c++ - 带有 C++ 模板的构建器模式

Question

我有一个高度可配置的类，其中包含许多模板参数，如下所示:

template<bool OptionA = false, bool OptionB = false, bool OptionC = false, class T = Class1B>
class MyClass
{
}

现在，如果我想创建类类型并且我只想将 OptionB 设置为 true，我必须执行以下操作:

MyClass<false, true>

特别是对于许多模板参数，这会变得很麻烦。

不，我的问题是，是否有任何示例可用于使用构建器模式创建基于模板的类类型？

我正在寻找这样的东西:

class Builder
{
    useOptionA();
    useOptionB();
    useOptionC();
    useClass2B(); //instead of Class1B
    create();
}

最后调用 Builder.useOptionB().useOptionC().useClass2B.create()应该返回 MyClass<false, true, true, Class2B> .这可能吗？

编辑:将类添加到模板参数列表。

Answer 1

正如其他人所说，最简单的方法是使用 enum 而不是 Builder。但是，如果您确实想要一个 Builder，您可以尝试这样的事情:

template<bool OptionA = false,
         bool OptionB = false,
         bool OptionC = false,
         typename T = Class1B>
struct Builder_t
{
    Builder_t() = default;
//    ~Builder_t() { std::cout << "Builder dtor." << std::endl; }

    auto useOptionA() -> Builder_t<true, OptionB, OptionC, T>          { return {}; }
    auto useOptionB() -> Builder_t<OptionA, true, OptionC, T>          { return {}; }
    auto useOptionC() -> Builder_t<OptionA, OptionB, true, T>          { return {}; }
    auto useClass2B() -> Builder_t<OptionA, OptionB, OptionC, Class2B> { return {}; }

    MyClass<OptionA, OptionB, OptionC, T> create() { return {}; }
};
using Builder = Builder_t<>;

// ...

// Build MyClass<true, false, false, Class2B>:
auto ma2 = Builder{}.useOptionA().useClass2B().create();

这会导致每个函数返回一个不同的Builder，其模板将被下一个函数使用；最终模板用作 MyClass 的模板。每个函数都会修改其指定的模板参数，从而允许生成器模式的编译时版本。不过，它确实有成本，如果取消注释用户定义的析构函数，这一点就会变得很明显。

---

考虑这个简单的测试程序:

#include <iostream>
#include <typeinfo>

class Class1B {};
class Class2B {};

template<bool OptionA = false,
         bool OptionB = false,
         bool OptionC = false,
         typename T = Class1B>
class MyClass
{
  public:
    MyClass() {
        std::cout << "MyClass<"
                  << OptionA << ", "
                  << OptionB << ", "
                  << OptionC << ", "
                  << "type " << typeid(T).name() << ">"
                  << std::endl;
    }
};

template<bool OptionA = false,
         bool OptionB = false,
         bool OptionC = false,
         typename T = Class1B>
struct Builder_t
{
    Builder_t() = default;
//    ~Builder_t() { std::cout << "Builder dtor." << std::endl; }

    auto useOptionA() -> Builder_t<true, OptionB, OptionC, T>          { return {}; }
    auto useOptionB() -> Builder_t<OptionA, true, OptionC, T>          { return {}; }
    auto useOptionC() -> Builder_t<OptionA, OptionB, true, T>          { return {}; }
    auto useClass2B() -> Builder_t<OptionA, OptionB, OptionC, Class2B> { return {}; }

    MyClass<OptionA, OptionB, OptionC, T> create() { return {}; }
};
using Builder = Builder_t<>;

int main()
{
    std::cout << std::boolalpha;

    std::cout << "Default:\n";
    std::cout << "Direct:  ";
      MyClass<> m;
    std::cout << "Builder: ";
      auto mdefault = Builder{}.create();
    std::cout << std::endl;

    std::cout << "Builder pattern:\n";
    std::cout << "A: ";
      auto ma = Builder{}.useOptionA().create();
    std::cout << "C: ";
      auto mc = Builder{}.useOptionC().create();
    std::cout << "---\n";

