为什么在C ++ 20中对lambda使用`std :: bind_front`？ [英] Why use `std::bind_front` over lambdas in C++20?

问题描述

如措辞类似的问题中所述（为什么在c语言中对lambda使用绑定++ 14？）答案是-没有理由（并且还提到了为什么最好使用lambdas）。

As mentioned in a similarly worded question (Why use bind over lambdas in c++14?) The answer was - no reason (and also mentioned why it would be better to use lambdas).

我的问题是-如果在C ++ 14中不再有使用绑定的理由，为什么标准委员会认为有必要添加 std :: bind_front 在C ++ 20中？

My question is - if in C++14 there was no longer a reason to use bind, why did the standards committee found it necessary to add std::bind_front in C++20?

与lambda相比，它现在具有任何新的优势吗？

Does it now have any new advantage over a lambda?

推荐答案

bind_front 绑定前X个参数，但如果可调用要获取更多参数，它们将附加在最后。当您仅绑定函数的前几个参数时，这使得 bind_front 非常可读。

bind_front binds the first X parameters, but if the callable calls for more parameters, they get tacked onto the end. This makes bind_front very readable when you're only binding the first few parameters of a function.

显而易见的示例是创建一个可用于绑定到特定实例的成员函数：

The obvious example would be creating a callable for a member function that is bound to a specific instance:

type *instance = ...;

//lambda
auto func = [instance](auto &&... args) -> decltype(auto) {return instance->function(std::forward<decltype(args)>(args)...);}

//bind
auto func = std::bind_front(&type::function, instance);

bind_front 版本是 lot 少吵。它指向正确的位置，恰好具有3个命名的事物： bind_front ，要调用的成员函数以及将在其上调用的实例。这就是我们情况所需要的：一个标记，表示我们正在创建一个函数的第一个参数，要绑定的函数以及要绑定的参数的绑定。没有多余的语法或其他细节。

The bind_front version is a lot less noisy. It gets right to the point, having exactly 3 named things: bind_front, the member function to be called, and the instance on which it will be called. And that's all that our situation calls for: a marker to denote that we're creating a binding of the first parameters of a function, the function to be bound, and the parameter we want to bind. There is no extraneous syntax or other details.

相比之下，lambda有很多我们不在乎的东西。 auto ... args 位， std :: forward 之类的东西，以此类推。这很难弄清楚

By contrast, the lambda has a lot of stuff we just don't care about at this location. The auto... args bit, the std::forward stuff, etc. It's a bit harder to figure out what it's doing, and it's definitely much longer to read.

请注意， bind_front 不允许 bind 的占位符，因此它并不是真正的替代。对于 bind 的最有用形式而言，它更是简写形式。

Note that bind_front doesn't allow bind's placeholders at all, so it's not really a replacement. It's more a shorthand for the most useful forms of bind.

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