Cd中带有std :: is_trivially_default_constructible的示例

头文件中提供了C++ STL的std :: is_trivially_default_constructible模板。 C++ STL的std :: is_trivially_default_constructible模板用于检查T是否是普通的默认可构造类型。如果T是琐碎的默认可构造类型，则返回布尔值true，否则返回false。

头文件：

#include

模板类别：

template< class T >
struct is_default_constructible;

句法：

std::is_trivially_default_constructible::value

参数：模板std :: is_trivially_default_constructible接受单个参数T(Trait类)，以检查T是否为平凡的默认可构造类型。

返回值：模板std :: is_trivially_default_constructible返回一个布尔变量，如下所示：

• 正确：如果类型T是平凡的默认可构造变量。
• False：如果类型T不是平凡的默认构造函数。

下面是演示C++中std :: is_trivially_default_constructible的程序：

程序1：

// C++ program to demonstrate
// std::is_trivially_default_constructible
#include 
#include 
using namespace std;
  
// Declaration of classes
class A {
};
  
class B {
    B() {}
};
  
enum class C : int { x,
                     y,
                     z };
  
class D {
    int v1;
    double v2;
  
public:
    D(int n)
        : v1(n), v2()
    {
    }
    D(int n, double f)
    noexcept : v1(n), v2(f) {}
};
  
int main()
{
    cout << boolalpha;
  
    // Check if int is trivially
    // default constructible or not
    cout << "int: "
         << is_trivially_default_constructible::value
         << endl;
  
    // Check if class A is trivially
    // default constructible or not
    cout << "class A: "
         << is_trivially_default_constructible::value
         << endl;
  
    // Check if class B is trivially
    // default constructible or not
    cout << "class B: "
         << is_trivially_default_constructible::value
         << endl;
  
    // Check if enum class C is trivially
    // default constructible or not
    cout << "enum class C: "
         << is_trivially_default_constructible::value
         << endl;
  
    // Check if class D is trivially
    // default constructible or not
    std::cout << "class D: "
              << is_trivially_default_constructible::value
              << endl;
    return 0;
}

输出：

int: true
class A: true
class B: false
enum class C: true
class D: false

程式2：

// C++ program to demonstrate
// std::is_trivially_default_constructible
#include 
#include 
using namespace std;
  
// Declare structures
struct Ex1 {
    Ex1() {}
    Ex1(Ex1&&)
    {
        cout << "Throwing move constructor!";
    }
    Ex1(const Ex1&)
    {
        cout << "Throwing copy constructor!";
    }
};
  
struct Ex2 {
    Ex2() {}
    Ex2(Ex2&&) noexcept
    {
        cout << "Non-throwing move constructor!";
    }
    Ex2(const Ex2&) noexcept
    {
        cout << "Non-throwing copy constructor!";
    }
};
  
// Driver Code
int main()
{
    cout << boolalpha;
  
    // Check if struct Ex1 is default
    // constructible or not
    cout << "Ex1 is default-constructible? "
         << is_trivially_default_constructible::value
         << '\n';
  
    // Check if struct Ex1 is trivially
    // defaultconstructible or not
    cout << "Ex1 is trivially default-constructible? "
         << is_trivially_default_constructible::value
         << '\n';
  
    // Check if struct Ex2 is trivially
    // defaultconstructible or not
    cout << "Ex2 is trivially default-constructible? "
         << is_trivially_default_constructible::value
         << '\n';
}

输出：

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

参考： http://www.cplusplus.com/reference/type_traits/is_trivially_default_constructible/

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