POD对象向量的内存布局

Question

假设我有一个简单的 C++ class,

class Data {

    public: 
         float data[3];         

         void clear() { data[0] = 0.0f; data[1] = 0.0f; data[2] = 0.0f }

}

还有一个数据向量，

std::vector<Data> v(10);

假设 &v[0].data[0] 指向 30 个浮点数的数组是否安全？

Answer 1

来自标准

23.3.6.1 Class template vector overview

The elements of a vector are stored contiguously, meaning that if v is a vector where T is some type other than bool, then it obeys the identity &v[n] == &v[0] + n for all 0 <= n < v.size()

所以&v[0]确实指向了10个连续Data对象的开始。

但是对于 Data 的布局我们有

9.2.13 Class members

Nonstatic data members of a (non-union) class with the same access control (Clause 11) are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members with different access control is unspecified (11). Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions (10.3) and virtual base classes (10.1).

所以我们不能确定sizeof(Data) == 3*sizeof(float)，所以一般的答案应该是：假设30个连续的浮点数是保存不了的。

POD对象向量的内存布局

Memory layout of vector of POD objects

c++

memory

vector