首先,我已经使该程序在一个cpp文件下正常运行,但是问题是按每个函数和头文件划分了该程序-正如我的课堂讲师在课堂上告诉我的那样,我试图将结构定义包含在头文件中,但我不断收到各种错误消息。我当前的头文件代码如下:

extern void threeLargest(Country countries[], Country fastGrowing[]);
extern void readData(Country countries[]);
extern void negGrowth(Country countries[]);

const int MAXCOUNTRIES = 229;
const int THREE = 3;
struct Country {
    string name;
    double pop1950;
    double pop1970;
    double pop1990;
    double pop2010;
    double pop2015;
    double growthRate;
    };
struct World {
    int numCountries;
    Country countries[MAXCOUNTRIES];
    Country fastGrowing[THREE];
    } myWorld;


现在,它给了我一个错误,内容如下(我只带了其中一些,你会明白为什么):

In file included from lab10_0.cpp:1:0:
lab10.h:6:2: error: ‘string’ does not name a type
  string name;
  ^
lab10_0.cpp: In function ‘void readData(Country*)’:
lab10_0.cpp:17:37: error: ‘struct Country’ has no member named ‘name’
  getline(ifstr,myWorld.countries[i].name);


在我看来,头文件无法识别字符串类型,其他使用头文件的cpp文件也无法识别。所以,我尝试包括

#include <string>
using namespace std;


在头文件的开头,但是我得到了完全不同的错误消息,说

/tmp/cclU6znx.o:(.bss+0x0): multiple definition of `myWorld'
/tmp/ccQ69Fio.o:(.bss+0x0): first defined here
/tmp/cckXoPSG.o:(.bss+0x0): multiple definition of `myWorld'
/tmp/ccQ69Fio.o:(.bss+0x0): first defined here
/tmp/cctaCWNQ.o:(.bss+0x0): multiple definition of `myWorld'
/tmp/ccQ69Fio.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status


我之前已经完成了将文件分离的操作,但是这次我必须在头文件中包含结构定义,而我却毫无头绪。请指教。实际上,我在进行谷歌搜索时尝试了很多事情,将头文件分为两个,一个头文件中的结构定义,另一个文件中的功能,但是还是没有运气。

请指教。

ps。如果需要,我可以发布完整程序。

================================================== ====================
经过多次对话并从中受益后添加了部分
我正在上传最初创建的原始cpp文件的代码,想知道这对于读者来说更容易观察到我的问题所在。

#include <string>
#include <iomanip>
#include <iostream>
#include <fstream>


using namespace std;

const int MAXCOUNTRIES = 229;
const int THREE = 3;

struct Country{
    string name;
    double pop1950;
    double pop1970;
    double pop1990;
    double pop2010;
    double pop2015;
    double growthRate;
    };
struct World{
    int numCountries;
    Country countries[MAXCOUNTRIES];
    Country fastGrowing[THREE];
    } myWorld;

void threeLargest(Country countries[], Country fastGrowing[]);
void readData(Country countries[]);
void negGrowth(Country countries[]);
int main() {

readData(myWorld.countries);
threeLargest(myWorld.countries,myWorld.fastGrowing);
negGrowth(myWorld.countries);

return 0;
}

void readData(Country countries[])
{

    fstream ifstr;
    ifstr.open("population.csv");

    for (int i=0; !(ifstr.eof()) && i < MAXCOUNTRIES; i++) {
    ifstr >> myWorld.countries[i].pop1950 >> myWorld.countries[i].pop1970
                >> myWorld.countries[i].pop1990 >> myWorld.countries[i].pop2010
                >> myWorld.countries[i].pop2015;
    getline(ifstr,myWorld.countries[i].name);
    myWorld.countries[i].growthRate = ((myWorld.countries[i].pop2015-myWorld.countries[i].pop1950)/myWorld.countries[i].pop1950)*100;}

    ifstr.close();
}

void threeLargest(Country countries[], Country fastGrowing[])
{
    myWorld.fastGrowing[THREE].growthRate = { };
    for (int i=0; i < MAXCOUNTRIES; i++) {
        if (myWorld.countries[i].growthRate > myWorld.fastGrowing[0].growthRate) {

            myWorld.fastGrowing[2].growthRate = myWorld.fastGrowing[1].growthRate;
            myWorld.fastGrowing[2].name = myWorld.fastGrowing[1].name;

            myWorld.fastGrowing[1].growthRate = myWorld.fastGrowing[0].growthRate;
            myWorld.fastGrowing[1].name = myWorld.fastGrowing[0].name;

            myWorld.fastGrowing[0].growthRate = myWorld.countries[i].growthRate;
            myWorld.fastGrowing[0].name = myWorld.countries[i].name;}

