C ++ static_cast运行时开销 [英] C++ static_cast runtime overhead

问题描述

请参阅下面的代码。

a）在这种情况下（简单继承，没有虚拟成员），B :: df任何开销（任何）？我发现了类似问题的一些冲突的答案，这就是为什么我问...

b）我正在考虑使 const M1 * func private在A中，并引入一个新的私有字段 const M2 * func 到B，以避免强制转换，但它使事情复杂化，智能指针更难。

  
 class M1 {
 public ：
 double f（）const; 
}; 
 
 
 
 class M2：public M1 {
 public：
 double df（）const; 
}; 
 
 
 
 class A {
 protected：
 const M1 * func; 
 public：
 A（const M1 * p）; 
〜A（）; 
 double f（）const; 
}; 
 
 
 
 class B：public A {
 public：
 B（const M2 * p）; 
 double df（）const; 
}; 
 

  
 M1 :: f（）const {return 1973.0; } 
 double M2 :: df（）const {return 0.0; } 
 
 
 
 A ::〜A（）{delete func; } 
 A :: A（const M1 * p）：func（p）{} 
 double A :: f（）const {return func-> f } 
 
 
 
 B :: B（const M2 * p）：A（p）{} 
 double B :: df（）const {return static_cast< const M2 *>（func）→df（）; } 
 
  
解决方案
  static_cast< T>（e）等价于以下列方式创建发明的临时变量v：
  T v（e）; //其中T是任意类型，e是任意表达式。 
 
 static_cast的运行时成本正是上述语句的成本
 
See the code below.

a) Does, in this case (simple inheritance, no virtual members), the static cast in B::df() have any overhead (whatsoever)? I found some conflicting answers to similar questions, that's why I am asking...

b) I was thinking about making const M1 * func private in A and introducing a new private field const M2 * func into B to avoid the cast, but it kind of complicates things up and makes use of smart pointers more difficult. Do you see a better way to avoid the cast?


class M1 {
public:
    double f() const;
};


class M2 : public M1 {
public:
    double df() const;
};

class A {
protected:
    const M1 * func;
public:
    A(const M1 * p);
    ~A();
    double f() const;
};

class B : public A {
public:
    B(const M2 * p);
    double df() const;
};



double M1::f() const { return 1973.0; }
double M2::df() const { return 0.0; }


A::~A() { delete func; }
A::A(const M1 * p) : func(p) {}
double A::f() const { return func->f(); }

B::B(const M2 * p) : A(p) {}
double B::df() const { return static_cast<const M2*>(func)->df(); }
解决方案
static_cast<T>(e) is equivalent to creating an invented temporary variable v in the following way:
T v(e); //where T is an arbitrary type  and e is an arbitrary expression.
The runtime cost of a static_cast is exactly the cost of the above statement

                        
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