blocks|key|2729846|text|是的，实例化该流是合法且定义良好的。您可以安全地将它与另一个流交换，或者在以后给它一个新的指针(这一次指向现有的缓冲区)。输出操作本身确实是一个no-op。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2729847|原因如下：|2729848|2729849|该构造没有非空的前置条件，只有这个后置条件：|ordered-list-item|2729850|2729851|rdbuf()+==+sb.后置条件：[C%2B%2B11:+27.7.3.2/2]:|offset|length|style|CODE|2729852|2729853|Interestingly，它明确指出，在构造函数中不应对sb执行任何操作：|2729854|2729855|[C%2B%2B11:+27.7.3.2/4]:备注:不对rdbuf().执行任何操作|2729856|2729857|，但也要注意：|2729858|2729859|[C%2B%2B11:+27.7.3.2/1]:效果:构造basic_ostream类的对象，通过调用basic_ios<charT,traits>::init(sb)+(27.5.5.2)将初始值分配给基类。|2729860|2729861|当sb为NULL时，init(sb)调用具有在流上设置badbit的效果：|2729862|2729863|[C%2B%2B11:+27.5.5.2/3]:后置条件:此函数的后置条件如表128所示。|2729864|[C%2B%2B11:+Table+128]:+..rdstate()：如果sb不是空指针，则为goodbit，否则为badbit。..|2729865|2729866|输出操作将导致等同于取消引用空指针的操作：|2729867|2729868|[C%2B%2B11:+27.7.3.1/2]:两组成员函数签名共享公共属性:格式化的输出函数(或插入器)和未格式化的输出函数。这两组输出函数都通过与调用rdbuf()->sputc(int_type)**.**等效的操作来生成(或插入)输出字符。它们可以使用basic_ostream的其他公共成员，但它们不应调用除rdbuf()、overflow()、xsputn()和sync().之外的任何rdbuf()虚拟成员|BOLD|2729869|除了它永远不会走到这一步，因为对于basic_ostream::sentry构造：|2729870|如果在完成任何准备之后，os.good()为true，则为[C%2B%2B11:+27.7.3.4/3]:；否则为ok_+==+false。|2729871|并且，对于explicit+operator+basic_ostream::sentry::bool()+const;|2729872|[C%2B%2B11:+27.7.3.4/5]:效果:返回ok_。|2729873|和：|2729874|通过构造sentry类的对象，[C%2B%2B11:+27.7.3.7/1]:每个未格式化的输出函数开始执行。如果此对象在转换为bool类型的值时返回true，该函数将努力生成所请求的输出。。。|2729875|…这意味着当已经设置了badbit时，根本不会发生输出操作。|2729876|This+was+also+the+case+in+C%2B%2B03.|2729877|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/6240980/560648^0|0|0|0|0|0|0|D|J|K|0|0|T|2|0|0|0|K|P|7|0|0|0|0|0|K|P|D|1B|X|0|0|1|2|A|8|R|6|0|0|0|K|0|0|J|M|9|Y|2|18|7|1J|6|0|0|0|0|0|K|21|O|3I|D|4B|7|4J|A|4U|8|53|6|5F|7|21|Q|0|H|L|0|C|9|M|4|T|K|1H|C|0|5|1I|0|0|K|P|3|0|0|4|6|F|K|1O|4|1Z|4|0|B|6|0|0|W|0|0^^$0|@$1|2|3|4|5|6|7|23|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|24|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|25|8|@]|9|@]|A|$]]|$1|E|3|F|5|G|7|26|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|27|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|28|8|@$K|29|L|2A|M|N]|$K|2B|L|2C|M|N]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|2D|8|@]|9|@]|A|$]]|$1|P|3|Q|5|G|7|2E|8|@$K|2F|L|2G|M|N]]|9|@]|A|$]]|$1|R|3|-4|5|6|7|2H|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|2I|8|@$K|2J|L|2K|M|N]|$K|2L|L|2M|M|N]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|2N|8|@]|9|@]|A|$]]|$1|V|3|W|5|G|7|2O|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|2P|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|2Q|8|@$K|2R|L|2S|M|N]|$K|2T|L|2U|M|N]|$K|2V|L|2W|M|N]]|9|@]|A|$]]|$1|10|3|-4|5|6|7|2X|8|@]|9|@]|A|$]]|$1|11|3|12|5|G|7|2Y|8|@$K|2Z|L|30|M|N]|$K|31|L|32|M|N]|$K|33|L|34|M|N]]|9|@]|A|$]]|$1|13|3|-4|5|6|7|35|8|@]|9|@]|A|$]]|$1|14|3|15|5|6|7|36|8|@$K|37|L|38|M|N]]|9|@]|A|$]]|$1|16|3|17|5|6|7|39|8|@$K|3A|L|3B|M|N]|$K|3C|L|3D|M|N]|$K|3E|L|3F|M|N]|$K|3G|L|3H|M|N]|$K|3I|L|3J|M|N]]|9|@]|A|$]]|$1|18|3|-4|5|6|7|3K|8|@]|9|@]|A|$]]|$1|19|3|1A|5|G|7|3L|8|@]|9|@]|A|$]]|$1|1B|3|-4|5|6|7|3M|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|6|7|3N|8|@$K|3O|L|3P|M|N]|$K|3Q|L|3R|M|N]|$K|3S|L|3T|M|N]|$K|3U|L|3V|M|N]|$K|3W|L|3X|M|N]|$K|3Y|L|3Z|M|N]|$K|40|L|41|M|N]|$K|42|L|43|M|N]|$K|44|L|45|M|1E]]|9|@]|A|$]]|$1|1F|3|1G|5|6|7|46|8|@$K|47|L|48|M|N]]|9|@]|A|$]]|$1|1H|3|1I|5|6|7|49|8|@$K|4A|L|4B|M|N]|$K|4C|L|4D|M|N]|$K|4E|L|4F|M|N]|$K|4G|L|4H|M|N]]|9|@]|A|$]]|$1|1J|3|1K|5|6|7|4I|8|@$K|4J|L|4K|M|N]]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|4L|8|@$K|4M|L|4N|M|N]|$K|4O|L|4P|M|N]]|9|@]|A|$]]|$1|1N|3|1O|5|6|7|4Q|8|@]|9|@]|A|$]]|$1|1P|3|1Q|5|6|7|4R|8|@$K|4S|L|4T|M|N]|$K|4U|L|4V|M|N]|$K|4W|L|4X|M|N]|$K|4Y|L|4Z|M|N]]|9|@]|A|$]]|$1|1R|3|1S|5|6|7|50|8|@$K|51|L|52|M|N]]|9|@]|A|$]]|$1|1T|3|1U|5|6|7|53|8|@]|9|@$K|54|L|55|1|56]]|A|$]]|$1|1V|3|-4|5|6|7|57|8|@]|9|@]|A|$]]]|1W|$1X|$5|1Y|1Z|20|A|$21|22]]]]

