is_trivial in C++

std::is_trivial function is used to check whether the given type T is a trivial class or not. It is a template declared in <type_traits> header file.

What is a Trivial Class in C++?

A trivial class type is a type whose storage is contiguous (trivially copyable) and which only supports static default initialization (trivially default constructible), either cv-qualified or not. It includes scalar types, trivial classes, and arrays of any such types. A trivial class is a class (defined with class, struct, or union) that is both trivially default constructible and trivially copyable, which implies that: 

• It uses the implicitly defined default, copy and move constructors, copy and move assignments, and destructors.
• It has no virtual members.
• It has no non-static data members with brace- or equal- initializers.
• Its base class and non-static data members (if any) are themselves also trivial types.

is_trivial Declaration

template< class T >
struct is_trivial;

Parameters

• T: The type that we want to check is if it is a trivial type class or not.

Return Type

To check if the T is a trivial type, we use std::is_trivial::value member:

• It is true if the class is trivial type.
• False otherwise.

Note: is_trivial inherits from integral_constant as being either true_type or false_type, depending on whether T is a trivial type.

Example of is_trivial

The below C++ program illustrates the usage of the is_trivial function to determine whether each class is trivial or not.

C++

// CPP program to illustrate is_trivial function
#include <iostream>
#include <type_traits>
using namespace std;

class A {
};
class B {
    B() {}
};
class C : B {
};
class D {
    virtual void fn() {}
};

// Driver Code
int main()
{
    cout << boolalpha;
    // Returns value in boolean type
    cout << "A: " << is_trivial<A>::value << endl;
    cout << "B: " << is_trivial<B>::value << endl;
    cout << "C: " << is_trivial<C>::value << endl;
    cout << "D: " << is_trivial<D>::value << endl;
    return 0;
}

Output

A: true
B: false
C: false
D: false

Explanation

Here, A is a class that has been passed as a parameter to the function is_trivial and it will return a value of integral constant type bool i.e. true or false.

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