std::underflow_error
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Defined in header <stdexcept> | ||
class underflow_error; | ||
Defines a type of object to be thrown as exception. It may be used to report arithmetic underflow errors (that is, situations where the result of a computation is a subnormal floating-point value).
The standard library components do not throw this exception (mathematical functions report underflow errors as specified inmath_errhandling). Third-party libraries, however, use this. For example,boost.math throwsstd::underflow_error ifboost::math::policies::throw_on_error is enabled (the default setting).
All member functions ofstd::underflow_error areconstexpr: it is possible to create and usestd::underflow_error objects in the evaluation of a constant expression.However, | (since C++26) |
Inheritance diagram
Contents |
[edit]Member functions
(constructor) | constructs a newunderflow_error object with the given message(public member function) |
operator= | replaces theunderflow_error object(public member function) |
std::underflow_error::underflow_error
underflow_error(conststd::string& what_arg); | (1) | (constexpr since C++26) |
underflow_error(constchar* what_arg); | (2) | (constexpr since C++26) |
underflow_error(const underflow_error& other); | (3) | (noexcept since C++11) (constexpr since C++26) |
std::underflow_error thenstd::strcmp(what(), other.what())==0. No exception can be thrown from the copy constructor.Parameters
| what_arg | - | explanatory string |
| other | - | another exception object to copy |
Exceptions
Notes
Because copyingstd::underflow_error is not permitted to throw exceptions, this message is typically stored internally as a separately-allocated reference-counted string. This is also why there is no constructor takingstd::string&&: it would have to copy the content anyway.
Before the resolution ofLWG issue 254, the non-copy constructor can only acceptstd::string. It makes dynamic allocation mandatory in order to construct astd::string object.
After the resolution ofLWG issue 471, a derived standard exception class must have a publicly accessible copy constructor. It can be implicitly defined as long as the explanatory strings obtained bywhat() are the same for the original object and the copied object.
std::underflow_error::operator=
underflow_error& operator=(const underflow_error& other); | (noexcept since C++11) (constexpr since C++26) | |
Assigns the contents with those ofother. If*this andother both have dynamic typestd::underflow_error thenstd::strcmp(what(), other.what())==0 after assignment. No exception can be thrown from the copy assignment operator.
Parameters
| other | - | another exception object to assign with |
Return value
*this
Notes
After the resolution ofLWG issue 471, a derived standard exception class must have a publicly accessible copy assignment operator. It can be implicitly defined as long as the explanatory strings obtained bywhat() are the same for the original object and the copied object.
Inherited fromstd::runtime_error
Inherited fromstd::exception
Member functions
[virtual] | destroys the exception object (virtual public member function of std::exception)[edit] |
[virtual] | returns an explanatory string (virtual public member function of std::exception)[edit] |
[edit]Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constexpr_exceptions | 202502L | (C++26) | constexpr exception types |
[edit]Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 254 | C++98 | the constructor acceptingconstchar* was missing | added |
| LWG 471 | C++98 | the explanatory strings ofstd::underflow_error'scopies were implementation-defined | they are the same as that of the original std::underflow_error object |
