使用名称空间会导致“未定义的引用”错误

Question

我正在学习Stephen Prata的书“ C ++ Primer Plus”中的C ++，并且目前正在进行名称空间练习。这是锻炼程序应该如何工作的：

  

第一个文件应该是包含名称空间的头文件。   第二个文件应该是扩展了   为三个原型函数提供定义的名称空间。   第三个文件应声明两个Sales对象。它应该使用   setSales（）的交互式版本，可为一个结构提供值   和setSales（）的非交互式版本，以提供以下值：   第二种结构。它应该显示两个的内容   使用showSales（）构建结构。

我写了两个cpp文件和一个头文件。我预先为我的意大利面条代码表示歉意。请记住，我只是一个初学者。 这是我的所有文件。

sales.h

#ifndef SALES_H_
#define SALES_H_

namespace SALES
{
    const int QUARTERS = 4;
    struct Sales {
        double sales[QUARTERS];
        double average;
        double max;
        double min;
    };
    // copies the lessoer of 4 or n items from the array ar
    // to the sales member of s and computes and stores the
    // average, maximum, and minimum values of the entered items;
    // remaining elements of sales, if any, set to 0
    void setSales(Sales & s, const double ar[], int n);
    // gathers sales for 4 quarters interactively, stores them
    // in the sales member of s and computes and stores the
    // average, maximum, and minimum values
    void setSales(Sales & s);
    // display all information in structure s
    void showSales(const Sales & s);
}

#endif

sales.cpp

#include <iostream>
#include "sales.h"

using namespace SALES;

void setSales(Sales & s, const double ar[], int n) {
    double tmp;
    double tmp_ar[n];
    //making temporary copy of an array
    for(int i = 0; i < QUARTERS; i++) {
        tmp_ar[i] = ar[i];
    }
    //bubble sort
    for(int i = 0; i < n; i++){
        for(int j = i + 1; j < n; j++){
            if(tmp_ar[i] > tmp_ar[j]){
                tmp = tmp_ar[i];
                tmp_ar[i] = tmp_ar[j];
                tmp_ar[j] = tmp;
            }
        }
    }
    //filling s.sales with 4 lesser elements
    for(int i = 0; i < QUARTERS; i++) {
        s.sales[i] = tmp_ar[i];
    }

    //finding and assigning minimum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++) {
        if(s.sales[i] < s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.min = tmp;

    tmp = s.sales[0];
    //finding and assigning maximum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++){
        if(s.sales[i] > s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.max = tmp;

    //calculating average
    double tmp_sum = 0;
    for(int i = 0; i < QUARTERS; i++) {
        tmp_sum += s.sales[i];
    }
    s.average = tmp_sum / QUARTERS;
}
void setSales(Sales & s) {
    using std::cin;
    using std::cout;
    using std::endl;
    cout << "Input sales for each quarter" << endl;
    double input;
    for(int i = 0; i < QUARTERS; i++) {
        cout << "Input sales for " << i + 1 << " quarter: ";

        while(!(cin >> input)) {
            cout << "Not an integer. Try again..." << endl;
        }
        s.sales[i] = input;
    }
    //temporary value for maximum and minimum
    double tmp;
    //finding and assigning minimum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++) {
        if(s.sales[i] < s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.min = tmp;

    tmp = s.sales[0];
    //finding and assigning maximum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++){
        if(s.sales[i] > s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.max = tmp;

    //calculating average
    double tmp_sum = 0;
    for(int i = 0; i < QUARTERS; i++) {
        tmp_sum += s.sales[i];
    }
    s.average = tmp_sum / 4;
}
void showSales(const Sales & s) {
    using std::cin;
    using std::cout;
    using std::endl;
    for(int i = 0; i < QUARTERS; i++)
        cout << "Quarter " << i + 1 << ": " << s.sales[i] << endl;

    cout << "Minimum: " << s.min << endl;
    cout << "Maximum: " << s.max << endl;
    cout << "Average: " << s.average << endl;
}

main.cpp

#include <iostream>
#include "sales.h"

using namespace SALES;

int main() {
    Sales year_one, year_two;
    setSales(year_one);
    showSales(year_one);
    int arr_size = 4;
    double arr[arr_size] = {400., 500., 200., 1000.};
    setSales(year_two, arr, arr_size);
    showSales(year_two);
    return 0;
}

