blocks|key|161540|text|默认构造的boost::optional为空-它不包含值，因此不能对其调用get()。你必须用一个有效的值初始化它：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|161541|boost::optional<myClass>+value+=+myClass();|code-block|syntax|javascript|161542|或者，您可以使用in-place+factory来避免复制初始化(但复制很可能无论如何都会被省略)；但是，我没有使用它的经验，所以我不能提供示例。|161543|161544|顺便说一下，您可以使用->代替get()，如下所示：|161545|value->setInt(10);|161546|但这只是一个风格偏好的问题，两者都是同样有效的。|161547|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_55_0/libs/optional/doc/html/boost_optional/detailed_semantics.html#reference_optional_constructor_factory^0|5|F|11|5|0|0|8|G|0|0|0|B|2|F|5|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|17|8|@]|D|@$9|18|A|19|1|1A]]|E|$]]|$1|M|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|P|3|Q|5|H|7|1H|8|@]|D|@]|E|$I|J]]|$1|R|3|S|5|6|7|1I|8|@]|D|@]|E|$]]|$1|T|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|U|$V|$5|W|X|Y|E|$Z|10]]]]

A default-constructed <code>boost::optional</code> is empty - it does not contain a value, so you can't call <code>get()</code> on it. You have to initialise it with a valid value:

<pre><code>boost::optional&lt;myClass&gt; value = myClass();
</code></pre>

Alternatively, you can use an <a href="http://www.boost.org/doc/libs/1_55_0/libs/optional/doc/html/boost_optional/detailed_semantics.html#reference_optional_constructor_factory" rel="noreferrer">in-place factory</a> to avoid copy initialisation (but the copy will most likely be elided anyway); however, I have no experience with that, so I can't provide an example.

<hr>

As a side note, you can use <code>-&gt;</code> in place of <code>get()</code>, like this:

<pre><code>value-&gt;setInt(10);
</code></pre>

But that's just a matter of stylistic preference, both are equally valid.

blocks|key|162824|text|两种简单的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|162825|boost::optional<myClass>+func(const+std::string+&str)
{
++boost::optional<myClass>+value;
++if(str.length()+>+5)+//+If+greater+than+5+length+string.+Set+value+to+10
++++value+=+10;
++else+if+(str.length()+<+5)+//+Else+set+it+to+0
++++value+=+0;
++else+//+If+it+is+5+set+the+value+to+5
++++value+=+5;

++return+value;
}

boost::optional<myClass>+func(const+std::string+&str)
{
++if(str.length()+>+5)+//+If+greater+than+5+length+string.+Set+value+to+10
++++return+10;
++else+if+(str.length()+<+5)+//+Else+set+it+to+0
++++return+0;
++else+//+If+it+is+5+set+the+value+to+5
++++return+5;
}|code-block|syntax|javascript|162826|请注意，从从不返回空可选项的函数返回optional不是一个好主意。|offset|length|style|CODE|162827|optional的行为类似于读访问上的指针--只有当您已经验证有需要读取的内容时，才能从它中读取值。您可以通过执行bool+something_to_read+=+opt;来检查是否有要读取的内容。|162828|但是，您可以随时对其进行写入。如果那里什么都没有，它就会创造一些东西。如果有什么东西在那里，它会覆盖它。|162829|.get()是一个读操作，而不是一个写操作。(它“读取”引用)只有当optional正在使用并且有数据时，才能安全地使用它。令人困惑的是，您可以写入“读访问”.get()返回值，因为它是一个非常数引用。|162830|所以“读”和“写”可能是不好用的词。:)|162831|有时，将optional看作是值和指针混合在一起会很有帮助。可能存在指向拥有的内存缓冲区的空指针，该缓冲区可能保存或不保存该类型的副本。|162832|如果optional中的指针为null，则缓冲区未初始化。如果它指向缓冲区，则初始化缓冲区。|162833|.get()取消引用该指针并返回结果引用而不进行检查。=检查指针，如果它是null，它执行从rhs到缓冲区的复制构造，并设置指针。如果不是，它只是分配给缓冲区。|162834|(指针是概念性的:通常作为bool标志实现)。|162835|我发现使用*optional比使用optional.get()更好，因为“在取消引用之前必须检查”这一点在使用取消引用操作符时更加明显。|162836|entityMap^0|0|0|I|8|0|0|8|1L|T|0|0|0|6|Y|8|27|6|0|0|0|0|0|6|R|1|0|D|4|0|5|9|H|E|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|18|8|@$I|19|J|1A|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|1B|8|@$I|1C|J|1D|K|L]|$I|1E|J|1F|K|L]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1G|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1H|8|@$I|1I|J|1J|K|L]|$I|1K|J|1L|K|L]|$I|1M|J|1N|K|L]]|9|@]|A|$]]|$1|S|3|T|5|6|7|1O|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1P|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1R|8|@$I|1S|J|1T|K|L]|$I|1U|J|1V|K|L]]|9|@]|A|$]]|$1|10|3|11|5|6|7|1W|8|@$I|1X|J|1Y|K|L]]|9|@]|A|$]]|$1|12|3|13|5|6|7|1Z|8|@$I|20|J|21|K|L]|$I|22|J|23|K|L]]|9|@]|A|$]]|$1|14|3|-4|5|6|7|24|8|@]|9|@]|A|$]]]|15|$]]

Two easy approaches:

<pre><code>boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 boost::optional&lt;myClass&gt; value;
 if(str.length() &gt; 5) // If greater than 5 length string. Set value to 10
 value = 10;
 else if (str.length() &lt; 5) // Else set it to 0
 value = 0;
 else // If it is 5 set the value to 5
 value = 5;

 return value;
}

boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 if(str.length() &gt; 5) // If greater than 5 length string. Set value to 10
 return 10;
 else if (str.length() &lt; 5) // Else set it to 0
 return 0;
 else // If it is 5 set the value to 5
 return 5;
}
</code></pre>

note that returning an <code>optional</code> from a function that never returns an empty optional is a bad idea.

<code>optional</code> behaves like a pointer on read access -- you can only read the value from it if you have already verified there is something there to read. You can check if there is something to read by doing <code>bool something_to_read = opt;</code>.

You can, however, write to it whenever. If there is nothing there, it creates something. If there is something there, it overwrites it.

<code>.get()</code> is a reading, not a writing, operation. (it "reads" the reference) It is only safe to use when the <code>optional</code> is engaged and has data. Confusingly, you can write to the "read access" <code>.get()</code> return value, as it is a non-const reference.

So maybe "read" and "write" are bad words to use. :)

It is sometimes helpful to think of optional as a value-and-pointer mixed together. There is a possibly null pointer to an owned buffer of memory that may, or may not hold a copy of the type.

If the pointer inside the optional is null, then the buffer is uninitialized. If it points at the buffer, then the buffer is initialized.

<code>.get()</code> dereferences that pointer and returns the resulting reference without checking. <code>=</code> checks the pointer, if it is null, it does a copy-construct from the rhs into the buffer and sets the pointer. If not, it just assigns to the buffer.

(The pointer is conceptual: usually implemented as a <code>bool</code> flag).

I find using <code>*optional</code> to be better than <code>optional.get()</code>, as the "you must check before you dereference" is more obvious with the dereference operator.

blocks|key|161554|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|161555|怎么做才是正确的呢？|blockquote|161556|161557|161558|boost::optional<myClass>+func(const+std::string+&str)
{
++++if(str.length()+>+5)
++++++++return+myClass{10};
++++if(str.length()+<+5)
++++++++return+myClass{0};
++++return+myClass{5};
}|code-block|syntax|javascript|161559|注意，这段代码不需要boost::optional，因为没有返回空对象的代码分支(它在语义上等同于返回myClass实例)。|161560|要返回空的可选参数，请使用以下命令：|161561|boost::optional<myClass>+func(const+std::string+&str)
{
++++if(str.length()+>+5)
++++++++return+myClass{10};
++++if(str.length()+<+5)
++++++++return+myClass{0};
++++return+boost::none;+//+return+empty+object
}|161562|惯用的客户端代码(不要预初始化值)：|161563|int+main()
{
++++if+(auto+v1+=+func("3214"))
++++++++//+use+*v1+to+access+value
++++++++std::cout+<<+"v1+is+valid"+<<+std::endl;
++++else
++++++++std::cout+<<+"v1+is+not+valid"+<<+std::endl;

++++return+0;
}|161564|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|-4|4|5|6|W|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|X|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|Y|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|Z|7|@]|8|@]|9|$]]|$1|F|3|G|4|H|6|10|7|@]|8|@]|9|$I|J]]|$1|K|3|L|4|5|6|11|7|@]|8|@]|9|$]]|$1|M|3|N|4|5|6|12|7|@]|8|@]|9|$]]|$1|O|3|P|4|H|6|13|7|@]|8|@]|9|$I|J]]|$1|Q|3|R|4|5|6|14|7|@]|8|@]|9|$]]|$1|S|3|T|4|H|6|15|7|@]|8|@]|9|$I|J]]|$1|U|3|-4|4|5|6|16|7|@]|8|@]|9|$]]]|V|$]]

<blockquote>
 How to do it the correct way?
</blockquote>

<pre><code>boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 if(str.length() &gt; 5)
 return myClass{10};
 if(str.length() &lt; 5)
 return myClass{0};
 return myClass{5};
}
</code></pre>

As a side note, this code doesn't need boost::optional, because there is no code branch that returns an empty object (it is semantically equivalent to returning a myClass instance).

To return an empty optional, use this:

<pre><code>boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 if(str.length() &gt; 5)
 return myClass{10};
 if(str.length() &lt; 5)
 return myClass{0};
 return boost::none; // return empty object
}
</code></pre>

Idiomatic client code (don't pre-initialize your values):

<pre><code>int main()
{
 if (auto v1 = func("3214"))
 // use *v1 to access value
 std::cout &lt;&lt; "v1 is valid" &lt;&lt; std::endl;
 else
 std::cout &lt;&lt; "v1 is not valid" &lt;&lt; std::endl;

 return 0;
}
</code></pre>

blocks|key|2752455|text|boost::optional<myClass>+func(const+std::string+&str)
{
++++boost::optional<myClass>+value;+//not+init+is+invalid
++++if(str.length()+>+5)+++++++//+If+greater+than+5+length+string.+Set+value+to+10
++++++++value+=+10;
++++else+if+(str.length()+<+5)+//+Else+set+it+to+0
++++++++value+=+0;

++++return+value;
}


v1+is+valid
v2+is+valid
v3+is+not+valid|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2752456|根据boost的说法，可选的默认ctor将创建一个无效的可选obj|unstyled|2752457|optional<T>+def+;+//not+initalize+with+a+obj+T
assert+(+!def+)+;|2752458|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 boost::optional&lt;myClass&gt; value; //not init is invalid
 if(str.length() &gt; 5) // If greater than 5 length string. Set value to 10
 value = 10;
 else if (str.length() &lt; 5) // Else set it to 0
 value = 0;

 return value;
}


v1 is valid
v2 is valid
v3 is not valid
</code></pre>

according to boost,optional default ctor will create an optional obj as invalid

<pre><code>optional&lt;T&gt; def ; //not initalize with a obj T
assert ( !def ) ;
</code></pre>

I am trying to use <a href="http://www.boost.org/doc/libs/1_62_0/libs/optional/doc/html/index.html" rel="noreferrer"><code>boost::optional</code></a> as below. 

<pre><code>#include &lt;iostream&gt;
#include &lt;string&gt;

#include &lt;boost/optional.hpp&gt;

struct myClass
{
 int myInt;
 void setInt(int input) { myInt = input; }
 int getInt(){return myInt; }
};

boost::optional&lt;myClass&gt; func(const std::string &amp;str)
{
 boost::optional&lt;myClass&gt; value;
 if(str.length() &gt; 5)
 {
 // If greater than 5 length string. Set value to 10
 value.get().setInt(10);
 }
 else if (str.length() &lt; 5)
 {
 // Else set it to 0
 value.get().setInt(0);
 }
 else
 {
 // If it is 5 set the value to 5
 value.get().setInt(5);
 }

 return value;
}


int main()
{
 boost::optional&lt;myClass&gt; v1 = func("3124");
 boost::optional&lt;myClass&gt; v2 = func("helloWorld");
 boost::optional&lt;myClass&gt; v3 = func("hello");

 if (v1)
 std::cout &lt;&lt; "v1 is valid" &lt;&lt; std::endl;
 else
 std::cout &lt;&lt; "v1 is not valid" &lt;&lt; std::endl;

 if (v2)
 std::cout &lt;&lt; "v2 is valid" &lt;&lt; std::endl;
 else
 std::cout &lt;&lt; "v3 is not valid" &lt;&lt; std::endl;

 if (v3)
 std::cout &lt;&lt; "v3 is valid" &lt;&lt; std::endl;
 else
 std::cout &lt;&lt; "v3 is not valid" &lt;&lt; std::endl;

 return 0;
 }
</code></pre>

I get following error

<blockquote>
 prog.exe:
 /usr/local/boost-1.55.0/include/boost/optional/optional.hpp:631:
 boost::optional::reference_type boost::optional::get() [with T =
 myClass; boost::optional::reference_type = myClass&amp;]: Assertion
 `this->is_initialized()' failed.
</blockquote>

Presumably, the optional variable is not initialized properly. How to do it the correct way?

EDIT:: Got some very good answers, just couple of more questions 1. Is it a good idea to use <code>make_optional</code> at the end of <code>'func'</code> function and return it? Also 2. I was thinking of assigning <code>boost::none</code> to emphasize that I have no value to assign and that's why <code>boost::none</code>. But not sure if that is valid?

How to use boost::optional

我正在尝试使用，如下所示。#include <iostream>#include <string>#include <boost/optional.hpp>struct myClass{ int myInt; void setInt(int input) { myInt = input; } int ge...

问如何使用boost::optional
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