        else if (myWorld.countries[i].growthRate > myWorld.fastGrowing[1].growthRate) {
            myWorld.fastGrowing[2].growthRate = myWorld.fastGrowing[1].growthRate;
            myWorld.fastGrowing[2].name = myWorld.fastGrowing[1].name;

            myWorld.fastGrowing[1].growthRate = myWorld.countries[i].growthRate;
            myWorld.fastGrowing[1].name = myWorld.countries[i].name;}

        else if (myWorld.countries[i].growthRate > myWorld.fastGrowing[2].growthRate) {
            myWorld.fastGrowing[2].growthRate = myWorld.countries[i].growthRate;
            myWorld.fastGrowing[2].name = myWorld.countries[i].name;}
    }


    cout << "The fastest growing country is " << myWorld.fastGrowing[0].name << ", which grew by "
         << fixed << setprecision(2) << myWorld.fastGrowing[0].growthRate << "% between 1950 and 2015.\n"

         << "The 2nd fastest growing country is " << myWorld.fastGrowing[1].name << " which grew by "
         << fixed << setprecision(2) << myWorld.fastGrowing[1].growthRate << "% between 1950 and 2015.\n"

         << "The 3rd fastest growing country is " << myWorld.fastGrowing[2].name << " which grew by "
         << fixed << setprecision(2) << myWorld.fastGrowing[2].growthRate << "% between 1950 and 2015.\n";
}

void negGrowth(Country countries[])
{
    cout << "The following countries' population shrunk between 1950 and 2015:" << endl;
    for (int i=0; i < MAXCOUNTRIES; i++) {
        if (myWorld.countries[i].growthRate < 0)
        cout << myWorld.countries[i].name << " shrunk by " << fixed << setprecision(2) << myWorld.countries[i].growthRate << "%." << endl;}
}


================================================== ======
第二次编辑/我的头文件看起来像以下情况:

#ifndef LAB10_H
#define LAB10_H

#include <string>

const int MAXCOUNTRIES = 229;
const int THREE = 3;

struct Country {
  std::string name;
  double pop1950;
  double pop1970;
  double pop1990;
  double pop2010;
  double pop2015;
  double growthRate;
  };

struct World {
  int numCountries;
  Country countries[MAXCOUNTRIES];
  Country fastGrowing[THREE];
};
extern World myWorld;

extern void threeLargest(Country countries[], Country fastGrowing[]);
extern void readData(Country countries[]);
extern void negGrowth(Country countries[]);


万一

正如我在某些人的评论中提到的那样,在头文件中使用extern World myWorld时,我可以看到头文件上的结构定义开始起作用,但是却出现了几行说undefined reference to 'myWorld'的错误。因此,我尝试在所有cpp文件中都包含World myWorld(大多数每个cpp文件都包含一个函数),最后我可以编译该程序了。

但是,程序无法正常运行-变量未正确存储,并且所有计算均不正确。

我的意思是,我没想到这个过程会如此痛苦,让我头疼不已。

请指教 :(

最佳答案

正如编译器指出的那样,问题是变量多重声明。在头文件中,您只需要定义类型和宏,而不必定义变量(外部声明除外)。

在您的情况下,您定义了myWorld类型的struct World全局变量。

struct World {
    int numCountries;
    Country countries[MAXCOUNTRIES];
    Country fastGrowing[THREE];
} myWorld;


我想在您的实现文件(.cpp)中,您会看到以下内容:

#include "myheader.h"
...
World myWorld;


发生的情况是,cpp预处理程序读取每个cpp文件,并在实际编译之前用该文件的完整内容替换文本#include "myheader.h"。这意味着cpp编译器对于每个.cpp都会看到类似以下内容的内容:

struct World {
    int numCountries;
    Country countries[MAXCOUNTRIES];
    Country fastGrowing[THREE];
} myWorld;
...
World myWorld;


现在,您也可以看到同一变量的两个声明,就像de cpp编译器一样。

您需要删除这些声明之一,通常是标头的声明。

当有很多文件.cpp包含相同的文件myheader.h时,这是最糟糕的,它们都将定义一个具有相同名称的全局变量。

当需要在多个.cpp文件中使用相同的全局变量时,可以在头文件中包含该变量的定义,但要使用修饰符extern,例如:

struct World {
    int numCountries;
    Country countries[MAXCOUNTRIES];
    Country fastGrowing[THREE];
};
extern World myWorld;


这将使所有.cpp文件都知道变量myworld的存在。变量的实际定义必须仅在一个.cpp文件中,否则在链接阶段会出现错误。

关于c++ - 在头文件的结构定义中进行的字符串声明,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/40797034/

10-15 00:49
查看更多