[Standard library]_[Primary]_[calculates the offset of struct members]

Source: Internet
Author: User


1. It is not necessary to calculate the offset of a struct in a C structure, but it is necessary to put it into the design of the plug-in structure, such as a scenario that can only be accessed using offsets, rather than a scene referencing member variables.

2. In the design of a consistent interface, the common interface does not change, but the structure of the plug-in module can not be designed according to the unified structure, they only need to provide offset to the public interface call on the line, different plug-ins

Possible offsets are inconsistent because they can be implemented independently. The public interface can access different variables through offsets.

3. You can use the Offsetof in the Stddef.h file

/* DEFINE OFFSETOF Macro */#ifdef __cplusplus#ifdef _win64#define offsetof (s,m) (size_t) (ptrdiff_t)   & Reinterpret_cast<const volatile Char&> (((((((s) 0)->m)) #else # define OFFSETOF (s,m)   (size_t) & Reinterpret_cast<const volatile Char&> ((((((s) 0)->m) #endif #else#ifdef  _win64#define offsetof (s,m )   (size_t) ((ptrdiff_t) & (((s) 0)->m)) #else # define OFFSETOF (s,m)   (size_t) & ((((s) 0)->m) # endif

FFmpeg part of the code:

Notice the offset here.

static const avoption options[]={{"B", "Set bitrate (in bits/s)", OFFSET (bit_rate), Ff_opt_type_int, Av_codec_default_ Bitrate, Int_min, Int_max, v| e},{"AB", "set bitrate (in bits/s)", OFFSET (bit_rate), Ff_opt_type_int, 64*1000, Int_min, Int_max, a| e},{"BT", "Set video bitrate tolerance (in bits/s)", OFFSET (bit_rate_tolerance), Ff_opt_type_int, Av_codec_default_ BITRATE*20, 1, Int_max, v| e},{"Flags", NULL, OFFSET (Flags), Ff_opt_type_flags, DEFAULT, 0, Uint_max, v| a| e| D, "Flags"},{"MV4", "use four motion vectors by Macroblock (MPEG4)", 0, Ff_opt_type_const, CODEC_FLAG_4MV, Int_min, Int_max , v| E, "Flags"},{"OBMC", "use overlapped block motion compensation (h263+)", 0, Ff_opt_type_const, CODEC_FLAG_OBMC, Int_min, I Nt_max, v| E, "Flags"},{"Qpel", "use section PEL Motion compensation", 0, Ff_opt_type_const, Codec_flag_qpel, Int_min, Int_max, v| E, "Flags"},{"loop", "Use Loop filter", 0, Ff_opt_type_const, Codec_flag_loop_filter, Int_min, Int_max, v| E, "Flags"},{"Qscale", "Use fixed Qscale", 0, Ff_opT_type_const, Codec_flag_qscale, Int_min, Int_max, 0, "Flags"},{"GMC", "use GMC", 0, Ff_opt_type_const, CODEC_FLAG_GMC, I Nt_min, Int_max, v| E, "Flags"},{"Mv0", "always try a megabyte with mv=<0,0>", 0, Ff_opt_type_const, Codec_flag_mv0, Int_min, Int_max, v| E, "Flags"},{"part", "Use data partitioning", 0, Ff_opt_type_const, Codec_flag_part, Int_min, Int_max, v| E, "Flags"},{"input_preserved", NULL, 0, Ff_opt_type_const, codec_flag_input_preserved, Int_min, Int_max, 0, "flags"},

To test the code, compile with vs2010:

void Testoffset () {//1. The compiler automatically aligns a multiple of its type size value to the structure body data type. struct T{char c;char b;int e;int e1;double D;};  cout << offsetof (t,b) << endl;cout << offsetof (t,e) << endl;cout << offsetof (t,e1) << Endl;cout << offsetof (t,d) << Endl;} int _tmain (int argc, _tchar* argv[]) {testoffset (); return 0;}



[Standard library]_[Primary]_[calculates the offset of struct members]

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