传递派生列表以作为成员存储(Pass a list of derived types in order to store them as a member)

我有一个类widget 。我有一个抽象类库，派生的是derived_a ， derived_b等。

我希望widget可以保存从base派生的任意数量的对象，以便稍后以多态方式使用它们。

我的第一次尝试看起来像这样：

#include <vector> #include <ostream> #include <iostream> #include <memory> class widget { public: explicit widget(std::vector<std::unique_ptr<base>>&& params) : members {std::move (params)} { } private: std::vector<std::unique_ptr<base>> members; };

会被称为这样：

std::vector<std::unique_ptr<base>> v; v.push_back(std::move(std::make_unique<derived_a>())); widget w (std::move(v));

但是，这个解决方案难以接受冗长而且用户友好，特别是在提供多种类型时：

std::vector<std::unique_ptr<base>> v; v.push_back(std::move(std::make_unique<derived_a>())); v.push_back(std::move(std::make_unique<derived_b>())); v.push_back(std::move(std::make_unique<derived_c>())); v.push_back(std::move(std::make_unique<derived_a>())); v.push_back(std::move(std::make_unique<derived_b>())); v.push_back(std::move(std::make_unique<derived_c>())); widget w {std::move(v)};

相反，我更喜欢使用的方式

widget w {derived_a(), derived_b(), derived_c(), derived_a(), derived_b(), derived_c()};

这样widget就会提供一个rvalues列表，然后它可以变成std::vector<unique_ptr<base>> 。我的印象是，这可以通过模仿ctor来实现，但是，尽管谷歌搜索量很大，但我并不知道如何准确地实现我的目标。

请注意，类模板解决方案看起来像这样：

widget<derived_a, derived_b, derived_c, derived_a, derived_b, derived_c> w;

是不可取的，因为我需要提供一些参数派生。

I have a class widget. I have an abstract class base with derivates derived_a, derived_b, etc.

I want widget to hold an arbitrary amount of objects that are derivated from base in order to later use them polymorphically.

My first attempt looks like this:

#include <vector> #include <ostream> #include <iostream> #include <memory> class widget { public: explicit widget(std::vector<std::unique_ptr<base>>&& params) : members {std::move (params)} { } private: std::vector<std::unique_ptr<base>> members; };

And would be called like this:

std::vector<std::unique_ptr<base>> v; v.push_back(std::move(std::make_unique<derived_a>())); widget w (std::move(v));

However, this solution seams unnessesarry verbose and not user friendly at all, especially when providing multiple types:

std::vector<std::unique_ptr<base>> v; v.push_back(std::move(std::make_unique<derived_a>())); v.push_back(std::move(std::make_unique<derived_b>())); v.push_back(std::move(std::make_unique<derived_c>())); v.push_back(std::move(std::make_unique<derived_a>())); v.push_back(std::move(std::make_unique<derived_b>())); v.push_back(std::move(std::make_unique<derived_c>())); widget w {std::move(v)};

Instead, I would prefer usage along the lines of

widget w {derived_a(), derived_b(), derived_c(), derived_a(), derived_b(), derived_c()};

so that widget is provided with a list of rvalues that it then can turn into std::vector<unique_ptr<base>>. I have the impression that this can be achieved through templating the ctor, but, despite intensive googling, I have no clue on how to achieve my goal exactly.

Please note that the class template solution that would look tike this:

widget<derived_a, derived_b, derived_c, derived_a, derived_b, derived_c> w;

Is undesirable, as I need to provide some deriveds with parameters.

最满意答案

不幸的是，你不能使用initializer_list ，但你可以使用variadic模板：

class widget { public: template <typename ... Ts> explicit widget(Ts&&... params) { int dummy[] = {0, (members.emplace_back(std::make_unique<Ts>(std::forward<Ts>(params))), 0)...}; static_cast<void>(dummy); // avoid unused variable warning } private: std::vector<std::unique_ptr<base>> members; };

演示

或者在C ++ 17中，使用折叠表达式：

template <typename ... Ts> explicit widget(Ts&&... params) { (members.emplace_back(std::make_unique<Ts>(std::forward<Ts>(params))), ...); }

Unfortunately, you cannot use initializer_list, but you can use variadic template:

class widget { public: template <typename ... Ts> explicit widget(Ts&&... params) { int dummy[] = {0, (members.emplace_back(std::make_unique<Ts>(std::forward<Ts>(params))), 0)...}; static_cast<void>(dummy); // avoid unused variable warning } private: std::vector<std::unique_ptr<base>> members; };

Demo

or in C++17, with folding expression:

template <typename ... Ts> explicit widget(Ts&&... params) { (members.emplace_back(std::make_unique<Ts>(std::forward<Ts>(params))), ...); }

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