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Showing content from https://cplusplus.com/DefaultConstructible below:

class template

<type_traits>

std::is_default_constructible

template <class T> struct is_default_constructible;

Is default constructible

Trait class that identifies whether T is a default constructible type.

A default constructible type is a type which can be constructed without arguments or initialization values, either cv-qualified or not. This includes scalar types, default constructible classes and arrays of such types.

A default constructible class is a class that has a default constructor (either its implicit constructor or a custom defined one).

The is_default_constructible class inherits from integral_constant as being either true_type or false_type, depending on whether T is default constructible.

Template parameters

T: A complete type, or void (possible cv-qualified), or an array of unknown bound.

Member types Inherited from integral_constant:

Member constants Inherited from integral_constant:
member constant definition value either true or false
Member functions Inherited from integral_constant:

operator bool: Returns value (public member function)

Example

// is_default_constructible example
#include <iostream>
#include <type_traits>

struct A { };
struct B { B(){} };
struct C { C(int){} };

int main() {
  std::cout << std::boolalpha;
  std::cout << "is_default_constructible:" << std::endl;
  std::cout << "int: " << std::is_default_constructible<int>::value << std::endl;
  std::cout << "A: " << std::is_default_constructible<A>::value << std::endl;
  std::cout << "B: " << std::is_default_constructible<B>::value << std::endl;
  std::cout << "C: " << std::is_default_constructible<C>::value << std::endl;
  return 0;
}

Output:

is_default_constructible:
int: true
A: true
B: true
C: false