我有一个名为"obj"的类,它有两个数据类型,一个是整型,一个是双精度。我试着读第三个对象,但似乎不能弄清楚。在我将其中一个数据类型更改为double之前,它正在工作。我觉得这和角色的类型有关。总而言之,我不能让它在文件写入后只输出第三个对象。有什么建议吗?
#include<iostream>
#include<fstream>
using namespace std;
class Data {
public:
int num1;
double num2;
Data() {}
~Data() {}
void setData();
void getData();
};
void Data::getData()
{
cout << "Enter first number: ";
cin >> num1;
cout << "Eneter second number: ";
cin >> num2;
}
#include "class.h"
const int SIZE = 5;
int main()
{
Data obj[SIZE];
for (int i = 0; i < SIZE; i++)
{
cout << "Enter numbers of object " << i+1 << endl;
obj[i].getData();
}
ofstream outFile;
outFile.open("C:\Users\juan\Desktop\26.bin", ios::out | ios::binary);
for (int i = 0; i < SIZE; i++)
{
outFile.write(reinterpret_cast<char *> (&obj[i].num1), sizeof(obj[i].num1));
outFile.write(reinterpret_cast<char *> (&obj[i].num2), sizeof(obj[i].num2));
}
cout << "Writing to file...." << endl;
outFile.close();
ifstream inFile;
inFile.open("C:\Users\juan\Desktop\26.bin", ios::in | ios::binary);
for (int i = 0; i < SIZE; i++)
{
inFile.read(reinterpret_cast<char *> (&obj[i].num1), sizeof(obj[i].num1));
inFile.read(reinterpret_cast<char *> (&obj[i].num2), sizeof(obj[i].num2));
}
for (int i = 0; i < SIZE; i++)
{
cout << obj[i].num1 << endl;
cout << obj[i].num2 << endl;
cout << endl << endl;
}
inFile.close();
Data third;
fstream seekfile;
seekfile.open("C:\Users\juan\Desktop\26.bin", ios::in | ios::binary);
seekfile.seekg(2 * sizeof(Data), ios::beg);
seekfile.read(reinterpret_cast<char *> (&third.num1), sizeof(third.num1));
seekfile.read(reinterpret_cast<char *> (&third.num2), sizeof(third.num2));
cout << endl << endl;
cout << third.num1 << endl;
cout << third.num2 << endl;
seekfile.close();
}
问题是sizeof(Data)
不是 sizeof(int) + sizeof(double)
的和,因此seekfile.seekg(2 * sizeof(Data), ios::beg)
是不正确的。这个
seekfile.seekg(2 * (sizeof(third.num1) + sizeof(third.num2))
应该能解决这个问题。
注意,由于填充,sizeof(Data)
大于其组成部分的总和。查看更多信息:
为什么结构体的n't sizeof等于每个成员的sizeof之和?
还请注意,如果您将num2
重新定义为int
,则不需要填充,在这种情况下,您的原始代码可以正常工作