Are these three default constructors equivalent in C++?

Consider the following code:

#include <type_traits>

template<typename T>
struct A1 {
    T t;

    // implicitly-declared default constructor
};

template<typename T>
struct A2 {
    T t;

    // explicitly-declared default constructor without noexcept
    A2() = default;
};

template<typename T>
struct A3 {
    T t;

    // explicitly-declared default constructor with noexcept
    A3() noexcept(std::is_nothrow_default_constructible<T>::value) = default;
};

Are these three default constructors equivalent in C++?

>Solution :

Are these three default constructors equivalent in C++?

Yes the default ctors are equivalent in the context of noexcept specification.

This can be seen from default ctor:

The implicitly-declared (or defaulted on its first declaration) default constructor has an exception specification as described in dynamic exception specification(until C++17) noexcept specification(since C++17).

So we move onto noexcept specification:

Every function in C++ is either non-throwing or potentially throwing:

• potentially-throwing functions are:
  • functions declared without noexcept specifier except for
    • default constructors, copy constructors, move constructors that are implicitly-declared or defaulted on their first declaration unless
      • a constructor for a base or member that the implicit definition of the constructor would call is potentially-throwing (see below)