c++ - C++ CRTP初始化

Question

我遇到了运行以下程序的段错误

#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;
}

当我用 vec类型的成员替换 int时，它不再存在段错误。为什么？

Answer 1

您的代码具有未定义的行为。问题来自order of the initialization;对于派生类CRTPChild，首先调用基类CRTPBase<CRTPChild>的构造函数，然后初始化vec的数据成员CRTPChild。调用_func时(从基类的构造函数中)，vec根本没有初始化。

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.

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