高效的将数据从boost :: asio :: streambuf复制到std :: string [英] efficient copy of data from boost::asio::streambuf to std::string

问题描述

我需要将（boost :: asio：:) streambuf的内容复制到std :: string。

I need to copy the content of a (boost::asio::)streambuf to an std::string.

以下代码可以工作，在_msg和临时std :: string之间有一个不必要的副本：

The following code works, but I think that there's an unnecessary copy between _msg and the temporary std::string:

Msg (boost::asio::streambuf & sb, size_t bytes_transferred) :
    _nBytesInMsg    (bytes_transferred)
{
    boost::asio::streambuf::const_buffers_type buf = sb.data();

    _msg = std::string(
        boost::asio::buffers_begin(buf),
        boost::asio::buffers_begin(buf) + _nBytesInMsg);
}

我尝试用以下代替：

     _msg.reserve(_nBytesInMsg);
     std::copy(
        boost::asio::buffers_begin(buf),
        boost::asio::buffers_begin(buf) + _nBytesInMsg,
        _msg.begin()
    );

在编译时，它不会将任何东西复制到_msg字符串。

While this compiles, it doesn't copy anything to the _msg string.

编译器（gcc4.4.7）会优化这种情况 - 例如将streambuf直接复制到_msg而不使用临时文件？

Will the compiler (gcc4.4.7) optimize this case - e.g. copy the streambuf straight to _msg without using a temporary?

是否有可能使用boost :: asio :: streambuf :: const_buffers_type来创建一个迭代器std :: copy工作？

Is there perhaps an iterator I can use with boost::asio::streambuf::const_buffers_type in order to make the std::copy work instead?

推荐答案

reserve 你认为它意味着什么。您可以确定阅读其文档的含义。它与 resize 不同;预留空间不会影响容器的大小。

reserve doesn't mean what you think it means. You can work out for sure what it means by reading its documentation. It is not the same as resize; reserving space does not affect the container's size.

您需要将元素实际插入到字符串中。执行此操作：

You need to actually insert elements into the string. Do this:

#include <algorithm>    // for copy
#include <iterator>     // for back_inserter

_msg.reserve(_nBytesInMsg);  // about to insert _nBytesInMsg elements

std::copy(
    boost::asio::buffers_begin(buf),
    boost::asio::buffers_begin(buf) + _nBytesInMsg,
    std::back_inserter(_msg)
);

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