错误：预期在 '.' 之前有主表达式代币

Question

我目前正在使用《A C++ for Dummies All-In One》自学 C++；第二版。为了创建这个程序，我使用 Qt。我认为在头文件中组织对象和类以及在 main.cpp 之外构建的 .cpp 文件中组织成员函数是一种很好的做法。在这方面，我尝试运行本书中的练习，但最近遇到了以下错误。

expected primary-expression before '.' token

此错误发生在第 31、32 和 37 行，因此它们似乎与我的类成员函数特别相关。

在我的 main.cpp 中，

#include "controlinginput.h"
#include <QtCore/QCoreApplication>
#include <iostream>
#include <sstream>


using namespace std;

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);


// just a basic name-entering
string name;
cout << "What is your name?";
cin >> name;
cout << "Hello " << name << endl;

/* now you are asked for a number
  but the computer will allow you to enter anything*/
int x;
cout << endl << "Enter a number! Any Number!" << endl;
cin >> x;
cout << "You choose " << x << endl;

/* now youll be asked for a number again
  but the computer will only allow numbers */
cout << endl<< "This time you will ONLY be able to enter a number! " << endl;
cout << "SO, Pick a number! any number!" << endl;
string entered = ControlingInput.enterOnlyNumbers(); // ###Error###        
int num = ControlingInput.stringToANumber(entered); // ###Error###
cout << endl << "You entered " << num << endl; // value is displayed
//Now finally we enter the password
cout << endl;
cout << "Please enter a password" << endl;
string password = ControlingInput.EnterPassword(); // ###Error###
cout << "shh... your password is " << password << endl;
return a.exec();
}

我做了一些研究，发现此错误表明存在相当广泛的语法滥用。不幸的是，我无法找到与我的具体相似的实例；我希望从一些更有经验的程序员那里获得一些见解。如果这是一个简单的问题，是由于我的疏忽造成的，我提前道歉并感谢您的反馈。如果它给我带来很多麻烦而不是一点麻烦，我会学得更好。

因为这些包括我的成员函数，所以我还包括了我的头文件和 .cpp

controlingInput.cpp （我已经包括了我的头文件和 iostream 和 sstream 在这里，但由于某种原因，编辑器在这里给我带来了问题）

using namespace std;

ControlingInput::ControlingInput()
{

}
int ControlingInput::stringToANumber(string MyString)
{
istringstream converter(MyString); //Holds the string that was passed to this function
int result;                        //Holds the integer result

//perform the conversion
converter >> result;
return result; //function completes and returns converted string

}

string ControlingInput::enterOnlyNumbers()
{
string numbAsString = ""; // this holds our numeric string
        char ch = getch();  // This gets a single character from our user
//Says to keep gettting characters from our user untill user presses enter
        while (ch != '\r') // \r is the enter key
        {
           //This says to add characters only if they are numbers
            if (ch >= '0' && ch <='9')
            {
                cout << ch; // show
                numbAsString += ch; // add character to the string
            }

            ch = getch(); // get the next character from the user

        }
        return numbAsString;

}

string ControlingInput::EnterPassword()
{
string numbAsString = ""; //this will hold our password string
char ch = getch(); // this gets a single char from our users just like before
//keep gettting characters from the user until enter/return is pressed
while (ch != '\r'); // \r is the enter or return key
{
    //for security passwords are displayed as asterisks instead of characters
    cout << '*';

    //add character input into the password string
    numbAsString += ch;

    //Get the next character from the user
    ch = getch();
}
return numbAsString; // return the user input from this function

这是我的 controlingInput.h

#ifndef CONTROLINGINPUT_H
#define CONTROLINGINPUT_H
#include <iostream>

using namespace std;

class ControlingInput
{
public:
int stringToANumber(string MyString);
string EnterPassword();
string enterOnlyNumbers();

};

#endif // CONTROLINGINPUT_H

提前感谢您的任何反馈。

Answer 1

您尝试使用类本身调用实例变量，就好像它们是静态的一样（这仍然是无效的语法）。为了使其正常工作，您需要一个 ControlingInput 实例。

int main(int argc, char *argv[])
{

    QCoreApplication a(argc, argv);

    ControlingInput ctrlInput; //Create instance
    ...

    string entered = ctrlInput.enterOnlyNumbers();        
    int num = ctrlInput.stringToANumber(entered);
    cout << endl << "You entered " << num << endl; // value is displayed
    ...

    string password = ctrlInput.EnterPassword();
    cout << "shh... your password is " << password << endl;
    return a.exec();

}