是否有任何种类的“表达式类” （C ++） [英] Is there any kind of "expression class" (C++)
问题描述
我创建一个游戏，让玩家输入输入，改变一些状态，然后检查一个目标值是否为真（明显这个描述被大大简化），我想能够有这个目标值如果玩家生命低于某个值，如果剩余的敌人的数量等于零，任何东西。是否有任何表达式类可以容纳一个简单的value1 compare-operator value2并检查？即：
表达式目标（x = 4）;
如果没有，是否有任何建议如何开发一个表达式类？
编辑：另一个（更倾向于我想要实现的）示例：
game.init（）{expression goal = FileRead（goalfile）; }
game.checkstate（）{if（goal）exit（1）; } // exit 1是游戏胜利状态
//另一个更具体的例子：
class level1 {public：expression goal（total_enemies == 0）; };
class level2 {public：expression goal（player.x == goal.x&& player.y == goal.y）; };
动态表达式 p>
如果你想从用户那里接收一个字符串并从中构建一个表达式，也许 C ++数学表达式库适合您的帐单？
template< typename T&
void trig_function（）
{
std :: string expression_string =clamp（-1.0，sin（2 * pi * x）+ cos（x / 2 * pi），+ 1.0） ;
T x;
exprtk :: symbol_table< T> symbol_table;
symbol_table.add_variable（x，x）;
symbol_table.add_constants（）;
exprtk :: expression< T>表达;
expression.register_symbol_table（symbol_table）;
exprtk :: parser< T>解析器
parser.compile（expression_string，expression）;
for（x = T（-5.0）; x <= T（+5.0）; x + = 0.001）
{
T y = expression.value ;
printf（％19.15f \t％19.15f \\\
，x，y）;
}
}
也有嵌入脚本语言的可能性，例如作为 Lua 或 Python ，这将给你（甚至）更多的权力。
如果你使用的是 Qt ，您可以使用 QtScript （Javascript-ish）运行从QObject派生对象读取（静态或动态）属性的表达式。
使用上面的一个可以让你不必编写自己的解析器，AST和赋值器，然而对于一小组操作符，如果你使用 Boost.Spirit 或其他一些体面的解析库。
静态表达式
要在一组预定义表达式之间进行选择（即在编译时已知），应将表达式存储在多态函数对象中。
对于C ++ 11，如果可以使用，请使用 std :: function 和lambda表达式。
std :: function< bool（int，int）> expr = []（int a，int b）{a * 2< b};
对于早期的编译器，我建议在 Boost （boost：:)或C ++ 0x TR1（std：:)，具体取决于您的编译器。此外，Boost.Lambda将在这里有帮助，因为它允许您构建和存储表达式以供以后评估。但是，如果你不熟悉C ++和模板（或函数式编程），它可能会吓得你有点。
这样你可以写
使用命名空间boost :: lambda;
boost :: function< bool（int，int）> myexpr1 =（_1 + _2）> 20;
boost :: function< bool（int，int）> myexpr2 =（_1 * _2）> 42;
std :: cout<< myexpr1（4,7）< < myexpr2（2,5）;
使用bind，它如下所示：
boost :: function< bool（Player&）> check = bind（& Player :: getHealth，_1）> 20;
玩家p1;
if（check（p1））{dostuff（）; }
check = bind（& Player :: getGold，_1）< 42;
if（check（p1））{doOtherStuff（）; }
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 want to be able to have that goal value be anything from if the players life is below a certain value to if the amount of enemies remaining is equal to zero. Is there any "expression class" that can hold a simple "value1 compare-operator value2" and check it? ie:
expression goal(x = 4);
if not, does anybody have any suggestions as to how I could develop an expression class?
EDIT: another (more towards what I am trying to achieve) example:
game.init(){ expression goal = FileRead(goalfile); }
game.checkstate(){ if(goal) exit(1); } //exit 1 is the games win state
//another more specific eg.:
class level1 { public: expression goal(total_enemies == 0); };
class level2 { public: expression goal(player.x == goal.x && player.y == goal.y); };
Dynamic expressions
If you want to receive a string from the user and built an expression from that, maybe the C++ Mathematical Expression Library fits your bill?
template<typename T>
void trig_function()
{
std::string expression_string = "clamp(-1.0,sin(2 * pi * x) + cos(x / 2 * pi),+1.0)";
T x;
exprtk::symbol_table<T> symbol_table;
symbol_table.add_variable("x",x);
symbol_table.add_constants();
exprtk::expression<T> expression;
expression.register_symbol_table(symbol_table);
exprtk::parser<T> parser;
parser.compile(expression_string,expression);
for (x = T(-5.0); x <= T(+5.0); x += 0.001)
{
T y = expression.value();
printf("%19.15f\t%19.15f\n",x,y);
}
}
There are also the possibility embed a scripting language, such as Lua or Python, which will give you (even) more power. This is something to consider if you're writing a game, since you'll likely want to script large parts of it.
If you're using Qt, you can use QtScript (Javascript-ish) to run expressions that read (static or dynamic) properties from your QObject-derived objects.
Using one of the above keeps you from having to write your own parser, AST and evaluator, however for a small set of operators it shouldn't be too hard to hack together something if you use Boost.Spirit or some other decent parsing library.
Static expressions
For selecting between a set of predefined expressions (i.e. known at compile time), you should store the expression in a polymorphic function object.
For C++11, if that's available to you, use std::function and lambda expressions.
std::function<bool (int, int)> expr = [](int a, int b) { a*2 < b };
For earlier compilers, I recommend function and bind, either in Boost (boost::) or C++0x TR1 (std::), depending on your compiler. Also, Boost.Lambda will be of help here, as it allows you to construct and store expressions for later evaluation. However, if you're not familiar with C++ and templates (or functional programming), it will likely scare you quite a bit.
With that you could write
using namespace boost::lambda;
boost::function<bool (int, int)> myexpr1 = (_1 + _2) > 20;
boost::function<bool (int, int)> myexpr2 = (_1 * _2) > 42;
std::cout << myexpr1(4,7) << " " << myexpr2(2,5);
with bind, it'd look like:
boost::function<bool (Player&)> check = bind(&Player::getHealth, _1) > 20;
Player p1;
if (check(p1)) { dostuff(); }
check = bind(&Player::getGold, _1) < 42;
if (check(p1)) { doOtherStuff(); }
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