# 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 in math_errhandling). Third-party libraries, however, use this. For example, boost.math throws `std::underflow_error` if `boost::math::policies::throw_on_error` is enabled (the default setting).

Inheritance diagram

### Member functions

(constructor) | constructs a new `underflow_error` object with the given message (public member function)
operator= | replaces the `underflow_error` object (public member function)

## std::underflow_error::underflow_error

underflow_error( const std::string& what_arg ); | (1) |
underflow_error( const char* what_arg ); | (2) |
| (3) |
underflow_error( const underflow_error& other ); |  | (until C++11)
underflow_error( const underflow_error& other ) noexcept; |  | (since C++11)

1) Constructs the exception object with what_arg as explanatory string. After construction, std::strcmp(what(), what_arg.c_str()) == 0.
2) Constructs the exception object with what_arg as explanatory string. After construction, std::strcmp(what(), what_arg) == 0.
3) Copy constructor. If *this and other both have dynamic type `std::underflow_error` then std::strcmp(what(), other.what()) == 0. No exception can be thrown from the copy constructor. (until C++11)

### Parameters

what_arg | - | explanatory string
other | - | another exception object to copy

### Exceptions

1-2) May throw std::bad_alloc

### Notes

Because copying `std::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 taking `std::string&&`: it would have to copy the content anyway.

Before the resolution of LWG issue 254, the non-copy constructor can only accept std::string. It makes dynamic allocation mandatory in order to construct a std::string object.

After the resolution of LWG 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 by `what()` are the same for the original object and the copied object.

## std::underflow_error::operator=

underflow_error& operator=( const underflow_error& other ); |  | (until C++11)
underflow_error& operator=( const underflow_error& other ) noexcept; |  | (since C++11)

Assigns the contents with those of other. If *this and other both have dynamic type `std::underflow_error` then std::strcmp(what(), other.what()) == 0 after assignment. No exception can be thrown from the copy assignment operator. (until C++11)

### Parameters

other | - | another exception object to assign with

### Return value

*this

### Notes

After the resolution of LWG 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 by `what()` are the same for the original object and the copied object.

## Inherited from std::runtime_error

## Inherited from std::exception

### Member functions

(destructor) [virtual] | destroys the exception object (virtual public member function of `std::exception`)
what [virtual] | returns an explanatory string (virtual public member function of `std::exception`)

### 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 accepting const char* was missing | added
LWG 471 | C++98 | the explanatory strings of `std::underflow_error`'s copies were implementation-defined | they are the same as that of the original `std::underflow_error` object

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