Enum hack for C + +

Transferred from: http://www.cnblogs.com/jiqingwu/p/cpp_enum_hack.html

Let's start with an example:

class Game {private:    staticconstintten;     int Scores[gameturn];};

For C + + compilers that support in-class initialization, this code can be compiled through.

However, older C + + compilers may not support in-class initialization, so that our static constants must be initialized outside of the class. As follows:

class Game {private:    staticconstint  gameturn;     int Scores[gameturn];}; Const int ten;

If not int scores[GameTurn]; , this code can be passed with a compiler that does not support in-class initialization.

But int scores[GameTurn]; GameTurn GameTurn The value cannot be determined because it is used. Therefore, the following error is reported.

enum_hack.cpp:5: error: array bound is not an integer constant

In this case, if we still do not want to specify the size of the array with hard-coded numbers, you can consider the main character of this article: Yes enum hack .

enum hackthe idea of using the technique is to GameTurn define a constant as an enumeration. The above code can be written as:

class Game {private:    //  static const int Gameturn;    enum Ten };     int Scores[gameturn];}; // const int game::gameturn = ten;

This will allow the code to be compiled and passed.

The benefits described in effective C + + enum hack :

1. enum hackThe behavior is more like #define , rather than const , if you don't want others to get pointers or references to your constant members, you can use enum hack them instead. (Why not use #define it directly?) First, because #define it is a string substitution, it is not conducive to program debugging. Second, #define the visual range is difficult to control, such as how do you make #define the defined constants visible only within a class? Unless you're using an ugly #undef .

2. Use does enum hack not result in "unnecessary memory allocation".

3. enum hackis a basic technique of template meta-programming, and a lot of code is using it. When you see it, you need to know it.

I do not know whether my understanding is wrong, welcome to discuss.

C + + enum hack (ext.)