回调在C++中

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英文:

Callback in C++

问题

我正在进行一个wasm项目,在其中我需要在C++头文件中定义一个empscripten fetch回调,因为我需要更新一个成员变量。
有人可以帮我解决在C++中如何定义一个回调函数,以便我能够更新一个局部成员变量吗?

在我的MyClass.CPP中,我有以下方法:
```cpp
void MyClass::Register()
{
    emscripten_fetch_attr_t attr;
    emscripten_fetch_attr_init(&attr);
    attr.onsuccess = DownloadSucceeded;
    attr.onerror = DownloadFailed;
}

我在头文件中定义了DownloadSucceededDownloadFailed

void DownloadSucceeded(emscripten_fetch_t* fetch)
{
    printf("从URL %s 下载完成 %llu 字节。\n", fetch->url, fetch->numBytes);
    emscripten_fetch_close(fetch); // 释放与fetch关联的数据。
    m_unlocked = true;
}

void DownloadFailed(emscripten_fetch_t* fetch)
{
    printf("从URL %s 下载失败,HTTP失败状态码:%d。\n", fetch->url, fetch->status);
    emscripten_fetch_close(fetch); // 失败时也释放数据。
    m_unlocked = false;
}

我得到了以下异常:

必须调用对非静态成员函数的引用。

根据上述异常,我尝试发送一个引用,但似乎也不起作用。如果有聪明的人能解释为什么以及如何解决这个问题,我将不胜感激!

我的天真的引用示例也不起作用:

attr.onsuccess = &DetectorBridge::DownloadSucceeded;

会得到:

从不兼容的类型‘void (mynamespace::MyClass::*)(emscripten_fetch_t*)’分配给‘void (*)(struct emscripten_fetch_t*)’。
英文:

I am working on a wasm project where I need to define a empscripten fetch callback in a C++ header file, because I need to update a member variable.
Can someone help me with how I define a callback function i C++, where I am able to update a local member variable?

In MyClass.CPP do I have the following method

void MyClass::Register()
{
    emscripten_fetch_attr_t attr;
    emscripten_fetch_attr_init(&attr);
    attr.onsuccess = DownloadSucceeded;
    attr.onerror = DownloadFailed;
}

I have defined DownloadSucceeded and DownloadFailed in my header file.

void DownloadSucceeded(emscripten_fetch_t* fetch)
{
    printf("Finished downloading %llu bytes from URL %s.\n", fetch->numBytes, fetch->url);
    emscripten_fetch_close(fetch); // Free data associated with the fetch.
    m_unlocked = true;
}

void DownloadFailed(emscripten_fetch_t* fetch)
{
    printf("Downloading %s failed, HTTP failure status code: %d.\n", fetch->url, fetch->status);
    emscripten_fetch_close(fetch); // Also free data on failure.
    m_unlocked = false;
}

I get the following exception:

reference to non-static member function must be called

From the above exception I tried to send a reference instead, but that didn't seem to work either. If some clever guy can explain why and how to solve this I would be grateful!

My naive reference example which does not work either:

attr.onsuccess = &DetectorBridge::DownloadSucceeded;

Gives:

assigning to 'void (*)(struct emscripten_fetch_t *)' from incompatible type 'void (mynamespace::MyClass::*)(emscripten_fetch_t *)'

答案1

得分: 1

Looking at the emscripten source, it appears that you need to perform something like this. You should save your MyClass pointer to a field designated by emscripten for this purpose.

void MyClass::Register()
{
    emscripten_fetch_attr_t attr;
    emscripten_fetch_attr_init(&attr);
    attr.onsuccess = DownloadSucceeded;
    attr.onerror = DownloadFailed;
    attr.userData = this; // save this object
}

Then, when you receive the callback, use the saved pointer to modify your variables.

void DownloadSucceeded(emscripten_fetch_t* fetch)
{
    printf("Finished downloading %llu bytes from URL %s.\n", 
        fetch->numBytes, fetch->url);
    static_cast<MyClass*>(fetch->userData)->m_unlocked = true; // get saved object
    emscripten_fetch_close(fetch); // Free data associated with the fetch.
}

I'm ignoring the issue of access to the m_unlocked member variable, but if that is a concern, there are typical solutions available.

英文:

Looking at the emscriptem source it seems you need to do something like this, where you save your MyClass pointer to a field that emscriptem has designed for this purpose.

void MyClass::Register()
{
    emscripten_fetch_attr_t attr;
    emscripten_fetch_attr_init(&amp;attr);
    attr.onsuccess = DownloadSucceeded;
    attr.onerror = DownloadFailed;
    attr.userData = this; // save this object
}

Then when you get the callback use that saved pointer to modify your variables

void DownloadSucceeded(emscripten_fetch_t* fetch)
{
    printf(&quot;Finished downloading %llu bytes from URL %s.\n&quot;, 
        fetch-&gt;numBytes, fetch-&gt;url);
    static_cast&lt;MyClass*&gt;(fetch-&gt;userData)-&gt;m_unlocked = true; // get saved object
    emscripten_fetch_close(fetch); // Free data associated with the fetch.
}

I'm ignoring the issue of access to the m_unlocked member variable, but if that is an issue there are the usual solutions.

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  • 本文由 发表于 2023年7月6日 15:34:21
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