String optimization

[chungphb]

Various ways to optimize spiderdb::string

[chungphb]

Since sizeof(char) is always 1, should we use std::malloc(len) instead of std::malloc(sizeof(char_t) * len)?

Should we use static_cast<char*> or change it to reinterpret_cast<char*>?

[chungphb]

Should we just remove the basic_string(char_t* data, size_t len) and basic_string(seastar::temporary_buffer<char_t>&& buffer) constructors and allow constructing by basic_string(const char_t* data, size_t len) only?

[chungphb]

For copy assignment operator and move assignment operator, we should call destructor to destroy the old object before constructing a new one.

For move constructor and move assignment operator, the moved object's destructor should also be called. After that, we won't need to reset its members anymore.

When we move a String object, we won't allocate a new block but reuse the old one and transfer its ownership to the new object. If we called the moved object's destructor, this block will be deallocated and the new object's _data member will be a dangling pointer.

[chungphb]

String errors should be preceded by "string: " or "spiderdb::string:" to remove capitalism altogether (no pun intended).

Also, why do we want to allow constructing a string with length 0?

[chungphb]

We should replace basic_string(std::basic_string<char_t> str) by basic_string(const std::basic_string&<char_t> str) and basic_string(const std::basic_string<char_t>& str). There's no need to create a new string just to deleted it right after.

[chungphb]

For operator[](size_t id)-s, there's no need to compare id to 0 since it's a size_t parameter.

For operator+(const basic_string& str), instead of constructing a new string using the basic_string(size_t len, char_t c) constructor so we could set all the characters to 0 for no reason before reset it immediately, we should just simply allocate the memory in place.

For operator==(const basic_string<char_t>& str), we could use std::equal instead of writing our own for loop for readability.

For operator>(const basic_string<char_t>& str) and operator<(const basic_string<char_t>& str), we could write a compare function (similar to seastar::sstring) to reduce redundancy.

All the comparison operators (==, !=, >, <. >=, <=) should be noexcept.

[chungphb]

We should move get_input_stream() out of String class.

Also, there's no need to make std::hash<basic_string> and operator<<(std::ostream& os, basic_string str) friends of String class when we already provided enough functions for them to use.

We also might want to be able to create an std::basic_string_view object from a String object to take advantages of its lightweight nature.