我应该在C ++中使用std :: function还是函数指针？

Question

在C ++中实现回调函数时，我仍应使用C样式函数指针：

void (*callbackFunc)(int);

或者我应该使用std :: function：

std::function< void(int) > callbackFunc;

Answer 1

简而言之，std::function除非有理由不要使用。函数指针的缺点是无法捕获某些上下文。例如，您将无法通过lambda函数作为捕获某些上下文变量的回调（但是如果不捕获任何上下文变量，它将起作用）。因此也不可能调用对象的成员变量（即非静态的），因为this需要捕获对象（-pointer）。（1）std::function（因为C ++ 11）主要用于存储函数（将其传递就不需要存储它）。因此，如果要将回调存储在例如成员变量中，则可能是最佳选择。但是，如果您不存储它，那么它是一个不错的“首选”，尽管它的缺点是在调用时会引入一些（非常小的）开销（因此，在性能非常关键的情况下，这可能是个问题，但在大多数情况下它不应该）。这是非常“通用的”：如果您非常关心一致且易读的代码，又不想考虑所做的每一个选择（即想保持简单），请使用std::function传递的每个函数。考虑第三个选项：如果您要实现一个小的函数，然后通过提供的回调函数报告某些内容，请考虑一个template参数，该参数可以是任何可调用的对象，即函数指针，函子，lambda，一个std::function缺点是这里的（外部）函数成为模板，因此需要在标头中实现。另一方面，您得到的好处是可以内联对回调的调用，因为（外部）函数的客户端代码“看到”了对回调的调用，将提供确切的类型信息。具有模板参数的版本的示例（对于C ++ 11之前的版本，请写&而不是&&）：template <typename CallbackFunction>void myFunction(..., CallbackFunction && callback) {&nbsp; &nbsp; ...&nbsp; &nbsp; callback(...);&nbsp; &nbsp; ...}正如您在下表中看到的，它们都有优点和缺点：+-------------------+--------------+---------------+----------------+|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;| function ptr | std::function | template param |+===================+==============+===============+================+| can capture&nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; no(1)&nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; || context variables |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| no call overhead&nbsp; |&nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;no&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; || (see comments)&nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| can be inlined&nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; no&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;no&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; || (see comments)&nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| can be stored&nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; no(2)&nbsp; &nbsp; &nbsp;|| in class member&nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| can be implemented|&nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp;no&nbsp; &nbsp; &nbsp; &nbsp;|| outside of header |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| supported without |&nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp;no(3)&nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp;yes&nbsp; &nbsp; &nbsp; || C++11 standard&nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+| nicely readable&nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; no&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; yes&nbsp; &nbsp; &nbsp; |&nbsp; &nbsp; &nbsp; (yes)&nbsp; &nbsp; &nbsp;|| (my opinion)&nbsp; &nbsp; &nbsp; | (ugly type)&nbsp; |&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;|&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; |+-------------------+--------------+---------------+----------------+（1）存在克服此限制的解决方法，例如，将其他数据作为进一步的参数传递给您的（外部）函数：myFunction(..., callback, data)will call callback(data)。这就是C样式的“带参数的回调”，这在C ++中是可行的（并且在WIN32 API中大量使用），但是应避免使用，因为我们在C ++中有更好的选择。（2）除非我们在谈论类模板，否则存储函数的类就是模板。但这意味着在客户端，函数的类型决定了存储回调的对象的类型，这在实际用例中几乎从来不是一个选择。（3）对于C ++ 11之前的版本，请使用 boost::function

Answer 2

使用std::function存储任意调用对象。它允许用户提供回调所需的任何上下文。一个普通的函数指针没有。如果由于某种原因（也许是因为您想要C兼容的API）而确实需要使用普通函数指针，则应添加一个void * user_context参数，以便至少有可能（尽管不方便）访问不直接传递给参数的状态。功能。