SoA._growthP

Growth factor P/Q. https://github.com/facebook/folly/blob/master/folly/docs/FBVector.md#memory-handling

Use 1.5 like Facebook's fbvector does.

  1. enum _growthP;
    struct SoA(S, Capacity = size_t, Allocator = Mallocator)
    enum _growthP = 3;
  2. enum _growthQ;

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