c++ - 对象表示和运算符sizeof的定义

Question

3.9/4给出了对象表示概念的定义:

[基本类型]

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).

但是sizeof操作符的定义如下:

[表达式大小]

The sizeof operator yields the number of bytes in the object representation of its operand.

特别是:

When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array

我想通过示例考虑该定义:

struct A {
    int a;  //sizeof(a) is implementation defined
    long b; //sizeof(b) is implementation defined
    char c; //sizeof(c) is 1 by definition 
    // Padding, problbly equals to 3 butes
}

sizeof(A) 是如何通过实现计算的？我们只计算 sizeof(a) + sizeof(b) + sizeof(c) + padding 的总和。

但是将那种类型的对象放入数组中所需的条件是关于什么的？

Answer 1

您可以简单地计算结构的大小:

long size = sizeof(struct A);  

如果你想要一个struct A的数组，

struct A arr[/* Size of array */];