Storing string into file

[hughLg]

I wanna write a program that storing the following struct of data into the binary file:

struct staff {
   string id; // maximum 8 characters.
   string name; // maximum 35 characters.
   string ic; // identity card number, maximum 14 character.
   unsigned short age;
   string remark; // no limit.
} employee;

I was noticed that the byte returned by sizeof(string) being 28, even though the string variable contains more then 28 characters. I tried the following:

write((const char *)&employee, sizeof(employee));

But next time when i retrieve by using function read(char *, size_t), the string fields are not what i expected.

So I break out the above structure by separating it from string fields, like this:

struct staff1 {
   unsigned id;
   unsigned name;
   unsigned ic;
   unsigned remark;
   
   unsigned short age;
} employee;

struct {
   string id; // maximum 8 characters.
   string name; // maximum 35 characters.
   string ic; // identity card number, maximum 14 character.
   string remark; // no limit.
} s;

I use the following way to collecting data and write to tge file:

#include <fstream>
#include <string>
#include <iostream>
using namespace std;

struct staff1 {
   unsigned id;
   unsigned name;
   unsigned ic;
   unsigned remark;
   
   unsigned short age;
} employee;

struct {
   string id; // maximum 8 characters.
   string name; // maximum 35 characters.
   string ic; // identity card number, maximum 14 character.
   string remark; // no limit.
} s;

int main() {
	if (0) {
		ofstream oStrF("str.dat", ios::out | ios::binary);
		ofstream oEmployeeF("employee.dat", ios::out | ios::binary);
		s.id = "01010101";
		employee.id = (s.id.length() + 1); // extra byte for null terminator.
		s.name = "ABC";
		employee.name = (s.name.length() + 1); // extra byte for null terminator.
		
		s.ic = "801010-12-5165";
		employee.ic = (s.ic.length() + 1); // extra byte for null terminator.
		s.remark = "OK";
		employee.remark = (s.remark.length() + 1); // extra byte for null terminator.
		employee.age = 21;

		oStrF.write((const char *)s.id.c_str(), employee.id);
		oStrF.write((const char *)s.name.c_str(), employee.name);
		oStrF.write((const char *)s.ic.c_str(), employee.ic);
		oStrF.write((const char *)s.remark.c_str(), employee.remark);
		oEmployeeF.write((const char *)&employee, sizeof(employee));
	}
	else {
		ifstream iStrF("str.dat", ios::in | ios::binary);
		ifstream iEmployeeF("employee.dat", ios::in | ios::binary);
		iEmployeeF.read((char *)&employee, sizeof(employee));
		// id.
		char *buf = new char[employee.id];
		iStrF.read((char *)buf, employee.id);
		cout << "ID: " << buf << endl;
		delete buf;
		// name.
		buf = new char[employee.name];
		iStrF.read((char *)buf, employee.name);
		cout << "NAME: " << buf << endl;
		delete buf;
		// ic.
		buf = new char[employee.ic];
		iStrF.read((char *)buf, employee.ic);
		cout << "IC: " << buf << endl;
		delete buf;
		// remark.
		buf = new char[employee.remark];
		iStrF.read((char *)buf, employee.remark);
		cout << "REMARK: " << buf << endl;
		delete buf;
		cout << employee.age << endl;
	}
}

Any better solution, i can't found any useful book for the I/O for it.

 

[Salem]

http://www.parashift.com/c++-faq-lite/serialization.html

--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[hughLg]

I still cannot get the answer. The problem is, the string size can only be known at runtime, otherwise I can just use the character array (fixed size) to eliminate all the problem. But doing that way will waste up space in the file.

 

[ArkM]

std::string binary serialization (unsafe!) variant:

void StrTo(const std::string& s, std::ostream& f)
{
  int sz = s.size();
  f.write((const char*)&sz,sizeof sz);
  if (sz)
     f.write(s.c_str(),sz);
}

void StrFrom(std::string& s, std::istream& f)
{
  int sz;
  f.read((char*)&sz,sizeof sz);
  if (sz > 0)
  {
     char* p = new char[sz];
     f.read(p,sz);
     s.assign(p,sz);
     delete [] p; // See [] before pointer in delete array.
  }
  else
     s.erase();
}

Add i/o error and EoF handling.
Binary mode stream assumed.
Remember that std::string may contain zero bytes.

 

[uolj]

Use data() instead of c_str(). Change StrFrom to use vector:

void StrTo(const std::string& s, std::ostream& f)
{
  int sz = s.size();
  f.write((const char*)&sz,sizeof sz);
  if (sz)
     f.write(s.data(),sz);
}

void StrFrom(std::string& s, std::istream& f)
{
  int sz;
  f.read((char*)&sz,sizeof sz);
  if (sz > 0)
  {
     std::vector<char> p(sz);
     f.read(p,sz);
     s.assign(p.begin(),p.end());
  }
  else
     s.erase();
}