feat: finish string helper except Split()

- add string lower upper function in string helper.
- add join function in string helper.
- add split function in string helper but implementation is not perfect and should be imporved in future.

src/StringHelper.hpp

@@ -1,6 +1,8 @@
 #pragma once
 #include <string>
 #include <cstdarg>
+#include <functional>
+#include <vector>
 
 namespace YYCC::StringHelper {
 	
@@ -10,6 +12,53 @@ namespace YYCC::StringHelper {
 	std::string Printf(const char* format, ...);
 	std::string VPrintf(const char* format, va_list argptr);
 
-	//std::string Join(const char* decilmer);
+	/**
+	 * @brief The data provider of general Join function.
+	 * This function pointer return non-null string pointer to represent a element of joined series.
+	 * otherwise return nullptr to terminate the joining process.
+	*/
+	using JoinDataProvider = std::function<const char*()>;
+	/**
+	 * @brief General Join function.
+	 * @details This function use function pointer as a general data provider interface,
+	 * so this function suit for all types container, the user only need write a little bit adaption code.
+	 * @param fct_data[in] The function pointer to data provider.
+	 * @param decilmer[in] The decilmer.
+	 * @return A std::string instance which containing the join result.
+	*/
+	std::string Join(JoinDataProvider fct_data, const char* decilmer);
+	/**
+	 * @brief Specialized Join function for common used container.
+	 * @param data 
+	 * @param decilmer 
+	 * @param reversed 
+	 * @return 
+	*/
+	std::string Join(const std::vector<std::string>& data, const char* decilmer, bool reversed = false);
 
+	/**
+	 * @brief Transform string to lower.
+	 * @param strl 
+	 * @return 
+	*/
+	std::string Lower(const char* strl);
+	void Lower(std::string& strl);
+	/**
+	 * @brief Transform string to upper.
+	 * @param strl 
+	 * @return 
+	*/
+	std::string Upper(const char* strl);
+	void Upper(std::string& strl);
+
+	/**
+	 * @brief General Split function.
+	 * @param _strl 
+	 * @param _decilmer 
+	 * @return 
+	 * @remarks This function may be low performance because it just a homebrew Split functon.
+	 * It can works in most toy cases but not suit for high performance scenario.
+	 * Also, this function will produce a copy of original string because it is not zero copy.
+	*/
+	std::vector<std::string> Split(const char* _strl, const char* _decilmer);
 }