Question

我在http://www.cplusplus.com/reference/vector/vector/data/中找到了一个std :: vector的例子。

// vector::data
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector (5);

  int* p = myvector.data();

  *p = 10;
  ++p;
  *p = 20;
  p[2] = 100;

  std::cout << "myvector contains:";
  for (unsigned i=0; i<myvector.size(); ++i)
    std::cout << ' ' << myvector[i];
  std::cout << '\n';

  return 0;
}

，结果是

myvector contains: 10 20 0 100 0

我的问题可能很愚蠢，但我真的不明白这里发生了什么。我们直接指向了向量的内存。然后我们为第一个元素（索引0）赋值10，移动到第二个元素并为其赋值20（索引1）。最后，我们为第三个元素（索引2）赋值100。答案应该如下吗？

10 20 100 0 0

Answer 1

这张图片可能有助于解释

int* p = myvector.data();
[   ] [   ] [   ] [   ] [   ]
  ^

*p = 10;
[10 ] [   ] [   ] [   ] [   ]
  ^

++p;
[10 ] [   ] [   ] [   ] [   ]
        ^

*p = 20;
[10 ] [20 ] [   ] [   ] [   ]
        ^

p[2] = 100; // [2] is relative to where p is
[10 ] [20 ] [   ] [100] [   ]
        ^

Answer 2

示例的输出是正确的。让我在下面演示 -

std::vector<int> myvector (5); // creating vector with size 5, elements are 0,0,0,0,0

  int* p = myvector.data(); //taking reference of the vector, p now points the first element of vector

  *p = 10; // it means first element is now 10
  ++p; // p now points second element of the vector
  *p = 20; // 2nd element is now 20
  p[2] = 100; //p[2] is the 4th element now because of two position shifting and it is now 100
  // vectors elements are now 10, 20, 0, 100, 0

Answer 3

std::vector<int> myvector (5); ==> [0,0,0,0,0]

int* p = myvector.data(); // p points to element 0

*p = 10;                  // element 0 is now 10
++p;                      // shift position of p to the next element
*p = 20;                  // element 1 is now 20
p[2] = 100;               // Using p as reference, go 2 positions to right and assign the value of 100 to the element found at that position.

输出正确[10,20,0,100,0]

C ++ std :: vector :: data为什么返回的指针索引与矢量索引不匹配？

3 个答案: