read/write index with std::function wrapper (#427)

* add access function to IndexIVF;

* - access for IndexIVF;
- write_index/read_index with std::function<...>;

* - fix test compile on mac;
- adjust write/read with std::function;

* replace std::function with IOReader/IOWriter;

* remove IndexIVF::access // tmp

* PFN_WRITE/READ => WRITE;

* revert mac compile fix;

* rename;

* fix compile;

* reset CMakeList;

* format; remove unused function/header;

IndexIVF.cpp

@@ -590,7 +590,6 @@ void IndexIVF::copy_subset_to (IndexIVF & other, int subset_type,
 }
 
 
-
 IndexIVF::~IndexIVF()
 {
     if (own_invlists) {

index_io.h

@@ -21,17 +21,21 @@ struct Index;
 struct VectorTransform;
 struct IndexIVF;
 struct ProductQuantizer;
+struct IOReader;
+struct IOWriter;
 
 void write_index (const Index *idx, FILE *f);
 void write_index (const Index *idx, const char *fname);
 
+void write_index (const Index *idx, IOWriter *writer);
+
 
 const int IO_FLAG_MMAP = 1;
 const int IO_FLAG_READ_ONLY = 2;
 
 Index *read_index (FILE * f, int io_flags = 0);
 Index *read_index (const char *fname, int io_flags = 0);
-
+Index *read_index (IOReader *reader, int io_flags = 0);
 
 
 void write_VectorTransform (const VectorTransform *vt, const char *fname);
@@ -55,6 +59,21 @@ struct Cloner {
     virtual ~Cloner() {}
 };
 
+struct IOReader {
+    // fread
+    virtual size_t operator()(
+        void *ptr, size_t size, size_t nitems) = 0;
+    virtual ~IOReader() {}
+};
+
+struct IOWriter {
+    // fwrite
+    virtual size_t operator()(
+        const void *ptr, size_t size, size_t nitems) = 0;   
+
+    virtual ~IOWriter() {}
+};
+
 }
 
 #endif