c++ - 发送/获取结构时使用 streambuf

Question

我正在研究 boost::asio::streambuf 并发现我可以使用它发送/获取一个结构，但是当我发送一个结构时我无法获取它我已经发送了。文档说应该使用 commit() 和 consume()，但是我应该在哪里使用它们呢？

struct person
{
    int m_id;
    std::string m_message;
};

std::istream& operator>>(std::istream& in, struct person& p)
{
    return in >> p.m_id >> p.m_message;
}

std::ostream& operator<<(std::ostream& out, struct person& p)
{
    return out << p.m_id << " " << p.m_message;
}

int main()
{
    boost::asio::streambuf buf;
    std::ostream out(&buf);

    person p;

    p.m_id = 1;
    p.m_message = "Hello World!";

    out << p;

    std::istream in(&buf);

    person p1;

    in >> p1;

    cout << "ID: " << p1.m_id << endl;
    cout << "Message: " << p1.m_message << endl;

return 0;
}

问题出在字符串上，所以当我只输入“hello”(没有 world)时，它工作正常，但如果我添加“world!”如上所示，它只是没有看到添加的“world!”，为什么？

Answer 1

有很多问题。

1. 首先，尽可能使参数const&:

   std::ostream &operator<<(std::ostream &out, person const &p) {
   

2. 其次，确保流刷新到缓冲区。我认为限制 ostream 或 istream 实例的生命周期是个好习惯

3. 第三，选择稳健的格式。当您有 m_id = 1 和 m_message = "123" 时，您的示例已经有更大的问题(您能看到吗？)。

   在文本格式中，您需要固定长度的字段或定界协议(protocol)。让我们修复它:

   std::ostream &operator<<(std::ostream &out, person const &p) {
       return out << p.m_id << ";" << p.m_message.length() << ";" << p.m_message;
   }
   

   现在，当你回头读它时，你会发现你需要更加精确:

   std::istream &operator>>(std::istream &in, person &p) {
       char separator;
       size_t length;
   
       bool ok = in >> p.m_id 
              && in >> separator && separator == ';' 
              && in >> length
              && in >> separator && separator == ';' 
              ;
   
       if (ok) {
           p.m_message.resize(length);
           in.read(&p.m_message[0], length);
   
           p.m_message.resize(in.gcount());
       }
   
       // ensure the expected number of bytes were read
       ok = ok && (p.m_message.length() == length);
   
       if (!ok)
           in.setstate(std::ios::failbit);
   
       return in;
   }
   

   哎呀。真的吗？对真的。至少!

4. 做错误处理

完整演示

Live On Coliru

#include <boost/asio.hpp>
#include <iostream>

struct person {
    int m_id;
    std::string m_message;
};

std::ostream &operator<<(std::ostream &out, person const &p) {
    return out << p.m_id << ";" << p.m_message.length() << ";" << p.m_message;
}

std::istream &operator>>(std::istream &in, person &p) {
    char separator;
    size_t length;

    bool ok = in >> p.m_id 
           && in >> separator && separator == ';' 
           && in >> length
           && in >> separator && separator == ';' 
           ;

    if (ok) {
        p.m_message.resize(length);
        in.read(&p.m_message[0], length);

        p.m_message.resize(in.gcount());
    }

    // ensure the expected number of bytes were read
    ok = ok && (p.m_message.length() == length);

    if (!ok)
        in.setstate(std::ios::failbit);

    return in;
}

int main() {
    boost::asio::streambuf buf;

    std::ostream(&buf) << person{ 1, "Hello World!" }; 

    person received;
    if (std::istream(&buf) >> received) {
        std::cout << "ID:      " << received.m_id << std::endl;
        std::cout << "Message: " << received.m_message << std::endl;
    } else {
        std::cout << "Couldn't receive person\n";
    }
}

打印

ID:      1
Message: Hello World!

奖金

C++14 添加了std::quoted:

#include <iomanip>
std::ostream &operator<<(std::ostream &out, person const &p) { return out << p.m_id << std::quoted(p.m_message); }
std::istream &operator>>(std::istream &in,  person       &p) { return in  >> p.m_id >> std::quoted(p.m_message); }

在这种情况下，它也可以完成工作: Live On Coliru