从boost :: optional中检索对象

Suppose a method returns something like this

假设一个方法返回这样的东西

boost::optional<SomeClass> SomeMethod()
{...}

Now suppose I have something like this

现在假设我有类似的东西

boost::optional<SomeClass> val = SomeMethod();

Now my question is how can I extract SomeClass out of val ?

现在我的问题是如何从val中提取SomeClass?

So that I could do something like this:

所以我可以这样做:

SomeClass sc = val ?

3 个解决方案

#1

You could use the de-reference operator:

您可以使用de-reference运算符:

SomeClass sc = *val;

Alternatively, you can use the get() method:

或者,您可以使用get()方法:

SomeClass sc = val.get();

Both of these return an lvalue reference to the underlying SomeClass object.

这两个都返回对底层SomeClass对象的左值引用。

#2

To check if the optional contains a value, and the optionally retrieve it:

要检查可选项是否包含值,并选择性地检索它:

boost::optional<SomeClass> x = SomeMethod();
if (x)
     x.get();

To get the optional value, or a default value if it does not exist:

要获取可选值,或者如果它不存在,则为默认值:

SomeMethod().get_value_or(/*default value*/)

#3

As mentioned in the previous answers, the de-reference operator and the function get() have the same functionality. Both require the optional to contain valid data.

如前面的答案所述,去引用运算符和函数get()具有相同的功能。两者都要求可选项包含有效数据。

if (val)
{
    // the optional must be valid before it can be accessed
    SomeClass sc1 = *val;
    SomeClass sc2 = val.get();
}

An alternative is the function value(), that throws an exception if the optional does not carry a value.

另一种方法是函数value(),如果可选项不带值,则抛出异常。

// throws if val is invalid
SomeClass sc3 = val.value();

Alternatively, the functions value_or and value_or_eval can be used to specify defaults that are returned in case the value is not set.

或者,函数value_or和value_or_eval可用于指定在未设置值的情况下返回的默认值。

#1