如何在c++中读取二进制文件中的小端整数

Question

我一直在尝试读取一个小端二进制文件。我希望首先找到文件中字符串的索引，从那里我可以开始读取数据。一旦获得索引，我将寻求提取学生信息。我成功地读取了字符串（名字和姓氏）。但是，我无法提取整数（年龄）。我感谢你的帮助:)

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

using namespace std;

class BinaryData
{
private:
   int      age;
   string   firstName;
   string   lastName;  
public:
   BinaryData(){}
   ~BinaryData(){}
   void readStudentData(string fileName)
   {
      //open the file and get its size
      ifstream in(fileName.c_str(), ios::in | ios::binary);
      in.seekg(0, ios::end);
      int fileSize = in.tellg();
      in.seekg(0, ios::beg);

      //read the entire file into a buffer
      string studentBuffer;
      studentBuffer.resize(fileSize);
      in.read(&studentBuffer[0], studentBuffer.size());
      
      //move to the position where to start reading student info
      int index = studentBuffer.find("StudentInfo");
      if(index != 0 && index != std::string::npos)
      {
          //read the student info if the header is found
          age = studentBuffer.substr(index, 3); //assume we are dedicating 3 characters for age
          index += 3;
          firstName = studentBuffer.substr(index, 10); //assume we are dedicating 10 characters for first name
          index += 10;
          lastName = studentBuffer.substr(index, 10); //assume we are dedicating 10 characters for first name 
          std::cout<< "The student name is: " << firstName << " " << lastName << ", they are " << age << " years old" << std::endl;
      }
      in.close();
   }
}

我的代码的输出是：

The student name is: John Doe, they are **(some character)** years old

年龄数据看起来像：二进制文件中的 13 00 00 00 应该转换为 19，因为这是小尾数。

如有任何帮助，我们将不胜感激。谢谢！

Answer 1

以下是与系统无关的转换整数的代码：

#include <iostream>
#include <string>
#include <cstdint>
#include <stdexcept>

/**
 * Converts a string containing binary data into a 64-bit unsigned integer.
 * The string is interpreted in little endian format.
 * If the string contains fewer than 1 byte or more than 8 bytes, an exception is thrown.
 * 
 * @param binary_data The string containing the binary data.
 * @return The 64-bit unsigned integer representation of the binary data.
 */
uint64_t convertToUInt64(const std::string& binary_data) {
    if (binary_data.empty()) {
        throw std::invalid_argument("Input string must contain at least 1 byte.");
    }
    if (binary_data.size() > sizeof(uint64_t)) {
        throw std::length_error("Input string must contain no more than 8 bytes.");
    }

    uint64_t value = 0;

    // Iterate over the string to construct the integer
    for (size_t i = 0; i < binary_data.size(); ++i) {
        // Cast each character to an unsigned byte and then shift it to the correct position
        value |= static_cast<uint64_t>(static_cast<uint8_t>(binary_data[i])) << (i * 8);
    }

    return value;
}

您可以使用

string

中的示例二进制数据来调用它：

int main() {
    // Binary data in a string, representing the value 19 in 3 bytes (little endian)
    std::string binary_data = "\x13\x00\x00";

    // Convert the binary data to a 64-bit unsigned integer
    uint64_t value = convertToUInt64(binary_data);

    // Output the integer value
    std::cout << "The 64-bit unsigned integer value is: " << value << std::endl;

    return 0;
}

我展示了一个通用函数，而不是仅接受 3 个字节的函数，以防还使用其他大小的字符串。如果你想要一个更小的整数，你可以检查该整数是否低于某个特定大小，然后分配。