如何创建一个可变的通用lambda？ [英] How to create a variadic generic lambda?

问题描述

由于C ++ 14，我们可以使用通用的lambdas：

  auto generic_lambda = []（auto param）{}; 
  

这基本上意味着其调用操作符是基于标记为auto的参数的模板。

问题是如何创建一个lambda，可以接受可变参数，类似于可变函数模板如何工作？如果这是不可能的最接近的事情，可以使用相同的方式是什么？
如何储存？是否可以在 std :: function ？

解决方案

/ h2>

如何创建可变参数lambda？

泛型lambda使用以下语法：

  auto variadic_generic_lambda = []（auto ... param）{}; 
  

基本上你只需要添加 ... auto （可能是ref限定的）和你的参数包名。

p>

  auto variadic_generic_lambda = []（auto&& ... param）{}; 
  

存储

您可以使用 auto 将一个lambda存储在一个可变的泛型lambda中

<变量，或者您可以将其存储在 std :: function ，但您只能使用给予 std :: function 的固定签名来调用它：

  auto variadic_generic_lambda = []（auto ... param）{}; 
 
 std :: function< void（int，int）> func = variadic_generic_lambda; 
 
 func（42，42）; //编译
 
 func（lol）; //不编译
  

---

关于可变常规lambda的集合？

由于每个lambda都有不同的类型，因此您不能将它们的直接类型存储在STL的通常的同类容器中。使用非通用lambdas的方法是将它们存储在相应的 std :: function 中，这将具有固定的签名调用，并且不会限制任何事情，因为您的lambda不是通用的，只能以这种方式调用：

  auto non_generic_lambda_1 = []（int，char）{ }; 
 auto non_generic_lambda_2 = []（int，char）{}; 
 
 std :: vector< std :: function< void（int，char）>> vec; 
 
 vec.push_back（non_generic_lambda_1）; 
 vec.push_back（non_generic_lambda_2）; 
 

如存储部分的第一部分所述，如果您可以限制

如果你不能，你将需要某种形式的异构容器，如：

p>

• std :: vector< boost :: variant>


• std :: vector< boost :: any>


• boost :: fusion :: >这个问题为异质容器的例子。
  
  
  

  还有什么？

  
  
  

  关于lambdas的一般信息和关于成员生成的细节参见：

  
  
  
  
  
  • http://en.cppreference.com/w/cpp/language/lambda
  
  
  • 通用lambda在C ++ 14中如何工作？
  
  
  
  

  Since C++14 we can use generic lambdas:

  auto generic_lambda = [] (auto param) {};
  

  This basically means that its call operator is templated based on the parameters marked as auto.

  The question is how to create a lambda that can accept a variadic number of parameters similarly to how a variadic function template would work ? If this is not possible what is the closest thing that could be used the same way ? How would you store it ? Is it possible in a std::function ?

  解决方案

  Syntax

  How do you create a variadic generic lambda ?

  You can create a variadic generic lambda with the following syntax:

  auto variadic_generic_lambda = [] (auto... param) {};
  

  Basically you just add ... between auto (possibly ref qualified) and your parameter pack name.

  So typically using universal references would give:

  auto variadic_generic_lambda = [] (auto&&... param) {};
  

  Storage

  How do you store a variadic generic lambda ?

  You can either use auto to store a lambda in a variable of its own type, or you can store it in a std::function but you will only be able to call it with the fixed signature you gave to that std::function :

  auto variadic_generic_lambda = [] (auto... param) {};
  
  std::function<void(int, int)> func = variadic_generic_lambda;
  
  func(42, 42); // Compiles
  
  func("lol"); // Doesn't compile
  

  ---

  What about collections of variadic generic lambdas ?

  Since every lambda has a different type you cannot store their direct type in the usual homogeneous containers of the STL. The way it is done with non generic lambdas is to store them in a corresponding std::function which will have a fixed signature call and that won't restrain anything since your lambda is not generic in the first place and can only be invoked that way:

  auto non_generic_lambda_1 = [] (int, char) {};
  auto non_generic_lambda_2 = [] (int, char) {};
  
  std::vector<std::function<void(int, char)>> vec;
  
  vec.push_back(non_generic_lambda_1);
  vec.push_back(non_generic_lambda_2);
  

  As explained in the first part of this storage section if you can restrain yourself to a given fixed call signature then you can do the same with variadic generic lambdas.

  If you can't you will need some form of heterogenous container like:

  • std::vector<boost::variant>
  • std::vector<boost::any>
  • boost::fusion::vector

  See this question for an example of heterogenous container.

  What else ?

  For more general informations on lambdas and for details on the members generated see:

  • http://en.cppreference.com/w/cpp/language/lambda
  • How does generic lambda work in C++14?

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