ROOT::RResult< T > Class Template Reference

template<typename T>
class ROOT::RResult< T >

The class is used as a return type for operations that can fail; wraps a value of type T or an RError.

The RResult<T> class and their related classes are used for call chains that can throw exceptions, such as I/O code paths. Throwing of the exception is deferred to allow for if (result) style error checking where it makes sense. If an RResult in error state leaves the scope unchecked, it will throw.

A function returning an RResult might look like this:

RResult<int> MyIOFunc()
{
   int rv = syscall(...);
   if (rv == -1)
      return R__FAIL("user-facing error message");
   if (rv == kShortcut)
      return 42;
   return R__FORWARD_RESULT(FuncThatReturnsRResultOfInt());
}

Code using MyIOFunc might look like this:

auto result = MyIOOperation();
if (!result) {
   // custom error handling or result.Throw()
}
switch (result.Inspect()) {
   ...
}

Note that RResult<void> can be used for a function without return value, like this

RResult<void> DoSomething()
{
   if (failure)
      return R__FAIL("user-facing error messge");
   return RResult<void>::Success();
}

RResult<T>::Unwrap() can be used as a short hand for "give me the wrapped value or, in case of an error, throw". For instance:

int value = FuncThatReturnsRResultOfInt().Unwrap();  // may throw

There is no implict operator that converts RResult<T> to T. This is intentional to make it clear in the calling code where an exception may be thrown.

Definition at line 197 of file RError.hxx.

Public Member Functions
	RResult (const RResult &other)=delete
	RResult (const T &value)
	RResult (RError &&error)
	RResult (RResult &&other)=default
	RResult (T &&value)
	~RResult ()=default
RResult &	Forward (RError::RLocation &&sourceLocation)
	Used by R__FORWARD_RESULT in order to keep track of the stack trace in case of errors.
const T &	Inspect ()
	If the operation was successful, returns a const reference to the inner type.
	operator bool ()
RResult &	operator= (const RResult &other)=delete
RResult &	operator= (RResult &&other)=default
T	Unwrap ()
	If the operation was successful, returns the inner type by value.
Public Member Functions inherited from ROOT::RResultBase
	RResultBase (const RResultBase &other)=delete
	RResultBase (RResultBase &&other)=default
	~RResultBase () noexcept(false)
std::optional< RError >	GetError () const
RResultBase &	operator= (const RResultBase &other)=delete
RResultBase &	operator= (RResultBase &&other)=default
void	Throw ()
	Throws an RException with fError.

Private Member Functions
void	ThrowOnError ()

Private Attributes
T	fValue
	The result value in case of successful execution.

Additional Inherited Members
Static Public Member Functions inherited from ROOT::RResultBase
static RError	ForwardError (RResultBase &&result, RError::RLocation &&sourceLocation)
	Used by R__FORWARD_ERROR in order to keep track of the stack trace.
Protected Member Functions inherited from ROOT::RResultBase
	RResultBase ()=default
	RResultBase (RError &&error)
bool	Check ()
	Used by the RResult<T> bool operator.
Protected Attributes inherited from ROOT::RResultBase
std::unique_ptr< RError >	fError
	This is the nullptr for an RResult representing success.
bool	fIsChecked {false}
	Switches to true once the user of an RResult object checks the object status.

#include <ROOT/RError.hxx>

Inheritance diagram for ROOT::RResult< T >:

Constructor & Destructor Documentation

RResult() [1/5]

template<typename T >

ROOT::RResult< T >::RResult ( const T &  value )

inline

Definition at line 218 of file RError.hxx.

RResult() [2/5]

template<typename T >

ROOT::RResult< T >::RResult ( T &&  value )

inline

Definition at line 219 of file RError.hxx.

RResult() [3/5]

template<typename T >

ROOT::RResult< T >::RResult ( RError &&  error )

inline

Definition at line 220 of file RError.hxx.

RResult() [4/5]

template<typename T >

ROOT::RResult< T >::RResult ( const RResult< T > &  other )

delete

RResult() [5/5]

template<typename T >

ROOT::RResult< T >::RResult ( RResult< T > &&  other )

default

~RResult()

template<typename T >

ROOT::RResult< T >::~RResult ( )

default

Member Function Documentation

Forward()

template<typename T >

RResult & ROOT::RResult< T >::Forward ( RError::RLocation &&  sourceLocation )

inline

Used by R__FORWARD_RESULT in order to keep track of the stack trace in case of errors.

Definition at line 230 of file RError.hxx.

Inspect()

template<typename T >

const T & ROOT::RResult< T >::Inspect ( )

inline

If the operation was successful, returns a const reference to the inner type.

If there was an error, Inspect() instead throws an exception.

Definition at line 239 of file RError.hxx.

operator bool()

template<typename T >

ROOT::RResult< T >::operator bool ( )

inlineexplicit

Definition at line 258 of file RError.hxx.

operator=() [1/2]

template<typename T >

RResult & ROOT::RResult< T >::operator= ( const RResult< T > &  other )

delete

operator=() [2/2]

template<typename T >

RResult & ROOT::RResult< T >::operator= ( RResult< T > &&  other )

default

ThrowOnError()

template<typename T >

void ROOT::RResult< T >::ThrowOnError ( )

inlineprivate

Definition at line 203 of file RError.hxx.

Unwrap()

template<typename T >

T ROOT::RResult< T >::Unwrap ( )

inline

If the operation was successful, returns the inner type by value.

For move-only types, Unwrap can only be called once, as it yields ownership of the inner value to the caller using std::move, potentially leaving the RResult in an unspecified state.

If there was an error, Unwrap() instead throws an exception.

Definition at line 252 of file RError.hxx.

Member Data Documentation

fValue

template<typename T >

T ROOT::RResult< T >::fValue

private

The result value in case of successful execution.

Definition at line 200 of file RError.hxx.