编译时出现错误：

sloth@sloth-commodore64:~/Programming/C++/src/Chapter9Exercise/ex4$ g++ main.cpp sales.cpp 
/tmp/ccl6q4PO.o: In function `main':
main.cpp:(.text+0x34): undefined reference to `SALES::setSales(SALES::Sales&)'
main.cpp:(.text+0x43): undefined reference to `SALES::showSales(SALES::Sales const&)'
main.cpp:(.text+0x152): undefined reference to `SALES::setSales(SALES::Sales&, double const*, int)'
main.cpp:(.text+0x15e): undefined reference to `SALES::showSales(SALES::Sales const&)'
collect2: error: ld returned 1 exit status

我也做了另一个main.cpp只是为了测试。我的代码版本既不使用多个文件也不使用名称空间，并且似乎可以正常工作。

我真的很困惑为什么它不起作用。我同时搜索了stackoverflow和Google，但似乎找不到解决方法。

编辑： 多亏@Justin和@OznOg，我找到了个人。在文件〜/ sales.h /中，我添加了一个带有SALES名称空间的前缀。现在〜/ sales.h /看起来像这样：

sales.h

#include <iostream>
#include "sales.h"

using namespace SALES;

void SALES::setSales(Sales & s, const double ar[], int n) {
    double tmp;
    double tmp_ar[n];
    //making temporary copy of an array
    for(int i = 0; i < QUARTERS; i++) {
        tmp_ar[i] = ar[i];
    }
    //bubble sort
    for(int i = 0; i < n; i++){
        for(int j = i + 1; j < n; j++){
            if(tmp_ar[i] > tmp_ar[j]){
                tmp = tmp_ar[i];
                tmp_ar[i] = tmp_ar[j];
                tmp_ar[j] = tmp;
            }
        }
    }
    //filling s.sales with 4 lesser elements
    for(int i = 0; i < QUARTERS; i++) {
        s.sales[i] = tmp_ar[i];
    }

    //finding and assigning minimum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++) {
        if(s.sales[i] < s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.min = tmp;

    tmp = s.sales[0];
    //finding and assigning maximum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++){
        if(s.sales[i] > s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.max = tmp;

    //calculating average
    double tmp_sum = 0;
    for(int i = 0; i < QUARTERS; i++) {
        tmp_sum += s.sales[i];
    }
    s.average = tmp_sum / 4;
}
void SALES::setSales(Sales & s) {
    using std::cin;
    using std::cout;
    using std::endl;
    cout << "Input sales for each quarter" << endl;
    double input;
    for(int i = 0; i < QUARTERS; i++) {
        cout << "Input sales for " << i + 1 << " quarter: ";

        while(!(cin >> input)) {
            cout << "Not an integer. Try again..." << endl;
        }
        s.sales[i] = input;
    }
    //temporary value for maximum and minimum
    double tmp;
    //finding and assigning minimum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++) {
        if(s.sales[i] < s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.min = tmp;

    tmp = s.sales[0];
    //finding and assigning maximum
    tmp = s.sales[0];
    for(int i = 1; i < QUARTERS; i++){
        if(s.sales[i] > s.sales[i - 1])
            tmp = s.sales[i];
    }
    s.max = tmp;

    //calculating average
    double tmp_sum = 0;
    for(int i = 0; i < QUARTERS; i++) {
        tmp_sum += s.sales[i];
    }
    s.average = tmp_sum / QUARTERS;
}
void SALES::showSales(const Sales & s) {
    using std::cin;
    using std::cout;
    using std::endl;
    for(int i = 0; i < QUARTERS; i++)
        cout << "Quarter " << i + 1 << ": " << s.sales[i] << endl;

    cout << "Minimum: " << s.min << endl;
    cout << "Maximum: " << s.max << endl;
    cout << "Average: " << s.average << endl;
}

此更改程序正常运行后。