Read/Write struct containing string property to binary file

问题

I am running in to an issue while reading/writing a struct having complex data specifically string. my struct looks like below.

struct MyRecord {
  char name[80];
  string location;     // <<== note this 
  double balance;
  unsigned long account_num;
};

I use different functions for reading and writing.

This is the code I am using to write the data to file

struct MyRecord acc;

strcpy(acc.name, "R");
acc.location = "newlocation ok";    // <<== note this 
acc.balance = 1.3;
acc.account_num = 34;

ofstream outbal("balance", ios::out | ios::binary);
outbal.write((char *) &acc, sizeof(struct MyRecord));

same like I am using the below code to read from file.

ifstream inbal("balance", ios::in | ios::binary);
inbal.read((char *) &acc, sizeof(struct MyRecord));

when I compile, it compiles nicely, but when executing I am getting the big list of error messages

./binaryfile_3[0x8048f6a]
./binaryfile_3[0x8048e38]
./binaryfile_3[0x8048e54]
/lib/libc.so.6(__libc_start_main+0xe6)[0x148d26]
./binaryfile_3[0x8048aa1]
======= Memory map: ========
00132000-002c3000 r-xp 00000000 fd:00 918500     /lib/libc-2.12.so
// continues....

although, it works in following 2 conditions.

Read write are in same function
If I remove the string location

I am new to c++, I tried writing using the manual and ended up in error. I searched through several solution and all end up in error while adding the string property. The above script also I grabbed from a site. I just modified it to match my issue which having different function to read/write and struct having string

Please help me to pin point the issue. Thanks.

回答1:

Add these two functions

std::ostream & operator << ( std::ostream & os, const MyRecord & rec ) {
    os.write( rec.name, sizeof( rec.name ) );
    size_t sz = rec.location.size();
    os.write( & sz, iszeof( sz ) );
    os.write( rec.location.c_str(), sz );
    os.write( (char*) & balance, sizeof( rec.balance ) );
    os.write( (char*) & account_num, sizeof( rec.account_num ) );
    return os;
}
std::istream & operator >> ( std::ostream & is, MyRecord & rec ) {
    is.read( rec.name, sizeof( rec.name ) );
    size_t sz;
    is.read( & sz, iszeof( sz ) );
    rec.resize( sz );
    if ( sz != 0 ) {
        is.read( & rec.location[ 0 ], sz );
    }
    is.read( (char*) & balance, sizeof( rec.balance ) );
    is.read( (char*) & account_num, sizeof( rec.account_num ) );
}

And here is a usage

ofstream outbal("balance", ios::out | ios::binary);
outbal << acc;

ifstream inbal("balance", ios::in | ios::binary);
inbal >> acc;

This is poor handmade similarity of Boost.Serialization. Indeed, it would be better to use boost, then my vehicle.

来源：https://stackoverflow.com/questions/22961017/read-write-struct-containing-string-property-to-binary-file

标签

c++

string

struct

ifstream

ofstream