Any of the functions defined in the C++ standard library can report a failure by throwing an exception of a type described in its Throws: paragraph. An implementation may strengthen the exception-specification for a non-virtual function by adding a non-throwing noexcept-specification.
A function may throw an object of a type not listed in its Throws clause if its type is derived from a type named in the Throws clause and would be caught by an exception handler for the base type.
Destructor operations defined in the C++ standard library shall not throw exceptions. Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification. Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified.193 An implementation may strengthen this implicit exception-specification by adding an explicit one.194
That is, the C library functions can all be treated as if they are marked noexcept. This allows implementations to make performance optimizations based on the absence of exceptions at runtime.
In particular, they can report a failure to allocate storage by throwing an exception of type bad_alloc, or a class derived from bad_alloc ([bad.alloc]). Library implementations should report errors by throwing exceptions of or derived from the standard exception classes ([bad.alloc], [support.exception], [std.exceptions]).