[common] adding HeapString class (heap allocated string) (#6732)

This commit adds a new class `HeapString` as heap allocated string.
The buffer to store the string is allocated from heap and is manged
by the `HeapString` class itself, e.g., it may be reused and/or freed
and reallocated when the string is set. The `HeapString` destructor
will always free the allocated buffer.

This commit also adds a unit test `test_heap_string` for the new
class.

tests/unit/test_heap_string.cpp

@@ -0,0 +1,156 @@
+/*
+ *  Copyright (c) 2021, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "test_platform.h"
+
+#include <string.h>
+
+#include <openthread/config.h>
+
+#include "test_util.hpp"
+#include "common/code_utils.hpp"
+#include "common/heap_string.hpp"
+
+namespace ot {
+
+void PrintString(const char *aName, const HeapString &aString)
+{
+    if (aString.IsNull())
+    {
+        printf("%s = (null)\n", aName);
+    }
+    else
+    {
+        printf("%s = [%zu] \"%s\"\n", aName, strlen(aString.AsCString()), aString.AsCString());
+    }
+}
+
+void VerifyString(const char *aName, const HeapString &aString, const char *aExpectedString)
+{
+    PrintString(aName, aString);
+
+    if (aExpectedString == nullptr)
+    {
+        VerifyOrQuit(aString.IsNull(), "IsNull() is incorrect");
+        VerifyOrQuit(aString.AsCString() == nullptr, "AsCString() is incorrect");
+        VerifyOrQuit(aString != "something", "operator!=() failed");
+    }
+    else
+    {
+        VerifyOrQuit(!aString.IsNull(), "IsNull() is incorrect");
+        VerifyOrQuit(aString.AsCString() != nullptr, "AsCString() is incorrect");
+        VerifyOrQuit(strcmp(aString.AsCString(), aExpectedString) == 0, "String content is incorrect");
+        VerifyOrQuit(aString != nullptr, "operator!=() failed");
+    }
+
+    VerifyOrQuit(aString == aExpectedString, "operator==() failed");
+}
+
+// Function returning a `HeapString` by value.
+HeapString GetName(void)
+{
+    HeapString name;
+
+    SuccessOrQuit(name.Set("name"), "Set() failed");
+
+    return name;
+}
+
+void TestHeapString(void)
+{
+    HeapString  str1;
+    HeapString  str2;
+    const char *oldBuffer;
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("After constructor\n\n");
+    VerifyString("str1", str1, nullptr);
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("Set(const char *aCstring)\n\n");
+    SuccessOrQuit(str1.Set("hello"), "Set() failed");
+    VerifyString("str1", str1, "hello");
+    oldBuffer = str1.AsCString();
+
+    SuccessOrQuit(str1.Set("0123456789"), "Set() failed");
+    VerifyString("str1", str1, "0123456789");
+    printf("\tDid reuse its old buffer: %s\n", str1.AsCString() == oldBuffer ? "yes" : "no");
+    oldBuffer = str1.AsCString();
+
+    SuccessOrQuit(str1.Set("9876543210"), "Set() failed");
+    VerifyString("str1", str1, "9876543210");
+    printf("\tDid reuse its old buffer (same length): %s\n", str1.AsCString() == oldBuffer ? "yes" : "no");
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("Set(const HeapString &)\n\n");
+    SuccessOrQuit(str2.Set(str1), "Set() failed");
+    VerifyString("str2", str2, str1.AsCString());
+
+    SuccessOrQuit(str1.Set(nullptr), "Set() failed");
+    VerifyString("str1", str1, nullptr);
+
+    SuccessOrQuit(str2.Set(str1), "Set() failed");
+    VerifyString("str2", str2, nullptr);
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("Free()\n\n");
+    str1.Free();
+    VerifyString("str1", str1, nullptr);
+
+    SuccessOrQuit(str1.Set("hello again"), "Set() failed");
+    VerifyString("str1", str1, "hello again");
+
+    str1.Free();
+    VerifyString("str1", str1, nullptr);
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("Set() move semantics\n\n");
+    SuccessOrQuit(str1.Set("old name"), "Set() failed");
+    PrintString("str1", str1);
+    SuccessOrQuit(str1.Set(GetName()), "Set() with move semantics failed");
+    VerifyString("str1", str1, "name");
+
+    printf("------------------------------------------------------------------------------------\n");
+    printf("operator==() with two null string\n\n");
+    str1.Free();
+    str2.Free();
+    VerifyString("str1", str1, nullptr);
+    VerifyString("str2", str2, nullptr);
+    VerifyOrQuit(str1 == str2, "operator==() failed with two null strings");
+
+    printf("\n -- PASS\n");
+}
+
+} // namespace ot
+
+int main(void)
+{
+    ot::TestHeapString();
+    printf("\nAll tests passed.\n");
+    return 0;
+}