Read binary bytes beginning at position N within ifstream? - c++

Question

I am writing an unknown number of structs to a binary file and then reinterpret_cast-ing the bytes back to the struct. I know how to write the bytes.
I am unsure how to iterate over the binary file. I would like to use std::ifstream. At some point I must need to increment a file pointer/index by sizeof(struct) bytes, but the only examples (of reading binary in to structs) I could find online were writing N structs and then reading N structs, they were not looping over the file, incrementing any file index.
Pseudo code of what I would like to achieve is:
std::ifstream file("test.txt", std::ifstream::binary);
const size_t fileLength = file.size();
size_t pos = 0;
while(pos < fileLength)
{
MyStruct* ms = &(reinterpret_cast<MyStruct&>(&file[pos]));
// Do whatever with my struct
pos += sizeof(MyStruct);
}
UPDATE:
My struct is POD

Answer 1

#include <fstream>
struct MyStruct{};
int main()
{
std::ifstream file("test.txt", std::ifstream::binary);
MyStruct ms;
//Evaluates to false if anything wrong happened.
while(file.read(reinterpret_cast<char*>(&ms),sizeof ms))
{
// Do whatever with my struct
}
if(file.eof())
;//Successfully iterated over the whole file
}
Please be sure not to do something like this:
char buffer[sizeof(MyStruct)];
file.read(buffer,sizeof(MyStruct));
//...
MyStruct* myStruct = reinterpret_cast<MyStruct*>(buffer);
It will likely work, but it breaks the aliasing rule and is undefined behaviour. If you truly need a buffer ( e.g. for small files it might be faster to read the whole file into the memory first and then iterate over that buffer) then the correct way is:
char buffer[sizeof(MyStruct)];
file.read(buffer,sizeof(MyStruct));
//...
MyStruct myStruct;
std::memcpy(&myStruct,buffer,sizeof myStruct);