小编Sog*_*ger的帖子

如果ProgressBar可以在需要之前消失,那将是很好的.在applyMenuChoice中使用setVisibility.progressBar有问题吗？在PrintStatusTask().execute()中使用setVisibility.progressBar的问题是它在运行时崩溃应用程序.

public class Controller extends Activity {
    private ProgressBar progressBar;
    ...

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.controller);
        progressBar = (ProgressBar)findViewById(R.id.progressBar);
        ...

    private boolean applyMenuChoice(MenuItem item) {
        switch (item.getItemId()) {
        case R.id.menuStatus:
            progressBar.setVisibility(View.VISIBLE);
            new PrintStatusTask().execute();
            progressBar.setVisibility(View.GONE);
            ...

我正在阅读Effective C++(Scott Meyers),并在尝试编译本书中的以下代码时得到错误"与运算符*不匹配":

rational.h

class rational
{
    private:
        int num;
        int den;
    public:
        rational(int n = 0, int d = 1);
        int getNum() const {return num;}
        int getDen() const {return den;}
};

rational.cpp

#include "rational.h"

rational::rational(int n,
                    int d)
    :num(n),
     den(d)
{}

const rational operator*(const rational &lhs, 
                         const rational &rhs)
{
    return  rational( lhs.getNum()*rhs.getNum(),
                      lhs.getDen()*rhs.getDen() );
}

main.cpp中

#include "rational.h"
int main()
{
    rational r1(1,2);
    rational r2;
    r2 = 2*r1;
    r2 = r1*3;
    return 0;
}

有人可以解释为什么会这样吗？