在C ++中使用数组或std ::载体，有什么性能上的差距？ [英] Using arrays or std::vectors in C++, what's the performance gap?

问题描述

在我们的C ++当然，他们建议不要使用C ++新项目阵列了。据我所知Stroustroup本人建议不要使用数组。但是否有显著的性能差异？

In our C++ course they suggest not to use C++ arrays on new projects anymore. As far as I know Stroustroup himself suggests not to use arrays. But are there significant performance differences?

推荐答案

使用C ++阵列，新（即使用动态数组）应尽量避免。还有就是你要跟踪的大小问题，你需要手动删除它们，并尽一切种类的看家。

Using C++ arrays with new (that is, using dynamical arrays) should be avoided. There is the problem you have to keep track of the size, and you need to delete them manually, and do all sort of housekeeping.

在堆栈中使用数组也沮丧，因为你没有范围检查，并围绕传递数组将失去它的大小（数组指针转换）的任何信息。你应该在这种情况下，它包装在一个小类C ++阵列，并提供了使用的boost ::阵列 尺寸功能和迭代器遍历它。

Using arrays on the stack is also discouraged because you don't have range checking, and passing the array around will lose any information about its size (array to pointer conversion). You should use boost::array in that case, which wraps a C++ array in a small class and provides a size function and iterators to iterate over it.

现在在的std ::向量与本地C ++数组（从互联网上获取）：

Now the std::vector vs. native C++ arrays (taken from the internet):

// Comparison of assembly code generated for basic indexing, dereferencing, 
// and increment operations on vectors and arrays/pointers.

// Assembly code was generated by gcc 4.1.0 invoked with  g++ -O3 -S  on a 
// x86_64-suse-linux machine.

#include <vector>

struct S
{
  int padding;

  std::vector<int> v;
  int * p;
  std::vector<int>::iterator i;
};

int pointer_index (S & s) { return s.p[3]; }
  // movq    32(%rdi), %rax
  // movl    12(%rax), %eax
  // ret

int vector_index (S & s) { return s.v[3]; }
  // movq    8(%rdi), %rax
  // movl    12(%rax), %eax
  // ret

// Conclusion: Indexing a vector is the same damn thing as indexing a pointer.

int pointer_deref (S & s) { return *s.p; }
  // movq    32(%rdi), %rax
  // movl    (%rax), %eax
  // ret

int iterator_deref (S & s) { return *s.i; }
  // movq    40(%rdi), %rax
  // movl    (%rax), %eax
  // ret

// Conclusion: Dereferencing a vector iterator is the same damn thing 
// as dereferencing a pointer.

void pointer_increment (S & s) { ++s.p; }
  // addq    $4, 32(%rdi)
  // ret

void iterator_increment (S & s) { ++s.i; }
  // addq    $4, 40(%rdi)
  // ret

// Conclusion: Incrementing a vector iterator is the same damn thing as 
// incrementing a pointer.

这篇关于在C ++中使用数组或std ::载体，有什么性能上的差距？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！