asio::buffer_cast 奇怪的输出

Question

我正在学习boost::asio，遇到了问题。基本上，asio::buffer_cast() 在我的机器上运行很奇怪。任何帮助将不胜感激。

我有如下示例代码，

#include <iostream>
#include <vector>
#include <sstream>
#include <iomanip>
#include <boost/asio.hpp>
int main ()
{  
std::ostringstream type_stream;
type_stream << std::setw(4) << 100;
std::cout<<"type_stream:"<<type_stream.str()<<std::endl;

std::ostringstream head_stream;
head_stream << std::setw(10) << 92;
std::cout<<"head_stream:"<<head_stream.str()<<std::endl;

std::vector<boost::asio::const_buffer> buffers;
buffers.push_back(boost::asio::buffer(type_stream.str()));
buffers.push_back(boost::asio::buffer(head_stream.str()));

auto test = buffers[0];
const unsigned char* p1 = boost::asio::buffer_cast<const unsigned char*>(test);
std::cout<<"type_stream again:"<<std::string(reinterpret_cast<const char*>(p1))<<std::endl;

auto test2 = buffers[1];
const unsigned char* p2 = boost::asio::buffer_cast<const unsigned char*>(test2);
std::cout<<"head_stream again:"<<std::string(reinterpret_cast<const char*>(p2))<<std::endl;

return 0;
}

如果您运行这里的代码：https://wandbox.org/permlink/4VkxJ4TFgjHzrath，它可以正常工作。输出是

type_stream: 100
head_stream:        92
type_stream again: 100
head_stream again:        92

但是当把代码放在函数中并运行在我的机器上时，我得到的输出是

type_stream: 100
head_stream:        92
type_stream again:        92
head_stream again:        92

我在代码中做错了什么吗？在我的机器上，第二个缓冲区似乎覆盖了第一个缓冲区。我有 gcc (Ubuntu 7.2.0-8ubuntu3.2) 7.2.0 和最新的 boost::asio.

Answer 1

是的，有 Undefined Behaviour。

问题在于

 buffers.push_back(boost::asio::buffer(type_stream.str()));

type_stream.str() returns 临时 std::string 因此缓冲区在推送后无效。

修复：

Live On Coliru

#include <boost/asio.hpp>
#include <boost/lexical_cast.hpp>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <vector>

std::string format_buf(int v, int width) {
    std::ostringstream oss;
    oss << std::setw(width) << v;
    return oss.str();
}

int main() {
    std::string type_str = format_buf(100, 4);
    std::string head_str = format_buf(92, 10);

    std::cout << "type_stream first: " << std::quoted(type_str) << std::endl;
    std::cout << "head_stream first: " << std::quoted(head_str) << std::endl;

    std::vector<boost::asio::const_buffer> buffers;
    buffers.push_back(boost::asio::buffer(type_str));
    buffers.push_back(boost::asio::buffer(head_str));

    auto test = buffers[0];
    const unsigned char *p1 = boost::asio::buffer_cast<const unsigned char *>(test);
    std::cout << "type_stream again:" << std::quoted(reinterpret_cast<const char *>(p1)) << std::endl;

    auto test2 = buffers[1];
    const unsigned char *p2 = boost::asio::buffer_cast<const unsigned char *>(test2);
    std::cout << "head_stream again:" << std::quoted(reinterpret_cast<const char *>(p2)) << std::endl;
}

版画

type_stream first: " 100"
head_stream first: "        92"
type_stream again:" 100"
head_stream again:"        92"

奖金

既然您正在有效地尝试进行固定宽度的格式化（？）为什么不让它更简单、更健壮：

Live On Coliru

char type[4] = {};
char head[10] = {};

bio::stream<bio::array_sink> tstream(type);
tstream << 100;

bio::stream<bio::array_sink> hstream(head);
hstream << 92;

它更健壮，因为您的代码字段可能太宽（例如，类型包含 12345）

Answer 2

在@sehe 的帮助下，以下代码有效。但是我有了新的担忧。在 prepareBuffers() 函数中创建 char type[4] = {}; char head[10] = {}; char type2[2] = {}; 可以吗？由于缓冲区是由 buffers.push_back(boost::asio::buffer(type)); 创建的，我怀疑当退出 prepareBuffers() 函数时类型将被破坏。但是在main() 中的程序仍然可以访问缓冲区的内容。有没有副本发生？

#include <boost/asio.hpp>
#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/stream.hpp>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <vector>

namespace bio = boost::iostreams;

void prepareBuffers(std::vector<boost::asio::const_buffer> & buffers){
char type[4] = {};
char head[10] = {};

bio::stream<bio::array_sink> tstream(type);
bio::stream<bio::array_sink> hstream(head);

tstream << 555555;
hstream << 923;

std::cout << "type_stream first: " << std::quoted(type) << std::endl;
std::cout << "head_stream first: " << std::quoted(head) << std::endl;

buffers.push_back(boost::asio::buffer(type));
buffers.push_back(boost::asio::buffer(head)); 

auto test = buffers[0];
const unsigned char* p1 = boost::asio::buffer_cast<const unsigned char*>(test);
std::cout<<"in function type_stream again:"<<std::string(reinterpret_cast<const char*>(p1))<<std::endl;

test = buffers[1];
const unsigned char* p2 = boost::asio::buffer_cast<const unsigned char*>(test);
std::cout<<"in function head_stream again:"<<std::string(reinterpret_cast<const char*>(p2))<<std::endl;
}

int main() {
std::vector<boost::asio::const_buffer> buffers2;
prepareBuffers(buffers2);

auto test21 = buffers2[0];
const unsigned char* p1 = boost::asio::buffer_cast<const unsigned char*>(test21);
std::cout<<"type_stream again:"<<std::string(reinterpret_cast<const char*>(p1))<<std::endl;

auto test22 = buffers2[1];
const unsigned char* p2 = boost::asio::buffer_cast<const unsigned char*>(test22);
std::cout<<"type_stream again:"<<std::string(reinterpret_cast<const char*>(p2))<<std::endl;

return 0;
}

asio::buffer_cast 奇怪的输出

asio::buffer_cast strange output

c++

buffer

boost-asio

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