回调在C++中

huangapple
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英文:

Callback in C++

问题

  1. 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。
  2. 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？
  4. 在我的MyClass.CPP中，我有以下方法：
  5. ```cpp
  6. void MyClass::Register()
  7. {
  8.     emscripten_fetch_attr_t attr;
  9.     emscripten_fetch_attr_init(&attr);
  10.     attr.onsuccess = DownloadSucceeded;
  11.     attr.onerror = DownloadFailed;
  12. }

我在头文件中定义了DownloadSucceeded和DownloadFailed：

  1. void DownloadSucceeded(emscripten_fetch_t* fetch)
  2. {
  3.     printf("从URL %s 下载完成 %llu 字节。\n", fetch->url, fetch->numBytes);
  4.     emscripten_fetch_close(fetch); // 释放与fetch关联的数据。
  5.     m_unlocked = true;
  6. }
  8. void DownloadFailed(emscripten_fetch_t* fetch)
  9. {
  10.     printf("从URL %s 下载失败，HTTP失败状态码：%d。\n", fetch->url, fetch->status);
  11.     emscripten_fetch_close(fetch); // 失败时也释放数据。
  12.     m_unlocked = false;
  13. }

我得到了以下异常：

  1. 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：

  1. attr.onsuccess = &DetectorBridge::DownloadSucceeded;

会得到：

  1. 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

  1. void MyClass::Register()
  2. {
  3.     emscripten_fetch_attr_t attr;
  4.     emscripten_fetch_attr_init(&attr);
  5.     attr.onsuccess = DownloadSucceeded;
  6.     attr.onerror = DownloadFailed;
  7. }

I have defined DownloadSucceeded and DownloadFailed in my header file.

  1. void DownloadSucceeded(emscripten_fetch_t* fetch)
  2. {
  3.     printf("Finished downloading %llu bytes from URL %s.\n", fetch->numBytes, fetch->url);
  4.     emscripten_fetch_close(fetch); // Free data associated with the fetch.
  5.     m_unlocked = true;
  6. }
  8. void DownloadFailed(emscripten_fetch_t* fetch)
  9. {
  10.     printf("Downloading %s failed, HTTP failure status code: %d.\n", fetch->url, fetch->status);
  11.     emscripten_fetch_close(fetch); // Also free data on failure.
  12.     m_unlocked = false;
  13. }

I get the following exception:

  1. 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:

  1. attr.onsuccess = &DetectorBridge::DownloadSucceeded;

Gives:

  1. 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.

  1. void MyClass::Register()
  2. {
  3.     emscripten_fetch_attr_t attr;
  4.     emscripten_fetch_attr_init(&attr);
  5.     attr.onsuccess = DownloadSucceeded;
  6.     attr.onerror = DownloadFailed;
  7.     attr.userData = this; // save this object
  8. }

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

  1. void DownloadSucceeded(emscripten_fetch_t* fetch)
  2. {
  3.     printf("Finished downloading %llu bytes from URL %s.\n", 
  4.         fetch->numBytes, fetch->url);
  5.     static_cast<MyClass*>(fetch->userData)->m_unlocked = true; // get saved object
  6.     emscripten_fetch_close(fetch); // Free data associated with the fetch.
  7. }

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.

  1. void MyClass::Register()
  2. {
  3.     emscripten_fetch_attr_t attr;
  4.     emscripten_fetch_attr_init(&amp;attr);
  5.     attr.onsuccess = DownloadSucceeded;
  6.     attr.onerror = DownloadFailed;
  7.     attr.userData = this; // save this object
  8. }

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

  1. void DownloadSucceeded(emscripten_fetch_t* fetch)
  2. {
  3.     printf(&quot;Finished downloading %llu bytes from URL %s.\n&quot;, 
  4.         fetch-&gt;numBytes, fetch-&gt;url);
  5.     static_cast&lt;MyClass*&gt;(fetch-&gt;userData)-&gt;m_unlocked = true; // get saved object
  6.     emscripten_fetch_close(fetch); // Free data associated with the fetch.
  7. }

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

huangapple
  • 本文由 发表于 2023年7月6日 15:34:21
  • 转载请务必保留本文链接：https://go.coder-hub.com/76626488.html
  • c++
  • callback
  • emscripten
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