Question

我正在尝试反向读取二进制文件（“example.dat”）并使用其内容填充记录结构。该文件包含10条记录，每条记录有三种数据类型。

#include <iostream>
#include <fstream>

using namespace std;

/* Gross Yearly Income */
const unsigned long int GYI = sizeof( unsigned long int );

/* Amortization Period in years as an unsigned integer */
const unsigned int APY = sizeof( unsigned int );

/* Year ly interest rate in double precision */
const double annualInterest = sizeof( double );

/*This is where I attempt to determine the size of the file and most likely a huge fail. */

/* Attempting to obtain file size */
const int RECORD_SIZE = GYI + APY + annualInterest;

/* There are ten records*/
const int RECORDS = 10;

struct record_t
{    
    unsigned long int grossAnnualIncome;
    unsigned int amortizationPeriod;
    double interestRate;

} total[RECORDS]; // a total of ten records

void printrecord (record_t *record);

int main()
{   
    record_t *details = new record_t[RECORDS];

    ifstream file; /* mortgage file containing records */

    file.open( "mortgage.dat", ios::binary );

/*This for loop is an attempt to read the .dat file and store the values found into the relevant    struct*/

    for ( int i = 0; i < RECORDS; i++)
    {           
        file.seekg( -( i + 1 ) * RECORD_SIZE, file.end);
        file.read( ( char * )( &details[i].grossAnnualIncome ), GYI );
        file.read( ( char * )( &details[i].amortizationPeriod ), APY );
        file.read( ( char * )( &details[i].interestRate ), annualInterest );    

        cout << i << " : " ; printrecord(details);
    }

    file.close();       

    return 0;       
}    

/* Display the file records according to data type */

void printrecord (record_t *record)
{
    cout << record -> grossAnnualIncome << endl;
    cout << record -> amortizationPeriod << endl;
    cout << record -> interestRate << endl;
}

/ *感谢任何帮助和反馈。 * /

Answer 1

为什么你会得到如此奇怪的数字，例如我看不到的利率。但是，为每个条目获取相同值的原因是因为行

cout << i << " : " ; printrecord(details);

始终在details中打印第一个条目。如果您将其更改为：

cout << i << " : " ; printrecord(details + i);

它会打印记录到details的实际值。

这样做的原因是数组的标识符将表现为指向数组的第一个元素的指针。此外，您可以对此指针执行指针运算。因此，以下两个陈述是等效的。

details + i
&details[i]
// This last one is just for fun, but is actually also equivalent to the other two.
&[i]details

使用二进制文件填充结构

1 个答案: