Question

D 2.0中有一个with关键字，但我不确定它的用途，或者如何使用它。我对文档的搜索毫无结果。有谁知道with关键字的用途是什么？（它是否像C＃的using语句？或者像Visual Basic的With子句？）

Answer 1

with语句是一种简化对同一对象的重复引用的方法。

它的用法如下：

with (expression)
{
  usage();
  writef("%s, %s", access, member);
}

Answer 2

如果您需要，可以使用with语句构造匿名对象：

class Foo { int x; }
void main()
{
    with (new Foo)
    {
        x = 5;
    }
}

This link包含与Scintilla一起使用的with关键字的示例代码。

Answer 3

从文档中： “with语句是一种简化对同一对象的重复引用的方法。”

当我在文档中提到它时，我的想法是，对于语法需要看起来很熟悉的数学，以及代码重复访问同一个对象，它会有所帮助。

struct Values
{
    double x,y,vx,vy,ax,ay,dt;

    int i;
    void set_i( int i )
    {
        this.i = i;
    }

    alias int NestedType;
};



void main()
{
    //Setup:
    Values vals;
    vals.i = 10;

    //Usage of with:
    with(vals)
    // with(otherVals)    // <<--  Easy to switch the 'with' scope to deal with a different instance:
    {
        // Imports all the member symbols of the struct or class into present scope
        // without needing to refer every time to vals:
        assert( i == 10 );

        // Good for "repeated references to the same object":   Helpful for maths where the syntax needs to look familiar, and repeatedly accesses the same object:
        x  += vx*dt;
        y  += vy*dt;
        vx += ax*dt;
        vy += ay*dt;

        // Call the member functions too:
        set_i(42);

        // ... and get nested types/enums etc etc
        NestedType ex;
    }
    // Results are persist (i.e.:  all above writes were by reference:
    assert( vals.i == 42 );


    //Equivalent usage without 'with':
    {
        Values* vp = &vals;      // get a reference to vals

        assert( vp.i == 42 );

        // This looks a lot Uglier than the previous one:
        vp.x  += vp.vx*vp.dt;
        vp.y  += vp.vy*vp.dt;
        vp.vx += vp.ax*vp.dt;
        vp.vy += vp.ay*vp.dt;

        vp.set_i(56);
        Values.NestedType ex;
    }

    assert( vals.i == 56 );

}

“with”关键字在D 2.0中有什么作用？

3 个答案: