Exceptions

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor Comments Keywords	ASCII chart Escape sequences History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
default initialization value initialization zero initialization copy initialization direct initialization	aggregate initialization list initialization (C++11) constant initialization reference initialization

Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
Class-specific function properties
virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
default constructor copy constructor move constructor (C++11)	copy assignment move assignment (C++11) destructor
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly

 

Exception handling provides a way of transferring control and information from some point in the execution of a program to an handler associated with a point previously passed by the execution (in other words, exception handling transfers control up the call stack)

An exception can be thrown by a throw-expression, and many standard library functions, including memory allocation functions, are specified to throw exceptions to signal certain error conditions.

In order for exception to be caught, the throw-expression has to be inside a try-block or inside a function called from a try-block, and there has to be a catch clause that matches the type of the exception object.

When declaring a function, exception specifications and noexcept specifiers may be provided to limit the types of the exceptions a function may throw.

Errors that arise during exception handling are handled by std::terminate and std::unexpected.

Usage

While throw-expression can be used to transfer control to an arbitrary block of code up the execution stack, for arbitrary reasons (similar to std::longjmp), its intended usage is error handling.

Error handling

Throwing an exception is used to signal errors from functions, where "errors" are typically limited to only the following:

1. Failures to establish invariants
2. Failures to meet the postconditions
3. Failures to meet the preconditions of the caller

In particular, this implies that the failures of constructors and most operators should be reported by throwing exceptions.

Exception safety

After the error condition is reported by a function, additional guarantees may be provided with regards to the state of the program. The following four levels of exception guarantee are generally recognized, which are strict supersets of each other:

1. Nothrow (or nofail) exception guarantee -- the function never throws exceptions. This is expected of destructors and, typically, of move constructors.
2. Strong exception guarantee -- If the function throws an exception, the state of the program is rolled back to the state just before the function call.
3. Basic exception guarantee -- If the function throws an exception, the program is in a valid state. It may require cleanup, but all invariants are intact.
4. No exception guarantee -- If the function throws an exception, the program may not be in a valid state: resource leaks, memory corruption, or other invariant-destroying errors may have occurred.

Exception objects

While objects of any complete type and cv pointers to void may be thrown as exception objects, all standard library functions throw anonymous temporary objects by value, and the types of those objects are derived (directly or indirectly) from std::exception. User-defined exceptions usually follow this pattern.

To avoid unnecessary copying of the exception object and object slicing, the best practice for catch clauses is to catch by reference.