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To default-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type, the default constructor for T is called (and the initialization is ill-formed if T has no default constructor or overload resolution results in an ambiguity or in a function that is deleted or inaccessible from the context of the initialization);
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.
If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

As a result, objects of type T with automatic or dynamic storage duration must be explicitly initialized before having their value read as part of an expression unless T is a class type or an array thereof or is an unsigned narrow character type. If T is an unsigned narrow character type, it may be used to initialize an object of unsigned narrow character type, which results in both objects having an indeterminate value. This technique can be used to implement copy operations such as std::memcpy() without triggering undefined behavior.

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