    std::cout << "AB: ";
      auto mab = Builder{}.useOptionA().useOptionB().create();
    std::cout << "B2: ";
      auto mb2 = Builder{}.useOptionB().useClass2B().create();
    std::cout << "---\n";

    std::cout << "ABC: ";
      auto mabc = Builder{}.useOptionA().useOptionB().useOptionC().create();
    std::cout << "AC2: ";
      auto mac2 = Builder{}.useOptionA().useOptionC().useClass2B().create();
    std::cout << "---\n";

    std::cout << "ABC2: ";
      auto mabc2 = Builder{}.useOptionA().useOptionB().useOptionC().useClass2B().create();
}

正常情况下，输出如下(使用GCC):

Default:
Direct:  MyClass<false, false, false, type 7Class1B>
Builder: MyClass<false, false, false, type 7Class1B>

Builder pattern:
A: MyClass<true, false, false, type 7Class1B>
C: MyClass<false, false, true, type 7Class1B>
---
AB: MyClass<true, true, false, type 7Class1B>
B2: MyClass<false, true, false, type 7Class2B>
---
ABC: MyClass<true, true, true, type 7Class1B>
AC2: MyClass<true, false, true, type 7Class2B>
---
ABC2: MyClass<true, true, true, type 7Class2B>

但是，如果我们取消注释析构函数...

Default:
Direct:  MyClass<false, false, false, type 7Class1B>
Builder: MyClass<false, false, false, type 7Class1B>
Builder dtor.

Builder pattern:
A: MyClass<true, false, false, type 7Class1B>
Builder dtor.
Builder dtor.
C: MyClass<false, false, true, type 7Class1B>
Builder dtor.
Builder dtor.
---
AB: MyClass<true, true, false, type 7Class1B>
Builder dtor.
Builder dtor.
Builder dtor.
B2: MyClass<false, true, false, type 7Class2B>
Builder dtor.
Builder dtor.
Builder dtor.
---
ABC: MyClass<true, true, true, type 7Class1B>
Builder dtor.
Builder dtor.
Builder dtor.
Builder dtor.
AC2: MyClass<true, false, true, type 7Class2B>
Builder dtor.
Builder dtor.
Builder dtor.
Builder dtor.
---
ABC2: MyClass<true, true, true, type 7Class2B>
Builder dtor.
Builder dtor.
Builder dtor.
Builder dtor.
Builder dtor.

Builder_t::create() 之前的每次调用都会创建一个不同的 Builder_t，所有这些都会在实例创建后销毁。这可以通过将 Builder_t 设为 constexpr 类来缓解，但如果有大量参数需要处理，这可能会减慢编译速度:

template<bool OptionA = false,
         bool OptionB = false,
         bool OptionC = false,
         typename T = Class1B>
struct Builder_t
{
// Uncomment if you want to guarantee that your compiler treats Builder_t as constexpr.
//    size_t CompTimeTest;

    constexpr Builder_t()
// Uncomment if you want to guarantee that your compiler treats Builder_t as constexpr.
//      : CompTimeTest((OptionA ? 1 : 0) +
//                     (OptionB ? 2 : 0) +
//                     (OptionC ? 4 : 0) +
//                     (std::is_same<T, Class2B>{} ? 8 : 0))
    {}

    constexpr auto useOptionA() -> Builder_t<true, OptionB, OptionC, T>          { return {}; }
    constexpr auto useOptionB() -> Builder_t<OptionA, true, OptionC, T>          { return {}; }
    constexpr auto useOptionC() -> Builder_t<OptionA, OptionB, true, T>          { return {}; }
    constexpr auto useClass2B() -> Builder_t<OptionA, OptionB, OptionC, Class2B> { return {}; }

    constexpr MyClass<OptionA, OptionB, OptionC, T> create() { return {}; }
};
using Builder = Builder_t<>;

// ....

// Uncomment if you want to guarantee that your compiler treats Builder_t as constexpr.
// char arr[Builder{}/*.useOptionA()/*.useOptionB()/*.useOptionC()/*.useClass2B()/**/.CompTimeTest];
// std::cout << sizeof(arr) << '\n';