为什么我的程序只更新 main() 中 3 个变量中的 1 个？

Question

所以我想修改我的部分程序。这涉及我在 buyCoffee 函数中创建另一个累加器。该程序运行完美，除了通过引用传递的 3 个主要函数变量中只有 1 个被更新。如果运行代码，购买所有 3 种尺寸的咖啡，然后检查 cupsSold，Small 是 main() 中唯一更新的咖啡。我很困惑，因为每种咖啡尺寸的代码都是相同的。可能有一些我不知道的潜在相互作用导致了这种情况。

// This program acts like a coffee shop, with multiple commands made for the manager, and one for the cashier. You can buy coffee(cashier), see how many sizes were sold for the day(manager), see how many ounces of coffee were sold for the day(manager), and view revenue for the day(manager). You can quit to close the shop.


#include <iostream>
#include <iomanip>
#include <string>
using namespace std;


//Global variables for prices and ounces.
double largeCoffee = 2, mediumCoffee = 1.90, smallCoffee = 1.75;
int smlOz = 9, medOz = 12, lrgOz = 15;


//Function initialization.
void showMainMenu();
void buyCoffee(double &totSmall, double &totMedium, double &totLarge);
void cupsSold(double totalSmall, double totalMedium, double totalLarge);
void coffeeSold(double totalSmall, double totalMedium, double totalLarge, int smlOz, int medOz, int lrgOz);
void totalRevenue(double totalSmall, double totalMedium, double totalLarge, double largeCoffee, double mediumCoffee, double smallCoffee);
void printReceipt(int &am1, int &am2, int &am3);


//Main handles the users choice, with if-else statements, after mainmenu has been called.
int main() {
  
  //Variable declaration.
  double totalSmall = 0, totalMedium = 0, totalLarge = 0;
  double& totSmall = totalSmall, totMedium = totalMedium, totLarge = totalLarge;
  string choice;

  //Loop will run until user enters "quit" on the main menu, closing the shop. 
  bool quit = true;
  while (quit == true){
    
    //Main menu function is always called before receiving input in choice.
    showMainMenu();
    cin >> choice;

    //Calls buyCoffee function if the choice is buyCoffee. Clears the variable 'choice' after the function has been run, taking you back the to main menu.
    if (choice == "buyCoffee"){
      //Passes the total coffee cup sizes by reference, so the program can run multiple times and retain and update values until the shop is closed.
      buyCoffee(totSmall, totMedium, totLarge);
      cin.clear();
    }
      
    //Calls cupsSold function if the choice is cupsSold. Clears the variable 'choice' after the function has been run, taking you back to the main menu. Displays the cup sizes sold for the day.
    else if (choice == "cupsSold"){
      //Passes the coffee cup sizes sold by current values.
      cupsSold(totalSmall, totalMedium, totalLarge);
      cin.clear();
    }
    //Calls coffeeSold function if the choice is coffeeSold. Clears the variable 'choice' after the function has been run, taking you back to the main menu.
    else if (choice == "coffeeSold"){
      //Passes the total coffee cup sizes sold by current values, and passes the global oz variables by value. Calculates the coffee oz sold for the day.
      coffeeSold(totalSmall, totalMedium, totalLarge, smlOz, medOz, lrgOz);
      cin.clear();
    }
      //Calls totalRevenue function if the choice is revenue. Clears the variable 'choice' after the function has been run, taking you back to main menu.
    else if (choice == "revenue"){
      //Passes the total cup sizes sold, and the price of each size of coffee to calculate the total revenue made for the day. 
      totalRevenue(totalSmall, totalMedium, totalLarge, largeCoffee, mediumCoffee, smallCoffee);
    }
      
    //Quits the program if the choice is 'quit'. Closes the shop.
    else if (choice == "quit"){
      cout << "The coffee shop is now closed. Run the program to open it again. Thank you!";
      //Exits the main menu loop since quit is now false.
      quit = false; 
    }
      
    //Will reject input if it's unknown. 
    else{
      cout << endl <<"Please input a valid command." << endl << endl;
    }  
  } 
}

//When the program starts, it shows the main menu, created in this function. Output is formatted with parameters left and right justified to view the commands easily on output.
void showMainMenu(){
  
  //The command output is left justified with 15 characters, and the description is right justified by 43 characters.
  cout << endl << setw(15) << left << "COMMAND";
  cout << setw(43) << right << "DESCRIPTION" << endl;
  cout << "==========================================================" << endl;

  cout << setw(15) << left << "buyCoffee";
  cout << setw(43) << right << "Buy coffee, and view the menu." << endl << endl;

  cout << setw(15) << left << "cupsSold";
  cout << setw(43) << right << "Coffee sizes sold for the day." << endl << endl;

  cout << setw(15) << left << "coffeeSold";
  cout << setw(43) << right << "View ounces of coffee sold for the day." << endl << endl;

  cout << setw(15) << left << "revenue";
  cout << setw(43) << right << "View revenue for the day." << endl << endl;

  cout << setw(15) << left << "quit";
  cout << setw(43) << right << "Close the shop for the day." << endl;

  cout << "==========================================================" << endl;
  //Command prompt.
  cout << "Enter Command: ";
}


