gecko-dev/dom/bindings/Nullable.h

89 lines
2.1 KiB
C++

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-*/
/* vim: set ts=2 sw=2 et tw=79: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef mozilla_dom_Nullable_h
#define mozilla_dom_Nullable_h
#include "mozilla/Assertions.h"
#include "nsTArrayForwardDeclare.h"
namespace mozilla {
namespace dom {
// Support for nullable types
template <typename T>
struct Nullable
{
private:
// mIsNull MUST COME FIRST because otherwise the casting in our array
// conversion operators would shift where it is found in the struct.
bool mIsNull;
T mValue;
public:
Nullable()
: mIsNull(true)
{}
explicit Nullable(T aValue)
: mIsNull(false)
, mValue(aValue)
{}
void SetValue(T aValue) {
mValue = aValue;
mIsNull = false;
}
// For cases when |T| is some type with nontrivial copy behavior, we may want
// to get a reference to our internal copy of T and work with it directly
// instead of relying on the copying version of SetValue().
T& SetValue() {
mIsNull = false;
return mValue;
}
void SetNull() {
mIsNull = true;
}
const T& Value() const {
MOZ_ASSERT(!mIsNull);
return mValue;
}
T& Value() {
MOZ_ASSERT(!mIsNull);
return mValue;
}
bool IsNull() const {
return mIsNull;
}
// Make it possible to use a const Nullable of an array type with other
// array types.
template<typename U>
operator const Nullable< nsTArray<U> >&() const {
// Make sure that T is ok to reinterpret to nsTArray<U>
const nsTArray<U>& arr = mValue;
(void)arr;
return *reinterpret_cast<const Nullable< nsTArray<U> >*>(this);
}
template<typename U>
operator const Nullable< FallibleTArray<U> >&() const {
// Make sure that T is ok to reinterpret to FallibleTArray<U>
const FallibleTArray<U>& arr = mValue;
(void)arr;
return *reinterpret_cast<const Nullable< FallibleTArray<U> >*>(this);
}
};
} // namespace dom
} // namespace mozilla
#endif /* mozilla_dom_Nullable_h */