feat: [structs] Add single malloc struct init

Author: manid2

cpp/structs/single-malloc.cpp

@@ -0,0 +1,74 @@
+/**
+ * Initialization of struct members with single malloc
+ * ===================================================
+ */
+
+#include "tests.h"
+
+/* ===========================================================================
+ * Algorithms implementation
+ * ===========================================================================
+ */
+typedef struct employee_ {
+	char *name;
+	int id;
+} ee_t;
+
+/*
+ * When a struct has pointers e.g. name in struct employee_ then it requires
+ * at least two malloc calls to allocate memory first for the struct and then
+ * for its pointer members. This can be a performance issue as malloc calls
+ * are expensive. To overcome that we allocate a single large block of memory
+ * with single malloc call and point struct pointer members to locations in
+ * that block.
+ *
+ * p0 is start of pointer to single large block of memory allocated using a
+ * single malloc. In this block struct pointer members can be pointed to at:
+ *
+ * +-------------------+------------------+
+ * | struct employee_  | remaining memory |
+ * +-------------+-----+------------------+
+ * | p0 (id, name_ptr) | p1 (name_val)    |
+ * +-------------------+------------------+
+ */
+ee_t *init_ee(int id, const char *name)
+{
+	size_t n0 = strlen(name), n1 = n0 + 1;
+	size_t sz = sizeof(ee_t), es = sz + n1;
+
+	char *p0 = (char *)malloc(es);
+	char *p1 = (char *)(p0 + sz);
+
+	memset(p1, 0, n1);
+	strncpy(p1, name, n0);
+
+	ee_t *ep = (ee_t *)p0;
+	ep->name = p1;
+	ep->id = id;
+	return ep;
+}
+
+/* ===========================================================================
+ * Test code
+ * ===========================================================================
+ */
+#define _sm_check(_id, _name)                                                \
+	{                                                                    \
+		ee_t *ep = init_ee(_id, _name);                              \
+		string em, am;                                               \
+		em = format("id: {}, name: {}", _id, _name);                 \
+		am = format("id: {}, name: {}", ep->id, ep->name);           \
+		CHECK_EQ(em, am);                                            \
+		SHOW_OUTPUT(em, am);                                         \
+		free(ep);                                                    \
+	}
+
+TEST(init_ee, "Initialization of struct members with single malloc")
+{
+	int _i[] = {2, 5};
+	const char *_n[] = {"abc", "new hello world, testing single malloc"};
+	int n = sizeof(_i) / sizeof(_i[0]);
+	fii (i, n) _sm_check(_i[i], _n[i]);
+}
+
+INIT_TEST_MAIN();