问题描述
对于每个参数，我需要应用两个嵌套函数:
For each argument I need apply two nested function:
obj.apply(someFilter(arg)); // arg is one argument, but here
// should be an unpacking of args
我不知道如何为这种情况编写解包.
I don't know how to write unpacking for such case.
我看到了:
pass{([&]{ std::cout << args << std::endl; }(), 1)...};
在维基上，但又不知道如何将其应用于我的案例.
on wiki, but again don't know how to apply this for my case.
推荐答案
其实很简单:您可以将任意表达式放入可变参数模板参数包的解包中:
It's actually quite simple:You can put arbitrary expression inside the unpack of an variadic templates argument pack:
obj.apply(someFilter(arg))...
这会给你 obj.apply 的结果作为一个昏迷分隔列表.然后你可以将它传递给一个虚拟函数:
This will give you the result of obj.apply as a coma seperated list. You can then pass it to a dummy function:
template<typename... Args> swallow (Args&&...) {}
swallow(obj.apply(someFilter(arg))...);
吞下逗号分隔的列表.
当然，这假设 obj.apply 返回某种对象.如果没有，您可以使用
Of course, this assumes that obj.apply returns some kind of object. If not you can use
swallow((obj.apply(someFilter(arg)), 0)...);
制作实际(非void)参数
如果您不知道 obj.apply` 返回什么(结果可能使逗号运算符重载)，您可以使用以下方法禁用自定义逗号运算符
If you don't know what obj.apply` returns (result might have overloaded the comma operator), you can disable the use of custom comma operators by using
swallow((obj.apply(someFilter(arg)), void(), 0)...);
如果您真的需要按顺序评估项目(从问题中看起来不太可能)，您可以滥用数组初始化语法而不是使用函数调用:
Should you actually need to evaluate the items in order (which doesn't seem very likely from the question), you can abuse array initialization syntax instead of using a function call:
using Alias=char[];
Alias{ (apply(someFilter(args)), void(), '\0')... };
这篇关于可变参数模板参数解包的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！