new operator - C++ new memory allocation fragmentation -

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i trying @ behavior of new allocator , why doesn't place data contiguously.

my code:

struct ci {     char c;     int i; }  template <typename t> void memtest() {     t * plast = new t();     for(int = 0; < 20; ++i) {          t * pnew = new t();          cout << (pnew - plast) << " ";          plast = pnew;     } }

so ran char, int, ci. allocations fixed length last, there odd jumps 1 available block another.

sizeof(char) : 1
average jump: 64 bytes

sizeof(int): 4
average jump: 16

sizeof(ci): 8 (int has placed on 4 byte align)
average jump: 9

can explain why allocator fragmenting memory this? why jump char larger ints , structure contains both int , char.

there 2 issues:

  • most allocators store additional data prior start of block (typically block size , couple of pointers)

  • there alignment requirements, e.g. linux typically allocates @ least 8 byte boundary, os x allocates @ least 16 byte boundary

so you'll kind of gap between successive allocations.

of course should never rely on specific behaviour this, implementation free pleases.


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