问题描述
很明显，如果使用C ++ 17或boost，std::optional是从函数返回可选值的最佳选择(另请参见 GOTW#90 )
Obviously, std::optional is the best choice to return an optional value from a function if one uses C++17 or boost (see also GOTW #90)
std::optional<double> possiblyFailingCalculation()
但是，如果一个人卡在一个较旧的版本中(并且不能使用boost)，那又是最好的选择呢?
But what and why would be the best alternative if one is stuck with an older version (and can't use boost)?
我看到一些选择:
-
STL智能指针(仅C ++ 11)
STL smart pointers (C++11 only)
std::unique_ptr<double> possiblyFailingCalculation();
- (+)几乎与可选的用法相同
- (−)令人困惑地拥有指向非多态类型或内置类型的智能指针
将其与bool配对
std::pair<double,bool> possiblyFailingCalculation();
旧样式
bool possiblyFailingCalculation(double& output);
- (−)与新的C ++ 11
auto value = calculation()样式不兼容 - (−) incompatible with new C++11
auto value = calculation()style - (−)有时不可能计算"是有效的返回值.
DIY模板:具有相同功能的基本模板很容易编写代码，但是是否有陷阱实现强大的std::optional<T>外观类似模板?
A DIY template: a basic template with the same functionality is easy enough to code, but are there any pitfalls to implement a robust std::optional<T> look-a-like template ?
引发异常
推荐答案
std::optional像其boost::optional父代一样，是一个非常基本的类模板.它是一个bool，一些存储和一堆便捷成员函数，其中大多数是一行代码和一个断言.
std::optional, like its boost::optional parent, is a pretty basic class template. It's a bool, some storage, and a bunch of convenience member functions most of which are one line of code and an assert.
DIY选项绝对是首选. (1)涉及分配，(2)，(3)涉及即使必须为空值也必须构造T-这对于double根本不重要，但对于更昂贵的类型确实重要.对于(5)，异常不能替代optional.
The DIY option is definitely preferred. (1) involves allocation and (2), (3) involve having to construct a T even if you want a null value - which doesn't matter at all for double but does matter for more expensive types. With (5), exceptions are not a replacement for optional.
您始终可以将您的实现与Boost的实现进行比较.毕竟，这是一个很小的仅标头的库.
You can always compare your implementation to Boost's. It's a small header-only library, after all.
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