D:使用带有名称的字符串变量调用函数

Question

D 学习者在这里...如果我有一个字符串(仅在运行时知道的值)是我要调用的函数的名称，我该怎么做？下面的例子...

void func001() {
//stuff
}

void func002() {
//stuff
}

// .........

void func100() {
//stuff
}

void main(char[][] args) {
  auto funcnum = to!uint(args[0]);
  auto funcname = format('func%03d', funcnum);
  ///// need to run the function named 'funcname' here
}

Answer 1

这是一个使用编译时反射的示例。与 __traits(allMembers) ，我们可以遍历聚合中所有成员的名称(模块、结构、类等)，并使用 __traits(getMember) ，我们可以按名称获取成员并执行诸如调用它之类的操作。

棘手的部分是getMember需要一个编译时间字符串，所以我们不能直接将命令行参数传递给它。相反，我们构建一个 switch从参数中分派(dispatch) - 几乎就像您手动一样，但不是自己编写所有名称，而是让循环处理它。

这里只有两个函数，但它可以扩展到任意数量而无需修改 main功能。

查看更多内联评论:

import std.stdio;

// I'm grouping all the commands in a struct
// so it is easier to loop over them without
// other stuff getting in the way
struct Commands {
    // making them all static so we don't need to instantiate it
    // to call commands. This is often not the best way but it makes
    // for an easy demo and does work well a lot of the time.
    static:

    // Also assuming they all return void and have no arguments.
    // It is possible to handle other things, but it gets a lot
    // more involved. (I think my book example goes partially into
    // it, or something like my web.d goes all the way and generates
    // web/http and javascript/json apis from a full signature but that
    // code is pretty unreadable...)

    void func001() {
        writef("func001 called\n");
    }
    void func002() {
        writef("func002 called\n");
    }
}

void main(string[] args) {
    if(args.length > 1)
    // we switch on the runtime value..
    // the label will be used below
    outer: switch(args[1]) {
        // then loop through the compile time options to build
        // the cases. foreach with a compile time argument works
        // a bit differently than runtime - it is possible to build
        // switch cases with it.
        //
        // See also: http://dlang.org/traits.html#allMembers
        // and the sample chapter of my book
        foreach(memberName; __traits(allMembers, Commands)) {
            case memberName:
                // get the member by name with reflection,
                // and call it with the parenthesis at the end
                __traits(getMember, Commands, memberName)();

            // breaking from the labeled switch so we don't fallthrough
            // and also won't break the inner loop, which we don't want.
            break outer;
        }

        default: // default is required on most D switches
            writef("No such function, %s!\n", args[1]);
            break;
    }
    else { // insufficient args given
        writeln("Argument required. Options are:");
        // we can also loop to list names at runtime
        foreach(memberName; __traits(allMembers, Commands)) {
            writeln(memberName);
        }
    }
}