c++ - 可以使用成员函数来初始化初始化列表中的成员变量吗？

Question

好的，成员变量can be used初始化初始化列表中的其他成员变量(注意初始化顺序等)。成员函数呢？具体来说，根据 C++ 标准，此代码段是否合法？

struct foo{
  foo(const size_t N) : N_(N),  arr_(fill_arr(N)) { 
    //arr_ = fill_arr(N); // or should I fall back to this one?
  }

  std::vector<double> fill_arr(const size_t N){
    std::vector<double> arr(N);
    // fill in the vector somehow
    return arr;
  }

  size_t N_;
  std::vector<double> arr_;
  // other stuff
};

Answer 1

是的，您在初始化列表中使用的成员函数是有效的并且符合标准。

数据成员按其声明的顺序进行初始化(这就是为什么它们应按其声明的顺序出现在初始化列表中的原因 - 您在示例中遵循的规则)。 N_ 首先初始化，您可以将此数据成员传递给 fill_arr。 fill_arr 在构造函数之前调用，但是因为这个函数不访问未初始化的数据成员(它根本不访问数据成员)，所以它的调用被认为是安全的。

以下是 C++ 标准最新草案 (N3242=11-0012) 中的一些相关异常(exception):

§ 12.6.2.13: Member functions (including virtual member functions, 10.3) can be called for an object under construction.(...) However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined. Example:

class A { public:    A(int); };

class B : public A {
   int j;
public:
   int f();
   B() : A(f()), // undefined: calls member function
                 // but base A not yet initialized
   j(f()) { }    // well-defined: bases are all initialized
};

class C {
public:
   C(int);
};

class D : public B, C {
   int i;
public:
   D() : C(f()), // undefined: calls member function
                 // but base C not yet initialized
   i(f()) { } // well-defined: bases are all initialized
};

§12.7.1: For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior. Example

struct W { int j; };
struct X : public virtual W { };
struct Y {
   int *p;
   X x;
   Y() : p(&x.j) { // undefined, x is not yet constructed
   }
};