C ++将结构附加到二进制文件中

Question

I have my struct:

struct a
{
  int    x;
  float  f;
  double d;
  char   c;
  char   s[50];
};

and I wish append each time into my timer schedule into a binary file.

// declaration
std::ofstream outFile;

// constructor:
outFile.open( "save.dat", ios::app );

// tick:
outFile << a << endl;

but inside the save.dat appears only this:

0C3A0000..0C3A0000..0C3A0000..0C3A0000..0C3A0000..0C3A0000..0C3A0000..0C3A0000..0C3A0000..

thanks in advance

Answer 1

What you're currently doing is writing the address of the struct definition.
What you want to do is use ostream::write

outfile.write(reinterpret_cast<char*>(&myStruct), sizeof(a));

This will work as long as your struct is a POD (Plain Old Data) type (which your example is). POD type means that all members are of fixed size.

If you on the other hand have variable sized members then you would need to write out each member one by one.

Answer 2

A sensible way to serialize custom objects is to overload your own output stream operator:

std::ostream & operator<<(std::ostream & o, const a & x)
{
  o.write(reinterpret_cast<char*>(&x.x), sizeof(int));
  o.write(reinterpret_cast<char*>(&x.f), sizeof(float));
  /* ... */
  return o;
}

a x;
std::ofstream ofile("myfile.bin", std::ios::binary | std::ios::app);
ofile << a;

This is still platform-dependent, so to be a bit safer, you should probably use fixed-width data types like int32_t etc.

It might also not be the best idea semantically to use << for binary output, since it's often used for formatted output. Perhaps a slightly safer method would be to write a function void serialize(const a &, std::ostream &);