blocks|key|1194344|text|这应该是您需要的全部内容：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1194345|ostringstream+os;
ifstream+file("name.txt");
os+<<+file.rdbuf();

string+s+=+os.str();|code-block|syntax|javascript|1194346|这将从file中读取字符并将它们插入到字符串流中。之后，它将获取在幕后创建的字符串。请注意，我落入了以下陷阱:使用提取操作符将跳过初始空格。必须使用如上的插入操作符，或使用noskipws操纵器：|offset|length|style|CODE|1194347|//+Beware,+skips+initial+whitespace!
file+>>+os.rdbuf();

//+This+does+not+skip+it
file+>>+noskipws+>>+os.rdbuf();+|1194348|这些函数被描述为逐个字符地读取流(虽然不确定这里可能进行哪些优化)，但我没有对它们进行计时以确定它们的速度。|1194349|entityMap^0|0|0|3|4|2E|8|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|R|$]]

This should be all you need:

<pre><code>ostringstream os;
ifstream file("name.txt");
os &lt;&lt; file.rdbuf();

string s = os.str();
</code></pre>

This reads characters from <code>file</code> and inserts them into the stringstream. Afterwards it gets the string created behind the scenes. Notice that i fell into the following trap: Using the extraction operator will skip initial whitespace. You have to use the insertion operator like above, or use the <code>noskipws</code> manipulator:

<pre><code>// Beware, skips initial whitespace!
file &gt;&gt; os.rdbuf();

// This does not skip it
file &gt;&gt; noskipws &gt;&gt; os.rdbuf(); 
</code></pre>

These functions are described as reading the stream character by character though (not sure what optimizations are possible here, though), i haven't timed these to determine their speed.

blocks|key|1194389|text|用于预分配的std::string+has+a+.reserve+method。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1194390|std::string+s;
s.reserve(1048576);+//+reserve+1+MB
read_file_into(s);|code-block|syntax|javascript|1194391|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/cpp/string/basic_string/reserve^0|6|B|O|8|I|L|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|H|7|10|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

<code>std::string</code> <a href="http://en.cppreference.com/w/cpp/string/basic_string/reserve" rel="noreferrer">has a <code>.reserve</code> method</a> for pre-allocation.

<pre><code>std::string s;
s.reserve(1048576); // reserve 1 MB
read_file_into(s);
</code></pre>

blocks|key|1194396|text|std::string::reserve()|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1194397|std::getline()|1194398|entityMap|0|LINK|mutability|MUTABLE|url|http://www.sgi.com/tech/stl/basic_string.html|1|http://www.cppreference.com/wiki/string/getline^0|0|M|0|0|0|E|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

<a href="http://www.sgi.com/tech/stl/basic_string.html" rel="nofollow noreferrer">std::string::reserve()</a>

<a href="http://www.cppreference.com/wiki/string/getline" rel="nofollow noreferrer">std::getline()</a>

blocks|key|1194426|text|为了好玩，这里有另一种方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1194427|//+Beware,+brain-compiled+code+ahead!

std::ifstream+ifs(+/*+...+*/+);
if(+!ifs.good()+)+return;+//+whatever

std::string+str;

ifs.seekg(+0,+std::ios_base::end+);
const+std::streampos+pos+=+ifs.tellg();
ifs.seekg(+0,+std::ios_base::beg+);
if(+pos!=std::streampos(-1)+)+//+can+get+stream+size?+
++str.reserve(static_cast<std::string::size_type>(pos));

str.assign(+std::istream_iterator<char>(ifs)
++++++++++,+std::istream_iterator<char>()+);|code-block|syntax|javascript|1194428|我希望我没有搞砸了。|1194429|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Just for fun, here's another way to do this: 

<pre><code>// Beware, brain-compiled code ahead!

std::ifstream ifs( /* ... */ );
if( !ifs.good() ) return; // whatever

std::string str;

ifs.seekg( 0, std::ios_base::end );
const std::streampos pos = ifs.tellg();
ifs.seekg( 0, std::ios_base::beg );
if( pos!=std::streampos(-1) ) // can get stream size? 
 str.reserve(static_cast&lt;std::string::size_type&gt;(pos));

str.assign( std::istream_iterator&lt;char&gt;(ifs)
 , std::istream_iterator&lt;char&gt;() );
</code></pre>

I hope I didn't blow it too badly.

blocks|key|980159|text|您似乎在问如何使用std::string进行CString::GetBuffer，ReleaseBuffer类型的操作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|980160|我不知道有什么方法可以直接做到这一点，一种简单的方法是创建一个原始的C样式缓冲区，读入缓冲区，然后使用赋值或其他方法将缓冲区复制到std::string中。当然，你必须担心缓冲区溢出等问题，我也会使用std::autoptr来管理原始缓冲区指针，在异常时重新分配等等。这比使用stringstream等要简单一些。如果需要，我可以提供一个例子。|980161|德文·艾林森|980162|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

It seems that you are asking how to do a CString::GetBuffer, ReleaseBuffer type operation with std::string. 

I don't know of any way to do this directly, an easy way would be to just create a raw C style buffer, read into the buffer, then copy the buffer to a std::string using assign or whatever. Of course you would have to worry about buffer overrun issues etc., also I would use a std::autoptr to manage the raw buffer pointer, to enusre deallocation on exception etc. This is a bit simpler than using stringstream etc. I can provide an example if needed.

Devin Ellingson

blocks|key|1194529|text|std::string::resize()实际上分配了所需的空间。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1194530|std::string::reserve()可能不会(这是一个请求)。|1194531|entityMap^0|0|L|0|0|M|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

<code>std::string::resize()</code> actually allocates the required space.

<code>std::string::reserve()</code> may not (it's a request).

I need to copy a file into a string. I need someway to preallocate memory for that string object and a way to directly read the file content into that string's memory?

how to pre-allocate memory for a std::string object?

我需要将一个文件复制到一个字符串中。我需要某种方法来为该字符串对象预分配内存，以及一种直接将文件内容读取到该字符串内存中的方法？

问如何为std::string对象预分配内存？
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

问如何为std::string对象预分配内存？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

问如何为std::string对象预分配内存？
EN