c++11 - 使用 std::unique_ptr 和 lambdas 推进对象的状态

Question

在推进对象的状态时，使用std::swap对简单对象和指针交换非常有效。对于其他就地操作，Boost.ScopeExit效果相当好，但如果您想跨函数共享退出处理程序，它并不是非常优雅。是否有 C++11 本机方式来完成类似Boost.ScopeExit但允许更好的代码重用的事情？

Answer 1

(Ab) 使用std::unique_ptr的自定义删除器作为ScopeExitVisitor或后置条件。向下滚动到第 7 行，main()看看它在呼叫站点的实际使用情况。以下示例允许使用不需要任何参数的/std::function或 lambdas，如果确实需要将参数传递给/ ，则允许使用嵌套类。DeleterScopeExitVisitorDeleterScopeExitVisitor

#include <iostream>
#include <memory>

class A {
 public:
  using Type = A;
  using Ptr = Type*;
  using ScopeExitVisitorFunc = std::function<void(Ptr)>;
  using ScopeExitVisitor = std::unique_ptr<Type, ScopeExitVisitorFunc>;

  // Deleters that can change A's private members. Note: Even though these
  // are used as std::unique_ptr<> Deleters, these Deleters don't delete
  // since they are merely visitors and the unique_ptr calling this Deleter
  // doesn't actually own the object (hence the label ScopeExitVisitor).
  static void ScopeExitVisitorVar1(Ptr aPtr) {
    std::cout << "Mutating " << aPtr << ".var1. Before: " << aPtr->var1;
    ++aPtr->var1;
    std::cout << ", after: " << aPtr->var1 << "\n";
  }

  // ScopeExitVisitor accessing var2_, a private member.
  static void ScopeExitVisitorVar2(Ptr aPtr) {
    std::cout << "Mutating " << aPtr << ".var2. Before: " << aPtr->var2_;
    ++aPtr->var2_;
    std::cout << ", after: " << aPtr->var2_ << "\n";
  }

  int var1 = 10;
  int var2() const { return var2_; }

  // Forward declare a class used as a closure to forward Deleter parameters
  class ScopeExitVisitorParamVar2;

 private:
  int var2_ = 20;
};

// Define ScopeExitVisitor closure. Note: closures nested inside of class A
// still have access to private variables contained inside of A.
class A::ScopeExitVisitorParamVar2 {
 public:
  ScopeExitVisitorParamVar2(int incr) : incr_{incr} {}
  void operator()(Ptr aPtr) {
    std::cout << "Mutating " << aPtr << ".var2 by " << incr_ << ". Before: " << aPtr->var2_;
    aPtr->var2_ += incr_;
    std::cout << ", after: " << aPtr->var2_ << "\n";
  }

 private:
  int incr_ = 0;
};

// Can also use lambdas, but in this case, you can't access private
// variables.
//
static auto changeStateVar1Handler = [](A::Ptr aPtr) {
  std::cout << "Mutating " << aPtr << ".var1 " << aPtr->var1 << " before\n";
  aPtr->var1 += 2;
};

int main() {
  A a;

  std::cout << "a: " << &a << "\n";

  std::cout << "a.var1: " << a.var1 << "\n";
  std::cout << "a.var2: " << a.var2() << "\n";

  { // Limit scope of the unique_ptr handlers. The stack is unwound in
    // reverse order (i.e. Deleter var2 is executed before var1's Deleter).
    A::ScopeExitVisitor scopeExitVisitorVar1(nullptr, A::ScopeExitVisitorVar1);
    A::ScopeExitVisitor scopeExitVisitorVar1Lambda(&a, changeStateVar1Handler);
    A::ScopeExitVisitor scopeExitVisitorVar2(&a, A::ScopeExitVisitorVar2);
    A::ScopeExitVisitor scopeExitVisitorVar2Param(nullptr, A::ScopeExitVisitorParamVar2(5));

    // Based on the control of a function and required set of ScopeExitVisitors that
    // need to fire use release() or reset() to control which visitors are used.
    // Imagine unwinding a failed but complex API call.
    scopeExitVisitorVar1.reset(&a);
    scopeExitVisitorVar2.release(); // Initialized in ctor. Use release() before reset().
    scopeExitVisitorVar2.reset(&a);
    scopeExitVisitorVar2Param.reset(&a);

    std::cout << "a.var1: " << a.var1 << "\n";
    std::cout << "a.var2: " << a.var2() << "\n";
    std::cout << "a.var2: " << a.var2() << "\n";
  }

  std::cout << "a.var1: " << a.var1 << "\n";
  std::cout << "a.var2: " << a.var2() << "\n";
}

产生：

a: 0x7fff5ebfc280
a.var1: 10
a.var2: 20
a.var1: 10
a.var2: 20
a.var2: 20
Mutating 0x7fff5ebfc280.var2 by 5. Before: 20, after: 25
Mutating 0x7fff5ebfc280.var2. Before: 25, after: 26
Mutating 0x7fff5ebfc280.var1 10 before
Mutating 0x7fff5ebfc280.var1. Before: 12, after: 13
a.var1: 13
a.var2: 26

从好的方面来说，这个技巧很好，因为：

• Deleters 中使用的代码可以访问私有变量
• 删除代码可以集中化
• 仍然可以使用 lambdas，尽管它们只能访问公共成员。
• 参数可以通过充当闭包的嵌套类传递给 Deleter
• 并非所有std::unique_ptr实例都需要分配一个对象（例如，将不需要的 Deleters 设置为 是完全可以接受的nullptr）
• 在运行时改变行为只是调用reset()或release()
• 根据您构建堆栈的方式，当std::unique_ptr(s) 的范围超出范围时，可以在编译时更改对象的安全保证

最后，使用Boost.ScopeExit您可以将调用转发到辅助函数或使用类似于Boost.ScopeExit文档建议的条件bool commit = ...;。类似于：

#include <iostream>
#include <boost/scope_exit.hpp>

int main() {
  bool commitVar1 = false;
  bool commitVar2 = false;
  BOOST_SCOPE_EXIT_ALL(&) {
    if (commitVar1)
      std::cout << "Committing var1\n"
    if (commitVar2)
      std::cout << "Committing var2\n"
  };
  commitVar1 = true;
}

这并没有错，但是就像在原始问题中提出的那样，您如何在不代理其他地方的调用的情况下共享代码？将std::unique_ptr的删除器用作ScopeExitVisitors.