attribute-specifier-seq decl-specifier-seq declarator virt-specifier-seq = default ;is called an explicitly-defaulted definition.
struct S { constexpr S() = default; // ill-formed: implicit S() is not constexpr S(int a = 0) = default; // ill-formed: default argument void operator=(const S&) = default; // ill-formed: non-matching return type ~S() noexcept(false) = default; // deleted: exception specification does not match private: int i; S(S&); // OK: private copy constructor }; S::S(S&) = default; // OK: defines copy constructor
struct trivial {
trivial() = default;
trivial(const trivial&) = default;
trivial(trivial&&) = default;
trivial& operator=(const trivial&) = default;
trivial& operator=(trivial&&) = default;
~trivial() = default;
};
struct nontrivial1 {
nontrivial1();
};
nontrivial1::nontrivial1() = default; // not first declaration