + ... + Tn().
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Indice

[modifica] Membri tipi

Nome
Original:
Name
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 Definition
type
il tipo comune per tutti T...
Original:
the common type for all T...
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[modifica] Specializzazioni

Specializzazioni personalizzate del tipo di carattere std::common_type sono ammessi. Le specializzazioni sono già fornite dalla libreria standard:
Original:
Custom specializations of the type trait std::common_type are allowed. The following specializations are already provided by the standard library:
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specializzata il tratto std::common_type
Original:
specializes the std::common_type trait
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(classe modello di specializzazione) [modifica]
specializzata il tratto std::common_type
Original:
specializes the std::common_type trait
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(classe modello di specializzazione) [modifica]

[modifica] Note

Con un argomento singolo modello, std::common_type<T>::type è esattamente T, che lo rende il C + + equivalente di boost::identity<T>
Original:
With a single template argument, std::common_type<T>::type is exactly T, which makes it the C++ equivalent of boost::identity<T>
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[modifica] Possibile implementazione

template<class ...T> struct common_type;
 
template<class T>
struct common_type<T> {
    typedef T type;
};
 
template<class T, class U>
struct common_type<T, U> {
    typedef decltype(true ? declval<T>() : declval<U>()) type;
};
 
template<class T, class U, class... V>
struct common_type<T, U, V...> {
    typedef typename common_type<typename common_type<T, U>::type, V...>::type type;
};

[modifica] Esempio

Viene illustrato in modalità mista aritmetica su una classe definita dall'utente
Original:
Demonstrates mixed-mode arithmetic on a user-defined class
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#include <iostream>
#include <type_traits>
 
template<class T>
struct Number { T n; };
 
template<class T, class U>
Number<typename std::common_type<T, U>::type> operator+(const Number<T>& lhs,
                                                        const Number<U>& rhs) 
{
    return {lhs.n + rhs.n};
}
 
int main()
{
    Number<int> i1 = {1}, i2 = {2};
    Number<double> d1 = {2.3}, d2 = {3.5};
    std::cout << "i1i2: " << (i1 + i2).n << "\ni1d2: " << (i1 + d2).n << '\n'
              << "d1i2: " << (d1 + i2).n << "\nd1d2: " << (d1 + d2).n << '\n';
}

Output: 

i1i2: 3
i1d2: 4.5
d1i2: 4.3
d1d2: 5.8
Estratto da "https://it.cppreference.com/mwiki/index.php?title=cpp/types/common_type&oldid=27006"