C ++ CRTP初始化 [英] C++ CRTP initialization

问题描述

我遇到了运行以下程序的段错误

  #include< iostream> 
 #include< vector> 
 
模板< typename Derived> 
结构CRTPBase {
 CRTPBase（）{
 func（）; 
} 
 void func（）{
 static_cast< Derived *>（this）-> _func（）; 
} 
}; 
结构CRTPChild：CRTPBase< CRTPChild> {
使用CRTPBase< CRTPChild> :: CRTPBase; 
 void _func（）{
 vec.resize（10）; 
 vec [0] = 2; 
} 
 std :: vector< int> vec; 
}; 
 int main（）
 {
 CRTPChild obj; 
 std :: cout<< obj.vec [0]<< std :: endl; 
} 
  

当我替换 vec 成员类型为 int 的成员，则不再进行段错误。为什么？

解决方案

您的代码具有未定义的行为。问题来自初始化顺序；对于派生类 CRTPChild ，首先调用基类 CRTPBase< CRTPChild> 的构造函数。 CRTPChild 的成员 vec 被初始化。从基类的构造函数调用 _func 时，根本不会初始化 vec 。

2）然后，直接基类按从左到右的顺序初始化为
，它们出现在此类的基本说明符列表中

3）然后，以类定义中的
声明的顺序初始化非静态数据成员。

将类型更改为 int 仍为 UB 。 UB意味着一切皆有可能，它可能导致段错误或可能不会。

i ran into a segfault running the following program

#include <iostream>
#include <vector>

template <typename Derived>
struct CRTPBase {
  CRTPBase() {
    func();
  }
  void func() {
    static_cast<Derived*>(this)->_func();
  }
};
struct CRTPChild : CRTPBase<CRTPChild>{
  using CRTPBase<CRTPChild>::CRTPBase;
  void _func(){
    vec.resize(10);
    vec[0] = 2;
  }
  std::vector<int> vec;
};
int main()
{
    CRTPChild obj;
    std::cout << obj.vec[0] << std::endl;
}

When i replace vec with a member of type int it doesn't segfault anymore. Why?

解决方案

Your code has undefined behavior. The problem comes from the order of the initialization; for the derived class CRTPChild, the constructor of the base class CRTPBase<CRTPChild> is invoked firstly, after that the data member vec of CRTPChild get initialized. When _func is invoked (from the constructor of the base class) vec is not initialized at all.

2) Then, direct base classes are initialized in left-to-right order as they appear in this class's base-specifier list

3) Then, non-static data members are initialized in order of declaration in the class definition.

Changing the type to int it's still UB. UB means anything is possible, it might lead to segfault or might not.

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