# std::runtime_error

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Defined in header `<stdexcept>` |  |
class runtime_error; |  |

Defines a type of object to be thrown as exception. It reports errors that are due to events beyond the scope of the program and can not be easily predicted.

Exceptions of type `std::runtime_error` are thrown by the following standard library components: std::locale::locale and std::locale::combine.

In addition, the following standard exception types are derived from `std::runtime_error`:

- std::range_error
- std::overflow_error
- std::underflow_error

std::regex_error std::system_error | (since C++11)

std::chrono::ambiguous_local_time std::chrono::nonexistent_local_time std::format_error | (since C++20)

Inheritance diagram

### Member functions

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

## std::runtime_error::runtime_error

runtime_error( const std::string& what_arg ); | (1) |
runtime_error( const char* what_arg ); | (2) |
| (3) |
runtime_error( const runtime_error& other ); |  | (until C++11)
runtime_error( const runtime_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::runtime_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::runtime_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::runtime_error::operator=

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

Assigns the contents with those of other. If *this and other both have dynamic type `std::runtime_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::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::runtime_error`'s copies were implementation-defined | they are the same as that of the original `std::runtime_error` object

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