C++，从函数返回字符串； boost::asio 读/写

Question

我得到一个编译错误，另外我不能在不给它数组元素的情况下 boost::asio::read buf。

std::string eport::read_data (void)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name
    error_code ec;  // address used for error checking
    std::string buf [100];  // data with crc on end

    try
    {
        read (port, buffer (buf), ec);
        std::cout << "eport::read: result: " << buf << std::endl;
    }
    catch (error_code &ec)
    {
        std::cout << "eport::read: ERROR: " << ec << std::endl; 
        return "error";
    }

    std::cout << "eport::read: SUCCESS" << std::endl;

    return buf;

错误：

eport.cc:83:9: error: could not convert ‘(std::string*)(& buf)’ from ‘std::string* {aka std::basic_string<char>*}’ to ‘std::string {aka std::basic_string<char>}’

函数是否需要转换为 const char* ？我不确定出了什么问题。任何帮助表示赞赏，谢谢。

更新代码

这是我的代码。我希望它可以帮助某人，因为 asio 在网络上缺乏好的例子。我知道我的 write 函数可以写得更好，而且这段代码还没有经过测试，所以我不确定我这样做是否正确。谢谢。

#include "../include/main.H"
#include <boost/asio.hpp>   // asynchronous input/output
#include <boost/crc.hpp>    // cyclic redundancy code (for data checking)

using namespace::boost::system;
using namespace::boost::asio;

const char *PORT = "/dev/ttyS0";

// serial port communication setup
serial_port_base::baud_rate BAUD (9600);    // what baud rate do we communicate at (default is 9600)
serial_port_base::character_size C_SIZE (8);    // how big is each "packet" of data (default is 8 bits)
serial_port_base::flow_control FLOW (serial_port_base::flow_control::none); // what flow control is used (default is none)
serial_port_base::parity PARITY (serial_port_base::parity::none);   // what parity is used (default is none)
serial_port_base::stop_bits STOP (serial_port_base::stop_bits::one);    // how many stop bits are used (default is one)

int eport::initialize (void)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name

    // set serial port options
    port.set_option (BAUD);
    port.set_option (C_SIZE);
    port.set_option (FLOW);
    port.set_option (PARITY);
    port.set_option (STOP);

    return 0;
}

int eport::write_data (std::string data)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name
    error_code ec;  // address used for error checking
    boost::crc_32_type crcresult;   // used for communication checking
    char buf [1024];    // buffer to hold data
    int crc;    // holds crc value
    std::ostringstream convert; // used to convert int to string
    std::string data_crc;   // data with crc on end
    std::stringstream ss;   // used to add strings

    strncpy (buf, data.c_str(), sizeof(buf));   // put data into buffer
    buf [sizeof(buf) - 1] = 0;  // make sure the last element has a null

    crcresult.process_bytes (buf, sizeof(buf)); // get crc value from buffer contents
    crc = crcresult.checksum(); // put crc value into integer
    convert << crc; // convert integer to string
    ss << data << convert.str ();   // add crc string to data string
    data_crc = ss.str ();   // data string with crc appended to be used in reading / writing
    std::cout << "eport::write: data with crc: " << data_crc << std::endl; 

    std::cout << "eport::write: writing: " << data_crc << std::endl; 
    write (port, buffer (data_crc, sizeof(data_crc)), ec);  // write data with crc to serial device

    if (ec) // if error code is true, print and return
    {
        std::cout << "eport::write: ERROR: " << ec << std::endl; 
        return -1;
    }

    std::cout << "eport::write: SUCCESS" << std::endl;

    return crc;
}

std::string eport::read_data (void)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name
    error_code ec;  // address used for error checking
    streambuf sb;   // asio stream buffer to hold read data
    std::string buf;    // read buffer will be put into this string

    size_t transferred = read (port, sb, ec);   // read data from serial device
    buf.resize (transferred);   // resize the string to the read data size
    sb.sgetn (&buf[0], buf.size ());    // stores characters from the stream to the array
    std::cout << "eport::read: result: " << buf << std::endl;

    if (ec)
    {
        std::cout << "eport::read: ERROR: " << ec << std::endl; 
        return "error";
    }

    std::cout << "eport::read: SUCCESS" << std::endl;

    return buf;
}

Answer 1

1. 最通用的方法是使用asio::streambuf

   streambuf sb;
   size_t transferred = read (port, sb, ec);
   

   根据文档：

   该函数用于从流中读取一定字节数的数据。调用将阻塞，直到满足以下条件之一：

   • 提供的缓冲区已满（即已达到最大大小）。
   • 发生错误。

   此操作是根据对流的read_some 函数的零次或多次调用来实现的。

   然后，将其复制到一个字符串中：

   std::string buf;
   buf.resize(transferred);
   sb.sgetn(&buf[0], buf.size());
   

   ---

2. 或者，预分配预期大小的缓冲区：

   std::string buf(100u, '\0');
   size_t transferred = read (port, buffer(buf), ec);
   buf.resize(transferred);
   

   ---

3. 对于更复杂的场景，使用read_until：

   streambuf sb;
   size_t transferred = read_until(port, sb, "\r\n", ec);
   

   这将一直读取到遇到"\r\n"（注意：可能会读取更多，但在看到分隔符后不会再次调用read_some）。

   ---

4. 更复杂的停止条件可以使用overload that takes a MatchCondition functor。

异常处理注意事项

如果您通过ec 接收error_code，则不会抛出异常

Answer 2

buf 是std::string 的数组。您应该更改原型或只返回一个字符串。以buf[0] 为例。

很可能你想要的是：

std::string buf;  // No [100]

Answer 3

您的代码存在需要回答的问题，更具体地说，您如何知道将发送到您的read 函数的字符数？

但是，您的问题的一般答案是使用字符数组，然后将其作为 std::string ：

std::string eport::read_data (void)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name
    error_code ec;  // address used for error checking
    char buf [100];  // data with crc on end

    try
    {
        read (port, buf, ec);
        std::cout << "eport::read: result: " << buf << std::endl;
    }
    catch (error_code &ec)
    {
        std::cout << "eport::read: ERROR: " << ec << std::endl; 
        return "error";
    }

    std::cout << "eport::read: SUCCESS" << std::endl;

    return buf;
}

std::string 构造函数将负责将末尾的 buf 复制到 std::string。

现在，如果有办法确定读取的字符数，那么函数必须以不同的方式编写。大多数read 函数都有一个参数指定要读取的最大字符数，并在某处返回读取的字符数。

假设您可以重写（或调用）具有这两个属性的不同读取函数，代码将如下所示：

std::string eport::read_data (void)
{
    io_service io;  // create the I/O service that talks to the serial device
    serial_port port (io, PORT);    // create the serial device, note it takes the io service and the port name
    error_code ec;  // address used for error checking
    char buf [100];  // data with crc on end
    int numCharsRead = 0;
    try
    {
        numCharsRead = read2 (port, buf, 100, ec);
        std::cout << "eport::read: result: " << buf << std::endl;
    }
    catch (error_code &ec)
    {
        std::cout << "eport::read: ERROR: " << ec << std::endl; 
        return "error";
    }

    std::cout << "eport::read: SUCCESS" << std::endl;

    return std::string(buf, numCharsRead);
}

注意return 的区别。 std::string 从字符数组构造，但最多只能有 numCharsRead 个字符。