cpp: ex2: add solution

c/ex2.c

@@ -0,0 +1,30 @@
+#include <stdio.h>
+#include <string.h>
+
+char* f(int m) {
+    char buf[6];
+    int x;
+
+    if (m == 1 && x-- /* variable is read before initialization */) {
+        strcpy(buf, "AAAAAA"); // null-byte copy overflows buffer at index 7
+        return buf;            // Return stack allocated array
+    } else if (m == 2) {
+        char* msg = (char*)malloc(100); // malloc is declared in <stdlib.h>
+        // While technically not UB, as it will be implicitly declared by the
+        // compiler, this is error-prone and the implicit declaration most
+        // likely does not match the actual function
+        strcpy(msg, "BBBBBB");
+        return msg;
+    }
+    // Missing return value
+}
+
+int main(int argc, char** argv) {
+    char* m;
+    m = f(argc);
+    putchar(m[0]); // if m != 2, either reads garbage stack memory or
+                   // non-sensical pointer due to missing return value
+    return 0;
+    // Potential memory leak, if m == 2
+    // While not UB, it is frowned upon
+}

c/ex3.c

@@ -0,0 +1,33 @@
+#include <limits.h>
+#include <stddef.h>
+
+static unsigned char reverse_byte(unsigned char c) {
+    unsigned char res = 0;
+    for (size_t i = 0; i < CHAR_BIT; ++i) {
+        res <<= 1;
+        res |= c & 1;
+        c >>= 1;
+    }
+    return res;
+}
+
+static void swap_bytes(unsigned char* lhs, unsigned char* rhs) {
+    unsigned char tmp = *lhs;
+    *lhs = *rhs;
+    *rhs = tmp;
+}
+
+void reverse_bytes(unsigned char* buf, size_t n) {
+    if (!buf || !n)
+        return;
+
+    // No need to worry about applying reverse_byte twice in the middle by
+    // mistake if done in a preliminary pass
+    for (size_t i = 0; i < n; ++i) {
+        buf[i] = reverse_byte(buf[i]);
+    }
+
+    for (size_t i = 0; i < ((n + 1) / 2); ++i) {
+        swap_bytes(&buf[i], &buf[n - 1 - i]);
+    }
+}

cpp/ex1.cc

@@ -0,0 +1,28 @@
+#ifdef USE_ATOMIC_N
+#include <atomic>
+#endif
+
+class CountInstances {
+public:
+    CountInstances() {
+        ++n_instances_;
+    }
+
+    ~CountInstances() {
+        --n_instances_;
+    }
+
+    int nobjs() const {
+        return n_instances_; // If using atomics, load could use a more relaxed
+                             // memory order, such as memory_order_acquire
+    }
+
+private:
+#ifdef USE_ATOMIC_N
+    // Can be used in a multi-thread program
+    static atomic<int> n_instances_;
+#else
+    // Using a bare int mean races if used in a multi-threaded context
+    static int n_instances_;
+#endif
+};

cpp/ex2.cc

@@ -0,0 +1 @@
+../c/ex1.c