正确使用std智能指针确保ptr安全

Question

这是使用std智能指针确保ptr安全的正确方法

这个例子可能不是最好的,但我试图模仿一些真正的代码.我遇到的问题是在实际代码中,通信器指针是一个原始指针,可以在任何时刻解除分配 - 导致使用指针崩溃.

所以我决定查看std :: shared_ptr和std :: weak_ptr以了解它应该如何设计现在我们有C++ 11.我在发送代码中使用weak_ptr来检查ptr是否仍然有效,然后才会取消引用ptr.这段代码是正确的方法吗？有什么改进？

#include <memory>
#include <iostream>
#include <string>

class communicator
{
public:
    communicator(const char* name, int comport, int speed) : name_(name), comport_(comport), speed_(speed) { }

    void send(const std::string& s) {
        std::cout << "sending " << s << " using " << name_ << " at " << speed_ << " rate and using com port " << comport_ << '\n';
    }

private:
    const char* name_;
    int comport_;
    int speed_;
};

class sender
{
public:
    sender() {}

    void set_communicator(std::weak_ptr<communicator> comms) {
        comms_ = comms;
    }

    void send(const std::string& s)
    {
        if (auto sh = comms_.lock())
            sh->send(s);
        else
            std::cout << "Attempting to send: " << s << " but ptr no longer exists\n";
    }

private:
    std::weak_ptr<communicator> comms_;
};

int main() {

    sender mysender;

    {
        // create comms object
        std::shared_ptr<communicator> comms(new communicator("myname", 3, 9600));

        mysender.set_communicator(comms);

        mysender.send("Hi guys!");

    }  // comms object gets deleted here

    mysender.send("Hi guys after ptr delete!");
}

输出:

sending Hi guys! using myname at 9600 rate and using com port 3
Attempting to send: Hi guys after ptr delete! but ptr no longer exists

Answer 1

可以随时取消分配的指针 - 导致使用指针时崩溃

这就是引入 ; 的理由背后的症状weak_ptr。因此我认为你weak_ptr基于 - 的方法是正确的。

然而，我发现有争议的是，与此相结合

sender() : comms_() {}

void set_communicator(std::weak_ptr<communicator> comms) {
    comms_ = comms;
}

sender内部资产的两阶段构建comms_ 一旦 lock() 失败，您不会将内部资产的状态重置为构建后状态

void send(const std::string& s)

但这本身并不是“错误”。这只是可以考虑用于完整应用程序的东西。

另一件事是，当失败时你不会throw（或者让ctor (#11)抛出shared_ptr(weak_ptr)）lock()，而只是if-else 处理它。我无法知道您的完整应用程序的要求，但根据您组装的摘录，基于异常的错误处理将改进设计。

例如：

#include <memory>
#include <stdexcept>
#include <iostream>
#include <string>

class communicator
{
public:
    communicator(const char* name, int comport, int speed) 
        : name_(name), comport_(comport), speed_(speed) { }

    void send(const std::string& s) {
        std::cout << "sending " << s << " using " << name_ << " at " 
                  << speed_ << " rate and using com port " << comport_ 
                  << '\n';
    }

private:
    const char* name_;
    int comport_;
    int speed_;
};

class sender
{
public:
    struct invalid_communicator : public std::runtime_error {
        invalid_communicator(const std::string& s) :
            std::runtime_error(
                std::string("Attempting to send: \"") + s 
                    + "\" but communicator is invalid or not set"
            ) {}
    };  

    sender() : comms_() {}

    void set_communicator(std::weak_ptr<communicator> comms) {
        comms_ = comms;
    }

    /* non-const */
    void send(const std::string& s) throw (invalid_communicator)
    {
        try {
            auto sh = std::shared_ptr<communicator>(comms_);
            sh->send(s);
        } catch (const std::bad_weak_ptr& e) {
            comms_ = decltype(comms_)();
            throw invalid_communicator(s);
        }
    }

private:
    std::weak_ptr<communicator> comms_;
};

int main() {
    int rv = -1;
    sender mysender;

    for (auto com : {1, 2, 3}) {
        try {
            { 
                // create comms object
                auto comms = std::make_shared<communicator>(
                    "myname", com, 9600
                );
                mysender.set_communicator(comms);
                mysender.send("Hi guys!");
            }// comms object gets deleted here

            mysender.send("Hi guys after ptr delete!"); 

            // never reached in this example; just to illustrate
            // how the story could continue  
            rv = EXIT_SUCCESS;            
            break; // it'd be not nice to "break", but I did not want to
                   // introduce another state variable
        } catch (const sender::invalid_communicator& e) {
            std::cerr << e.what() << std::endl;
        }
    }

    return rv;
}

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