列表中double和float对象的大小是否相等？

Question

我想知道浮动和双重对象的大小是否从std :: list的角度来看是相等的？

我在std :: list中分配了500万个Real(别名float或double)对象,并使用Valgrind监视内存使用情况.

在这两种情况下,使用的内存是相等的,尽管'double'(8字节)的大小是一个'float'对象(4字节)的大小的两倍!

顺便说一句,当我使用'new'运算符为相同数量的对象分配内存时,double数组的内存使用量是float数组的两倍,这似乎是正确的.我也期待使用std :: list.

我在Fedora 16.x86_64上使用gcc 4.6.2.

有任何想法,以帮助我了解这个谜.

这是我为测试编写的代码

#include <iostream>
#include <list>

typedef double Real;

int main(int argc, char** argv)
{
    std::list<Real> pts;
    int k;

    int npts = 5000000; // 5 mil

    std::cout << "sizeof(Real): " << sizeof(Real) << std::endl;
    for(k=0; k < npts;++k)
        pts.push_back(1.0);

    return 0;

}

如果我定义Real < - 双重Valgrind输出

==15335== Memcheck, a memory error detector
==15335== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==15335== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==15335== Command: /home/soheil/Workspace/tbin/test_memory_usage
==15335== 
sizeof(Real): 8
==15335== 
==15335== HEAP SUMMARY:
==15335==     in use at exit: 616 bytes in 6 blocks
==15335==   total heap usage: 5,000,053 allocs, 5,000,047 frees, 120,015,245 bytes allocated
==15335== 
==15335== LEAK SUMMARY:
==15335==    definitely lost: 0 bytes in 0 blocks
==15335==    indirectly lost: 0 bytes in 0 blocks
==15335==      possibly lost: 0 bytes in 0 blocks
==15335==    still reachable: 616 bytes in 6 blocks
==15335==         suppressed: 0 bytes in 0 blocks
==15335== Rerun with --leak-check=full to see details of leaked memory
==15335== 
==15335== For counts of detected and suppressed errors, rerun with: -v
==15335== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

如果我定义Real < - float,则Valgrind输出为

==15252== Memcheck, a memory error detector
==15252== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==15252== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==15252== Command: /home/soheil/Workspace/tbin/test_memory_usage
==15252== 
sizeof(Real): 4
==15252== 
==15252== HEAP SUMMARY:
==15252==     in use at exit: 616 bytes in 6 blocks
==15252==   total heap usage: 5,000,053 allocs, 5,000,047 frees, 120,015,245 bytes allocated
==15252== 
==15252== LEAK SUMMARY:
==15252==    definitely lost: 0 bytes in 0 blocks
==15252==    indirectly lost: 0 bytes in 0 blocks
==15252==      possibly lost: 0 bytes in 0 blocks
==15252==    still reachable: 616 bytes in 6 blocks
==15252==         suppressed: 0 bytes in 0 blocks
==15252== Rerun with --leak-check=full to see details of leaked memory
==15252== 
==15252== For counts of detected and suppressed errors, rerun with: -v
==15252== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

Answer 1

a中的每个元素std::list<T>都是一个链表节点,因此它是一个包含两个指针的结构,以及类型的有效负载数据T.例如,对于GCC 4.1.2,它如下:

  struct _List_node_base
  {
    _List_node_base* _M_next;
    _List_node_base* _M_prev;

    // *** Non-virtual member functions ***
  };

  template<typename _Tp>
    struct _List_node : public _List_node_base
    {
      _Tp _M_data;
    };

分配的大小将是该结构的大小; 如果T足够小,那么你可能会看到由struct padding主导的数字.

因此,对于GCC定义,这是两个64位指针(如此16个字节),加上4或8个字节T,最多填充8个字节,因此总共24个字节,与您测量的内容相匹配.

为了测试理论,尝试Real改为float[2]或double[2].