writing structures using fstream in c++

Question

Please tell me that the following line writes the address of the structure variable to the file or it writes the values of members of the structure.

 file_write.write((char*)&structure_data,sizeOfStructure);

Where file_write is fstream's object and "structure_data" is a struct having 3 members of integer datatypes. Thanks.

Answer 1

This line:

file_write.write((char*)&structure_data,sizeOfStructure);

takes whatever structure_data is, and just copies those blob of bytes that makes up structure_data to a file.

It doesn't figure out what the members are. Also, this is the cause for thousands of SO questions that erroneously do coding like this, where structure_data cannot be written to a file this way and have the file contents make sense. It is quickly discovered that the contents of the file are useless when an attempt to read back the data into a program is unsuccessful.

Most of the time in those scenarios structure_data would contain pointers, or members that are not C-layout compatible, i.e. non-POD types such as std::string or std::vector, that basically renders this technique of writing to a file like this totally useless (and invalid).

Look up object serialization such as this link on the topic