没有明显原因得到 "Multiple definition"

Question

从标题看问题已经很清楚了。这是代码：

TimeTask.hpp:

#pragma once
#include <locale>
#include <codecvt>
#include <chrono>

namespace TimeTask {

static std::wstring_convert<std::codecvt_utf8_utf16<wchar_t> > converter;

std::tm * to_date(
    int day, int month, int year,
    int hours = -1, int minutes = -1, int seconds = -1
) {
    time_t rawTime = time(0);
    std::tm * tInfo = localtime(&rawTime);
    tInfo->tm_mday  = day;
    tInfo->tm_mon   = month - 1;
    tInfo->tm_year  = year - 1900;

    if(hours >= 0)
        tInfo -> tm_hour = hours;
    if(seconds >= 0)
        tInfo -> tm_sec = seconds;
    if(minutes >= 0)
        tInfo -> tm_min = minutes;

    return tInfo;
}

std::string from_time_point(const std::chrono::system_clock::time_point &arg) {
    time_t time = std::chrono::system_clock::to_time_t(arg);
    return ctime(&time);
}

}

Task.hpp:

#pragma once

#include <string>
#include "TimeTask.hpp"

class Task
{
public:
    Task(
        const std::basic_string<wchar_t>&,
        const bool&,
        const std::chrono::system_clock::time_point&
    );
    Task();

    std::basic_string<wchar_t>                  description;
    bool                                        executed;
    std::chrono::system_clock::time_point       date;

    std::wstring towString();
};

Task.cpp:

#include "Task.hpp"
#include <sstream>    

Task::Task(
    const std::basic_string<wchar_t>& description,
    const bool& executed,
    const std::chrono::system_clock::time_point& date
):
    description(description),
    executed(executed),
    date(date) { }

Task::Task():
    Task( L"Task", false, std::chrono::system_clock::now() ) { }

std::wstring Task::towString() {
    std::wostringstream retval(L"");

    retval << L"Task:\t"
           << description
           << L"\nPerformed:\t"
           << std::boolalpha << executed;    

    retval << TimeTask::converter.from_bytes(TimeTask::from_time_point(date));

    return retval.str();
}

main.cpp:

#include    <iostream>
#include    <type_traits>
#include    <vector>
#include    <list>
#include    "Task.hpp"

int main () {
    Task t1(L"Lezione programmazione",
            false,
            std::chrono::system_clock::from_time_t(
                std::mktime(TimeTask::to_date( 5,5,2017)) ));

    Task t2(L"Lezione programmazione",
            false,
            std::chrono::system_clock::from_time_t(
                std::mktime(TimeTask::to_date( 1,10,2017)) ));

    Task t3(L"Lezione programmazione",
            false,
            std::chrono::system_clock::from_time_t(
                std::mktime(TimeTask::to_date( 1,17,2017 )) ));

    std::list   <Task>    agenda1 = { t1, t2, t3 };
    std::vector <Task>    agenda2 = { t1, t2, t3 };

    for( auto it : agenda1 )
        std::cout << TimeTask::converter.to_bytes(it.towString());

    return 0;
}

我在这里做错了什么？我没有看到 to_date 和 from_time_point 的任何多重定义（我的编译器 g++ 正在抱怨的功能）。

Answer 1

您应该将这些函数标记为 inline。因为你的头文件不仅声明了，还定义了这个函数。当从多个翻译单元包含时，这两个都会编译该函数的一个实例，并且在链接过程中会出错。

详见一定义规则（[basic.def.odr]）：

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required. The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined (see [class.ctor], [class.dtor] and [class.copy]). An inline function or variable shall be defined in every translation unit in which it is odr-used outside of a discarded statement.

/../

There can be more than one definition of a class type, enumeration type, inline function with external linkage ([dcl.inline]), inline variable with external linkage ([dcl.inline]), class template, non-static function template, static data member of a class template, member function of a class template, or template specialization for which some template parameters are not specified ([temp.spec], [temp.class.spec]) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements.

没有明显原因得到 "Multiple definition"

Getting "Multiple definition" with no apparent reason

c++

multiple-definition-error