在c ++ 11中实现boost :: optional [英] Implementing boost::optional in c++11
问题描述
我正在尝试使用c ++ 11功能实现boost :: optional数据结构。这里是我到目前为止:
I am experimenting with implementing boost::optional like data structure using c++11 features. Here is what I have so far :
template<typename T>
struct maybe {
bool valid;
union {
T value;
};
maybe() : valid(false) {}
maybe(const T& _v) {
valid = true;
new (&value) T(_v);
}
maybe(const maybe& other) {
if (other.valid) {
valid = true;
new (&value) T(other.value);
}
else valid = false;
}
~maybe() {
if (valid)
value.~T();
}
bool is_valid() { return valid; }
operator T&() {
if (valid) return value;
throw std::bad_exception();
}
};
我使用无限制联合功能为可选值创建一个正确对齐的空间,存储在原位,而不是动态分配空间。事情主要工作,除非我想创建一个可能<>与参考。例如也许< int&>
导致g ++ 4.7抱怨:
I make use of the unrestricted union feature to create a properly aligned space for the optional value that can be stored in-situ, instead of dynamically allocation space. Things work mostly, except when I want to create a maybe<> with a reference. For instance maybe<int&>
causes g++ 4.7 to complain :
error: ‘maybe<int&>::<anonymous union>::value’ may not have reference type ‘int&’
because it is a member of a union
我应该怎么做,可能是类存储引用?
What should I do to make the maybe class store references? Any other improvements/suggestions to the class are also welcome.
推荐答案
要使用引用,这绝对需要明确的专业化,因为你不能放置新的引用:你需要使用指针来存储。
To make this work with references you definitely need an explicit specialization, because you can't do placement new of a reference: you need to use pointers for storage.
除此之外,代码缺少一个副本赋值运算符。移动构造函数和移动赋值运算符也很好(特别是因为这是重新实现 boost :: optional
的第一个原因:boost中的一个缺少它们)。
Beyond that, the code is missing a copy assignment operator. A move constructor, and move assignment operator would also be nice (especially since that's the #1 reason to reimplement boost::optional
: the one in boost is lacking them).
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