std::is_default_constructible, std::is_trivially_default_constructible, std::is_nothrow_default_constructible - cppreference.com

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Elemento definito nell'header `<type_traits>`

`template< class T > struct is_default_constructible;`	(1)	(dal C++11)

`template< class T > struct is_trivially_default_constructible;`	(2)	(dal C++11)

`template< class T > struct is_nothrow_default_constructible;`	(3)	(dal C++11)

Verifica se un tipo è DefaultConstructible, cioè ha un costruttore di default accessibile esplicita o implicita. Se la condizione è soddisfatta, un membro costante value true parità è previsto, in caso contrario è value false.

Original:

Checks whether a type is DefaultConstructible, i.e. has an accessible explicit or implicit default constructor. If the requirement is met, a member constant value equal true is provided, otherwise value is false.

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Uguale a 1), ma l'espressione costruttore non chiama qualsiasi operazione che non è banale.

Original:

Same as 1), but the constructor expression does not call any operation that is not trivial.

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Uguale a 1), ma l'espressione costruttore è noexcept.

Original:

Same as 1), but the constructor expression is noexcept.

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Inherited from std::integral_constant

Member constants


	`true` se `T` is default-constructible , `false` altrimenti Original: `true` if `T` is default-constructible , `false` otherwise The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (pubblico membro statico costante)

Member functions


	converte l'oggetto in `bool`, restituisce `value` Original: converts the object to `bool`, returns `value` The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (metodo pubblico)

Member types


Tipo Original: Type The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.	Definition

`value_type`	`bool`

`type`	`std::integral_constant<bool, value>`

Possibile implementazione

template< class T>
struct is_default_constructible : std::is_constructible<T> {};

template< class T>
struct is_trivially_default_constructible : std::is_trivially_constructible<T> {};

template< class T>
struct is_nothrow_default_constructible : std::is_nothrow_constructible<T> {};

Esempio

#include <iostream>
#include <type_traits>

struct Ex1 {
    std::string str; // member has a non-trivial default ctor
};
struct Ex2 {
    int n;
    Ex2() = default; // trivial and non-throwing
};

int main() {
    std::cout << std::boolalpha << "Ex1 is default-constructible? "
              << std::is_default_constructible<Ex1>::value << '\n'
              << "Ex1 is trivially default-constructible? "
              << std::is_trivially_default_constructible<Ex1>::value << '\n'
              << "Ex2 is trivially default-constructible? "
              << std::is_trivially_default_constructible<Ex2>::value << '\n';
              << "Ex2 is nothrow default-constructible? "
              << std::is_nothrow_default_constructible<Ex2>::value << '\n';
}

Output:

Ex1 is default-constructible? true
Ex1 is trivially default-constructible? false
Ex2 is trivially default-constructible? true
Ex2 is nothrow default-constructible? true

Vedi anche


is_constructible is_trivially_constructible is_nothrow_constructible (C++11) (C++11) (C++11)	Verifica se un tipo ha un costruttore per argomenti specifici Original: checks if a type has a constructor for specific arguments The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe template) [modifica]

is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11) (C++11) (C++11)	Verifica se un tipo ha un costruttore di copia Original: checks if a type has a copy constructor The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe template) [modifica]

is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11) (C++11) (C++11)	Verifica se un tipo ha un costruttore mossa Original: checks if a type has a move constructor The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (classe template) [modifica]