//*totsmall, *totmedium, and *totallarge reference the cup sizes defined in main().
void buyCoffee(double &totSmall, double &totMedium, double &totLarge){
  //Variables initialized so the user can enter how many coffees they want to purchase.
  int numSmall = 0, numMedium = 0, numLarge = 0;
  string size;
  //Enters another loop for the coffee menu, until user exits.
  int amount1 = 0, amount2 = 0, amount3 = 0;
  int& am1 = amount1, am2 = amount2, am3 = amount3;
  bool quit2 = true;
  while(quit2 == true){
    //Displays menu with the coffees justified on the left 15 characters, as well as prices and ounces of the coffee on the right, justified by 15 characters.
    cout << endl << "MENU" << endl;
    cout << "==============================" << endl;
    cout << setw(15) << left << "Small Coffee";
    cout << setw(15) << right << "9oz - 1.75$" << endl << endl;
      
    cout << setw(15) << left << "Medium Coffee";
    cout << setw(15) << right << "12oz - 1.90$" << endl << endl;

    cout << setw(15) << left << "Large Coffee";
    cout << setw(15) << right << "15oz - 2.00$" << endl;
    cout << "==============================" << endl;

    //User prompt.
    cout << "Enter small, medium, or large. Enter quit to view your receipt (if you purchased anything) and/or return to the main menu: ";
    cin >> size;
    cout << endl;
    
    //If the size is small, it enters another loop for how many the user would like to purchase. 
    if (size == "small"){
      bool quit3 = true;
      while (quit3 == true){
        
        //User prompt.
        cout << "How many would you like to purchase? ";
        cin >> numSmall;
        cout << endl;
        
        //if input is a positive number, it'll add it to the total cup count in main() and call printReceipt function to display the receipt for the customer. If not a positive number, it asks again for correct input until it's correct. After printing the receipt, it'll set quit3 to false to exit the purchase number loop. This code repeats for all 3 sizes.
        if (numSmall > 0){
          
          totSmall += numSmall;
          am1 += numSmall;
          cout << "Anything else?";
          cin.clear();
          quit3 = false;
        }
        else if (numSmall != 0){
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
        else{
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
      }
    }
      
    //Another loop for the medium size. All three size loops work exactly the same.
    else if (size == "medium"){
      bool quit3 = true;
      while (quit3 == true){
        cout << "How many would you like to purchase? ";
        cin >> numMedium;
        cout << endl;
        if (numMedium > 0){
          totMedium += numMedium;
          am2 += numMedium;
          cout << "Anything else?";
          cin.clear();
          quit3 = false;
        }
        else if (numMedium != 0){
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
        else{
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
      }
    }
      
    //Another loop for the large size. All three size loops work exactly the same.
    else if (size == "large"){
      bool quit3 = true;
      while (quit3 == true){
        cout << "How many would you like to purchase? ";
        cin >> numLarge;
        cout << endl;
        if (numLarge > 0){
          totLarge += numLarge;
          am3 += numLarge;
          cout << "Anything else?";
          cin.clear();
          quit3 = false;
        }
        else if (numLarge != 0){
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
        else{
          cout << "Invalid input." << endl;
          cin.clear();
          cin.get();
        }
      }
    }
    else if (size == "quit"){
      if (numLarge > 0 || numMedium > 0 || numSmall > 0){
        printReceipt(am1, am2, am3);
        cin.clear();
        quit2 = false;
      }
      else{
        cin.clear();
        quit2 = false;
      }
    }
      
    //Safety for invalid input.
    else{
      cout << "Invalid input.";
      cin.clear();
      cout << endl;
      }
    }
}

//Prints the receipt after purchasing coffee. Passes the the size purchased to calculate the total cost for the receipt.
void printReceipt(int &am1, int &am2, int &am3){
  double orderTotal = 0;
  
  //Prints receipt for the large coffee bought.

  orderTotal += (am3 * largeCoffee);
  
  //Again, formats the output like the rest of the program. Same for the next 2 receipts.
  cout << "RECEIPT" << endl;
  cout << "==============================" << endl;
  cout << am3 << " Large Coffee";
  cout << setw(15) << right << "$2.00/each.";
  cout << endl;

  
//Prints receipt for the medium coffee bought. Same as the first one.

  orderTotal += (am2 * mediumCoffee);
  cout <<endl;
  cout << am2 << " Medium Coffee";
  cout << setw(15) << right << "$1.90/each.";
  cout << endl;


  
//Prints receipt for the small coffee bought. Same as the first one.

  orderTotal += (am1 * smallCoffee);
  cout << endl;
  cout << am1 << " Small Coffee";
  cout << setw(15) << right << "$1.75/each." << endl << endl;
  cout << "Order Total: ";
  cout << fixed << setprecision(2) << setw(12) << right << "$" << orderTotal << endl << endl;
  cout << "Thank you, please come again!" << endl;
  cout << "==============================" << endl << endl;

}


//Shows the cups sold for the day, passing the total cup sizes by value.
void cupsSold(double totalSmall, double totalMedium, double totalLarge){
  
  //Formatting like the rest of the program.
  cout << endl << "COFFEE SIZES SOLD TODAY:" << endl;
  cout << "========================" << endl;
  cout << setw(8) << left << "Small";
  cout << fixed << setprecision(0) << setw(16) << right << totalSmall << endl << endl;

  cout << setw(8) << left << "Medium";
  cout <<  fixed << setprecision(0) << setw(16) << right << totalMedium << endl << endl;

  cout << setw(8) << left << "Large";
  cout <<  fixed << setprecision(0) << setw(16) << right << totalLarge << endl;

  cout << "========================" << endl;
  }

//Calculates coffee sold in oz for the day. Passes the cup sizes sold, as well as the size oz amounts by value.
void coffeeSold(double totalSmall, double totalMedium, double totalLarge, int smlOz, int medOz, int lrgOz){
  int ozTotal = (totalSmall * smlOz) + (totalMedium * medOz) + (totalLarge * lrgOz);
  
  //Formatting like the rest of the program.
  cout << endl << "COFFEE OUNCES SOLD TODAY:" << endl;
  cout << "========================" << endl;
  
  cout << setw(16) << left << "Today's Total:";
  cout << fixed << setprecision(0) << setw(8) << right << ozTotal << endl;

  cout << "========================" << endl;
}

//Calculates the revenue (coffee sold) for the day and displays it.
void totalRevenue(double totalSmall, double totalMedium, double totalLarge, double largeCoffee, double mediumCoffee, double smallCoffee){

  double totalRev = (totalSmall * smallCoffee) + (totalMedium * mediumCoffee) + (totalLarge * largeCoffee);
  
  //Formatting like the rest of the program.
  cout << endl << "TOTAL REVENUE TODAY" << endl;
  cout << "=============================" << endl;
  
  cout << setw(16) << left << "Today's Revenue:";
  cout << fixed << setprecision(2) << setw(8) << right << "$" << totalRev << endl;

  cout << "=============================" << endl;
  
}

Answer 1

问题是，当您在同一行声明多个指针或引用时，* 或 & 绑定到变量，而不是类型。

这会创建第一个变量的引用，但第二个变量只是一个double。

double& a = something, b = somethingElse;

这会创建两个引用。

double& a = something, &b = somethingElse;

避免此问题的一个好方法是始终每行声明一个变量。我觉得这也有助于使您的代码更具可读性。

double& a = something;
double& b = somethingElse;

基于此，我将您的程序修改为更小的程序，以专注于该问题，并解释说您根本不需要创建这些引用。

#include <iostream>
#include <string>

void buyCoffee(double& totSmall, double& totMedium, double& totLarge) 
{
    totSmall = 5;
    totMedium = 10;
    totLarge = 15;
}

void print(const std::string& header, double totalSmall, double totalMedium,
           double totalLarge)
{
    std::cout << header << "\n";
    std::cout << "totalSmall = " << totalSmall
        << ", totalMedium = " << totalMedium
        << ", totalLarge = " << totalLarge << "\n";
}

int main() 
{
    // This uses explicit references as you had in your example with
    // a correction to the second and third variables
    {
        double totalSmall = 0, totalMedium = 0, totalLarge = 0;

        print("Before: ", totalSmall, totalMedium, totalLarge);

        double &totSmall = totalSmall, &totMedium = totalMedium, &totLarge = totalLarge;
        buyCoffee(totSmall, totMedium, totLarge);

        print("After: ", totalSmall, totalMedium, totalLarge);
    }

    // This just passes the variables to the function
    // Since the function accepts them by reference it modifies
    // the original variable inside the function
    {
        double totalSmall = 0, totalMedium = 0, totalLarge = 0;

        print("Before: ", totalSmall, totalMedium, totalLarge);

        buyCoffee(totalSmall, totalMedium, totalLarge);

        print("After: ", totalSmall, totalMedium, totalLarge);
    }
}

如果运行此命令，您将看到两者给出相同的结果，原始变量被函数修改。

示例 @ ideone

输出：

Before: 
totalSmall = 0, totalMedium = 0, totalLarge = 0
After: 
totalSmall = 5, totalMedium = 10, totalLarge = 15
Before: 
totalSmall = 0, totalMedium = 0, totalLarge = 0
After: 
totalSmall = 5, totalMedium = 10, totalLarge = 15