Yes, it is legal and well-defined to instantiate that stream. You can safely swap it with another stream, or give it a new pointer (this time to an extant buffer) at a later time. The output operation itself is indeed a no-op.

Here's why:

<ol>
<li>The construction has no non-null precondition, and has only this postcondition:

<blockquote>
 <code>[C++11: 27.7.3.2/2]:</code> Postcondition: <code>rdbuf() == sb</code>.
</blockquote></li>
<li>Interestingly, it makes an explicit point that no operation shall be performed on <code>sb</code> within the constructor:

<blockquote>
 <code>[C++11: 27.7.3.2/4]:</code> Remarks: Does not perform any operations on <code>rdbuf()</code>.
</blockquote></li>
<li>But note also:

<blockquote>
 <code>[C++11: 27.7.3.2/1]:</code> Effects: Constructs an object of class <code>basic_ostream</code>, assigning initial values to the base class by calling <code>basic_ios&lt;charT,traits&gt;::init(sb)</code> (27.5.5.2).
</blockquote></li>
<li>That <code>init(sb)</code> call has the effect of setting <code>badbit</code> on the stream when <code>sb</code> is NULL:

<blockquote>
 <code>[C++11: 27.5.5.2/3]:</code> Postconditions: The postconditions of this function are indicated in Table 128.
 
 <code>[C++11: Table 128]:</code> [..] <code>rdstate()</code>: <code>goodbit</code> if <code>sb</code> is not a null pointer, otherwise
 <code>badbit</code>. [..]
</blockquote></li>
<li>The output operation would result in actions equivalent to dereferencing a null pointer:

<blockquote>
 <code>[C++11: 27.7.3.1/2]:</code> Two groups of member function signatures share common properties: the formatted output functions (or inserters) and the unformatted output functions. Both groups of output functions generate (or insert) output characters by actions equivalent to calling <code>rdbuf()-&gt;sputc(int_type)</code>. They may use other public members of <code>basic_ostream</code> except that they shall not invoke any virtual members of <code>rdbuf()</code> except <code>overflow()</code>, <code>xsputn()</code>, and <code>sync()</code>.
</blockquote>

except it never gets this far, because for <code>basic_ostream::sentry</code> construction:

<blockquote>
 <code>[C++11: 27.7.3.4/3]:</code> If, after any preparation is completed, <code>os.good()</code> is <code>true</code>, <code>ok_ == true</code> otherwise, <code>ok_ == false</code>.
</blockquote>

and, for <code>explicit operator basic_ostream::sentry::bool() const;</code>:

<blockquote>
 <code>[C++11: 27.7.3.4/5]:</code> Effects: Returns <code>ok_</code>.
</blockquote>

and:

<blockquote>
 <code>[C++11: 27.7.3.7/1]:</code> Each unformatted output function begins execution by constructing an object of class <code>sentry</code>. If this object returns <code>true</code>, while converting to a value of type <code>bool</code>, the function endeavors to generate the requested output. [..]
</blockquote>

&hellip;the implication being that no output operation takes place at all when <code>badbit</code> is already set.</li>
</ol>

<a href="https://stackoverflow.com/a/6240980/560648">This was also the case in C++03.</a>

Consider the following:

<pre><code>std::ostream out(nullptr);
</code></pre>

Is this legal and well-defined?

<hr>

How about if I now do:

<pre><code>out &lt;&lt; "hello world\n";
</code></pre>

Is this legal and well-defined? If so, presumably it's a no-op of sorts?

Is it valid to construct an `std::ostream` from a null buffer?

请考虑以下几点：std::ostream out(nullptr);这是合法且定义明确的吗？如果我现在这样做：out << "hello world\n";这是合法且定义明确的吗？如果是这样的话，想必这是一种无操作吧？

问从null buffer构造一个` `std::ostream`是否有效？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问从null buffer构造一个` `std::ostream`是否有效？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问从null buffer构造一个` `std::ostream`是否有效？
EN