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std::is_trivially_default_constructible in C++ with Example
  • Last Updated : 12 Jun, 2020

The std::is_trivially_default_constructible template of C++ STL is present in the <type_traits> header file. The std::is_trivially_default_constructible template of C++ STL is used to check whether T is trivially default constructible type or not. It return the boolean value true if T is trivially default constructible type, otherwise return false.

Header File:

#include<type_traits>

Template Class:

template< class T >
struct is_default_constructible;

Syntax:

std::is_trivially_default_constructible<T>::value

Parameter: The template std::is_trivially_default_constructible accepts a single parameter T(Trait class) to check whether T is trivially default constructible type or not.



Return Value: The template std::is_trivially_default_constructible returns a boolean variable as shown below:

  • True: If the type T is a trivially default constructible.
  • False: If the type T is not a trivially default constructible.

Below is the program to demonstrate std::is_trivially_default_constructible in C++:

Program 1:

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// C++ program to demonstrate
// std::is_trivially_default_constructible
#include <iostream>
#include <type_traits>
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<int>::value
         << endl;
  
    // Check if class A is trivially
    // default constructible or not
    cout << "class A: "
         << is_trivially_default_constructible<A>::value
         << endl;
  
    // Check if class B is trivially
    // default constructible or not
    cout << "class B: "
         << is_trivially_default_constructible<B>::value
         << endl;
  
    // Check if enum class C is trivially
    // default constructible or not
    cout << "enum class C: "
         << is_trivially_default_constructible<C>::value
         << endl;
  
    // Check if class D is trivially
    // default constructible or not
    std::cout << "class D: "
              << is_trivially_default_constructible<D>::value
              << endl;
    return 0;
}

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Output:

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

Program 2:

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// C++ program to demonstrate
// std::is_trivially_default_constructible
#include <iostream>
#include <type_traits>
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<Ex1>::value
         << '\n';
  
    // Check if struct Ex1 is trivially
    // defaultconstructible or not
    cout << "Ex1 is trivially default-constructible? "
         << is_trivially_default_constructible<Ex1>::value
         << '\n';
  
    // Check if struct Ex2 is trivially
    // defaultconstructible or not
    cout << "Ex2 is trivially default-constructible? "
         << is_trivially_default_constructible<Ex2>::value
         << '\n';
}

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Output:

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

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

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