Is there any kind of “expression class” (C++)

Question

I am creating a game that lets the player enter input, changes some states, then checks if a \"goal value\" is true (obviously this description is muchly simplified), and I

Answer 1

I'm thinking you can define your own class and work around using the 'assert' keyword, but I may have understood the question wrong.

http://www.cplusplus.com/reference/clibrary/cassert/assert/

Answer 2

There's no standard way to do it in C++. One solution is to write your own parser.

Another solution, that I'd recommend: embed a Lua interpreter in your program. Lua is a simple yet powerful programming language, that also has an extremely lightweight (<300kB) and easy-to-use interpreter. Read an introductory article here: http://www.ibm.com/developerworks/linux/library/l-embed-lua/index.html

Having Lua embedded in your game has a number of nice side advantages:

• you can use it as a powerful configuration language for your game
• with Lua, you can easily create a command-line interactive environment, which is nice for testing and experiements. For example, you will be able to change the game engine parameters and see the effect immediately, without recompiling. That's especially convenient for "researchy" projects or game programming.

Answer 3

There is no standard way to compile expressions during runtime. You'll have to do it some other way.

You might consider using a scripting language, like Lua or Python, and embed it in your C++. That would allow your players the ability to program to the extent you want them to.

Answer 4

C++ doesn't have this as part of the language -- there is no way to, at runtime, get access to the same stuff that parsed your program.

I'm sure there are numerous third-party arithmetic parser libraries you could use, however.

Answer 5

Why not build your own expression classes?

class GoalBase
{
    virtual bool goal() = 0;
};

class Enemies : public GoalBase 
{
   // ..
   private:
      int enemies_;

   public:
      Enemies(int start) : enemies_(start) {}
      void kill() { if (enemies_) --enemies_; }
      bool goal() { return enemies_ == 0; }
};

int main()
{
    Enemies enemiesToKill(5);
    enemiesToKill.kill();    

    // ..
    if (enemiesToKill.goal()) {
        // ..
    }

    return 0;
}

Other classes could have other methods, parameters, operators etc. Use your imagination.

Answer 6

Static expressions

(amendment to Macke's post)

When your expressions are known at compile time, you can use std::function. However, the performance might not be optimal.

You can automatically register test at compile-time and execute them at run-time with (presumambly) minimal runtime overhead, using C++11 templates and macros. A proof-of-concept implementation can be found here.

In the long run, a language feature named "Contracts" could do the job. (N4415, N4435, N4378) Today, there are various libraries available to support contract programming.