Question

使用下面的代码，当我尝试编译

时

clang++ -std=c++11 -stdlib=libc++ -o test sales_function.cpp sales_prog.cpp

我收到以下错误

duplicate symbol __ZN10Sales_data7combineERKS_ in:
/var/folders/7f/9r4z5bs90bjfm3dy1k_g03xc0000gn/T/sales_functions-5G1FRA.o
/var/folders/7f/9r4z5bs90bjfm3dy1k_g03xc0000gn/T/sales_prog-82wDRv.o
ld: 1 duplicate symbol for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

现在要使sales_functions.cpp文件中的非成员函数起作用，我包含了“sales_dat.h”标题，因为函数使用的struct Sales_data在那里定义。但是在那个文件中，我有非成员函数'read'，它在结构Sales_data的一个构造函数中调用。因此，为了避免我转发声明的struct并在sales_dat.h中的struct定义之前声明了read函数。

我一直在尝试不同的方法，例如将struct的定义放在它自己的文件中，并将声明只放在头文件中。但这给了我其他问题，编译器无法在我的非成员函数文件中使用它的对象。

我得到的错误，但我不明白为什么我得到上述错误。我想也许它与sales_prog.cpp和sales_function.cpp中的“sales_dat.h”标题有关，但似乎我必须将它放在sales_function.cpp中才能使非成员函数使用struct Sales_data对象。

这到底发生了什么？ sales_functions.cpp

#include <iostream>
#include "sales_dat.h"
std::istream &read(std::istream &is, Sales_data &item)
{
    double price = 0;
    is >> item.bookName >> item.books_sold >> price;
    item.revenue = price * item.books_sold;
    return is;
}
std::ostream &print(std::ostream &os, Sales_data &item)
{
    os << item.isbn() << " " << item.books_sold << " " << item.revenue;
    return os;
}
Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
{
    Sales_data sum = lhs;
    sum.combine(rhs);
    return sum;
}

sales_dat.h

#ifndef SALES_DAT_H
#define SALES_DAT_H
#include <iostream>
#include <string>
struct Sales_data;
std::istream &read(std::istream &, Sales_data &);
struct Sales_data {
    std::string bookName;
    std::string isbn() const { return bookName; }
    Sales_data &combine(const Sales_data &);
    unsigned books_available = 10;
    unsigned books_sold = 0;
    double revenue = 0;
    unsigned total_sold = 0;
    unsigned count = 1;
    Sales_data() = default;
    Sales_data(unsigned c) : count(c) {}
    Sales_data(const std::string &s) : bookName(s) {}
    Sales_data(const std::string &s, unsigned m, unsigned n, double p) : bookName(s), books_sold(m), books_available(n), revenue(p*m) {}
    Sales_data(std::istream &inpst) { read(inpst, *this); }
};
std::ostream &print(std::ostream &, Sales_data &);
Sales_data add(const Sales_data &, const Sales_data &);
Sales_data &Sales_data::combine(const Sales_data &rs)
{
    count += rs.count;
    books_sold += rs.books_sold;
    revenue += rs.revenue;
    return *this;
}
#endif

sales_prog.cpp

#include <iostream>
#include <string>
#include "sales_dat.h"
int main()
{
    std::cout << "Enter a transaction" << std::endl;
    Sales_data total(std::cin);
    if(std::cin) {
        Sales_data trans;
        while(read(std::cin,trans)) {
            if(total.isbn() == trans.isbn()) {
                total = add(total,trans);
            }
            else {
                std::cout << "Number of " << total.isbn() << " transactions: " << total.count << std::endl;
                std::cout << "Number of " << total.isbn() << " sold: " << total.books_sold << std::endl;
                std::cout << "Revenue: " << total.revenue << std::endl;
                total = trans;
            }
        }
        if(total.books_sold != 0) {
            std::cout << "Number of " << total.isbn() << " transactions: " << total.count << std::endl;
            std::cout << "Number of " << total.isbn() << " sold: " << total.books_sold << std::endl;
            std::cout << "Revenue: " << total.revenue << std::endl;
        }
        else
            std::cout << "Entry: " << total.isbn() << " has zero sold" << std::endl;
    }
    else
        std::cerr << "No transaction" << std::endl;
    return 0;
}

Answer 1

Sales_data::combine函数是在头文件中定义的，但你没有使它inline，因此包含该头文件的每个源文件都定义了该函数。

链接器因重复符号而失败

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