How to output a std::map to a binary file?

Question

How can I output a std::map to a binary file?

The map declaration looks like this.

map accounts;

ofstream os(o         


        

Answer 1

Since you're storing it as pointers, you will have to iterate over the map, storing first the key, and then the pointed-to Account, one by one.

This seems like something that could be done better with a database. Especially if you're going to need to do this in more than one place.

The practice of doing this with objects is known as serialization.

If your Account class is what's known as a plain old data class (i.e. it contains no pointers and no classes or structs apart from other plain old data classes and structs) you can simply write its memory directly to a file. In that case, an approach like the following would be acceptable:

int32_t sizeAccount = sizeof(Account); // this should always be a 32 bit int
for (map::iterator i = accounts.begin(); i != accounts.end(); ++i)
{
    int32_t sizeStr = i->first.length() + 1; // this should always be a 32 bit int

    os.write(&sizeStr, sizeof(sizeStr)); // 4 byte length of string
    os.write(i->first.c_str(), sizeStr); // null terminated string

    os.write(&sizeAccount, sizeof(sizeAccount)); // 4 byte size of object
    os.write(i->second, sizeAccount);    // object data itself
}

If, however, your object has any pointer members, or any members of a type that have pointer members, or any subclasses or superclasses, or any members of types that have subclasses or superclasses, etc, this approach may not be sufficient and may yield either nonsensical or plain incorrect output.