5.6.3. Standard Exceptions

The C++ library defines several classes that it uses to report problems encountered in the functions in the standard library. These exception classes are also intended to be used in the programs we write. These classes are defined in four headers:

• The exception header defines the most general kind of exception class named exception. It communicates only that an exception occurred but provides no additional information.

• The stdexcept header defines several general-purpose exception classes, which are listed in Table 5.1.

Table 5.1. Standard Exception Classes Defined in <stdexcept>

• The new header defines the bad_alloc exception type, which we cover in § 12.1.2 (p. 458).

• The type_info header defines the bad_cast exception type, which we cover in § 19.2 (p. 825).

The library exception classes have only a few operations. We can create, copy, and assign objects of any of the exception types.

We can only default initialize (§ 2.2.1, p. 43) exception, bad_alloc, and bad_cast objects; it is not possible to provide an initializer for objects of these exception types.

The other exception types have the opposite behavior: We can initialize those objects from either a string or a C-style string, but we cannot default initialize them. When we create objects of any of these other exception types, we must supply an initializer. That initializer is used to provide additional information about the error that occurred.

The exception types define only a single operation named what. That function takes no arguments and returns a const char* that points to a C-style character string (§ 3.5.4, p. 122). The purpose of this C-style character string is to provide some sort of textual description of the exception thrown.

The contents of the C-style string that what returns depends on the type of the exception object. For the types that take a string initializer, the what function returns that string. For the other types, the value of the string that what returns varies by compiler.