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[英]c++ read and write struct array with std::string into binary file
[英]How to read/write string type member of a struct using binary file in/out in c++?
我有 2 c++ 代碼:一個用於將數據寫入二進制文件,另一個用於讀取該文件。
write.cpp
代碼如下:
#include <iostream>
#include <fstream>
using namespace std;
const int NAME_SIZE = 51;
struct Data
{
char name[NAME_SIZE];
int age;
};
int main()
{
Data person;
char again;
fstream people("people.db", ios::out | ios::binary);
do
{
cout << "Enter the following data about a "<< "person:\n";
cout << "Name: ";
cin.getline(person.name, NAME_SIZE);
cout << "Age: ";
cin >> person.age;
cin.ignore();
people.write(reinterpret_cast<char *>(&person),sizeof(person));
cout << "Do you want to enter another record? ";
cin >> again;
cin.ignore();
} while (again == 'Y' || again == 'y');
people.close();
return 0;
}
read.cpp
代碼如下:
#include <iostream>
#include <fstream>
using namespace std;
const int NAME_SIZE = 51;
struct Data
{
char name[NAME_SIZE];
int age;
};
int main()
{
Data person;
char again;
fstream people;
people.open("people.db", ios::in | ios::binary);
if (!people)
{
cout << "Error opening file. Program aborting.\n";
return 0;
}
cout << "Here are the people in the file:\n\n";
people.read(reinterpret_cast<char *>(&person),sizeof(person));
while (!people.eof())
{
cout << "Name: ";
cout << person.name << endl;
cout << "Age: ";
cout << person.age << endl;
cout << "\nPress the Enter key to see the next record.\n";
cin.get(again);
people.read(reinterpret_cast<char *>(&person),sizeof(person));
}
cout << "That's all the data in the file!\n";
people.close();
return 0;
}
上面提到的代碼工作正常。 當我在結構中使用字符串類型成員時出現問題:
新的write.cpp
:
#include <iostream>
#include <fstream>
using namespace std;
struct Data
{
string name;
int age;
};
int main()
{
Data person;
char again;
fstream people("people.db", ios::out | ios::binary);
do
{
cout << "Enter the following data about a "<< "person:\n";
cout << "Name: ";
cin>>person.name;
cout << "Age: ";
cin >> person.age;
cin.ignore();
people.write(reinterpret_cast<char *>(&person),sizeof(person));
cout << "Do you want to enter another record? ";
cin >> again;
cin.ignore();
} while (again == 'Y' || again == 'y');
people.close();
return 0;
}
新的read.cpp
:
#include <iostream>
#include <fstream>
using namespace std;
struct Data
{
string name;
int age;
};
int main()
{
Data person;
char again;
fstream people;
people.open("people.db", ios::in | ios::binary);
if (!people)
{
cout << "Error opening file. Program aborting.\n";
return 0;
}
cout << "Here are the people in the file:\n\n";
people.read(reinterpret_cast<char *>(&person),sizeof(person));
while (!people.eof())
{
cout << "Name: ";
cout << person.name << endl;
cout << "Age: ";
cout << person.age << endl;
cout << "\nPress the Enter key to see the next record.\n";
cin.get(again);
people.read(reinterpret_cast<char *>(&person),sizeof(person));
}
cout << "That's all the data in the file!\n";
people.close();
return 0;
}
現在,當我運行read.cpp
時,程序無法讀取字符串並且程序崩潰了。 我必須使用字符串作為結構的成員。 如何解決這個問題呢?
想到的唯一方法是分別寫入以下數據:
並分別閱讀。
創建用於寫入/讀取Data
實例的函數,以使它們知道彼此的實現策略。
std::ostream& write(std::ostream& out, Data const& data)
{
size_t len = data.name.size();
out.write(reinterpret_cast<char const*>(&len), sizeof(len));
out.write(data.name.c_str(), len);
out.write(reinterpret_cast<char const*>(&data.age));
return out;
}
std::istream& read(std::istream& in, Data& data)
{
size_t len;
in.read(reinterpret_cast<char*>(&len), sizeof(len));
char* name = new char[len+1];
in.read(name, len);
name[len] = '\0';
data.name = name;
delete [] name;
in.read(reinterpret_cast<char*>(&data.age));
return in;
}
並與您的第一種方法類似地使用它們。
而不是使用
people.write(reinterpret_cast<char *>(&person),sizeof(person));
采用
write(people, person);
而不是使用
people.read(reinterpret_cast<char *>(&person),sizeof(person));
采用
read(people, person);
一個問題是sizeof(person.Name)
不能提供您認為的功能。 無論您分配給person.Name字符串是什么字符,它始終具有相同的大小(在我的情況下為28個字節)。 這是因為std :: string至少包含:
因此,您無法調用people.write(reinterpret_cast<char *>(&person),sizeof(person));
。 字符串的內容不位於&person
(它位於std :: string中的指針所指向的位置)
因此,當執行cout << person.name << endl;
時會發生什么? 從您的文件中讀取后? 當您將人寫到people.db時,您實際上已經讀取了person.name
的字符串指針指向的地址(而不是內容)。 再次從文件中讀取后,這當然不是有效的內存位置。
以下代碼片段可能對您的情況有所幫助。 可以使用定界符和預定義的字符串長度來代替寫入字符串的長度。
constexpr char delimiter = '\0';
constexpr uint32_t maxStringSize = 1024;
struct Data
{
string name;
int age;
};
寫入文件時,在字符串后放置一個delimiter
。
假設我們有一個Data structure {"John", 42}
那么我們會這樣寫:
std::ofstream outStream(filename, std::ios::binary);
outStream << structure.name << delimiter << structure.age;
outStream.close();
讀取文件不是寫入的鏡像(不幸的是)。
我們將使用std::ifstream::getline
在不知道字符串大小的情況下讀取字符串。 (省略錯誤檢查)
std::ifstream istrm(filename, std::ios::binary);
Data dataRead;
// string input - use a buffer and look for the next delimiter
char* buf = new char[maxStringSize];
istrm.getline(buf, maxStringSize, delimiter);
dataRead.name = std::string(buf);
// the number input
istrm >> dataRead.age;
有關如何讀/寫此struct
的向量的靈感,您可以查看我的存儲庫。
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