And as long as there is no special value assigned to the first enumerator of an enumeration, its value is zero too (please refer to the C++ Standard for more on this). In relation to our sample above this means, any time we go thru the switch and the value of szInput is new to s_mapStringValues, it is inserted (this is why I added the print out of the map’s size) and the value of the new element is set to the initial value of an integral type, which is 0 by default. The most important sentence of its description is: ‘If this element (key the value ) does not exist, it is inserted.’. ![]() Why this? std::map::operator can be used for two things: To set a value of a key (as you can see in Initialize()) and to retrieve the value associated with it (look at the switch statement in the main() function). In looking at the enum definition, you see that the first value is evNotDefined (ev means ‘enum value’). Map to associate the strings with the enum valuesĬout << "Please enter a string (end to terminate): " Ĭout << "Detected the first valid string." << endl Ĭout << "Detected the second valid string." << endl Ĭout << "Detected the third valid string." << endl Value-Defintions of the different String values Switch on String in C++ ExampleĪnd here comes a sample Switch On String implementation I will use for later discussion: #include There is no need to declare them as members of any class since normally the connection between a string value and the action to be taken is very specific to the respective code. cpp file, which helps to avoid global namespace pollution (for more on this see John Lakos, Large Scale C++ Software Design, Addison-Wesley). Both, the enum and the std::map, should be declared as static to make them visible only to the code of this particular. Using enum and std::map in C++ to Switch Over StringsĪfter thinking about the problem a little bit, there is only one place where to put the enum definition and the std::map definition: The. The string is the key of the map, the enumerator the value. The std::map contains the link between the valid string values you want to compare some runtime data against, and the numeric enum values you can make a switchon. The enumeration defines the numeric values use in the switch statement. You need an enumeration and a std::map, and that’s it. ![]() The Standard Template Library (STL), part of the ANSI/ISO C++ Standard, offers everything needed to get really close to the C# sample. I can’t give you a solution to do exactly what you can do in C#. In this article I will show you a way to implement a switch on strings using pure standard C++. Too many! And those of you who had the chance to take a look at C# might think ‘Why can’t I code something similar in C++ like this: switch(strValue) How many times have you written code like this: if (!strcmp(pszValue, "Value X")) So you do not need any special environment. Integrated Development Environment: This is standard C++. If you are struggling with learning C++, consider visiting the TechRepublic Academy! Be weary of hash collision when using this technique, as collisions will cause compile-time warnings and errors. This method has its flaws, but it serves a functional purpose in terms of tasks such as parsing input from the user. Using constexpr you could create a hash function and then hash the strings, which would, in turn, make them integral types. ![]() If the compiler s able to evaluate it as a constant expression, then the compiler will take that function and and replace it with the constant value. However, using constexpr it is possible to cause your functions to be evaluated at compile time. To be fair, it is still technically the case, in that the C++ standard states that you can only switch over integral types. There was a time – not so long ago – when you could not switch on or over string literals in C++. In fact, this was the case case prior to the release of C++11. Using enum and std::map in C++ to Switch Over Strings.This is why the code in case 6 was executed. The value of month is 6 so it matches with case 6. The value of month, which is acting as the expression for the switch statement, will be compared with every case value in the code. Here's what the syntax looks like: switch(expression). You use the switch statement in Java to execute a particular code block when a certain condition is met.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |