Question

I’m trying to get fixed sized floats and ints across all windows computers. As in, I have a program, I compile it and distribute the executable. I want the data types to be of constant bit size/ordering across all windows computers.

My first question is whether windows types defined at http://msdn.microsoft.com/en-us/library/aa383751(v=vs.85).aspx have fixed sizes across all windows computers (let’s say running the same OS- Windows 7).

Basically, I’d like to transfer data contained in a struct over a network and I want to avoid having to put it all in a string or encode it into a portable binary form.

Edit: What about floats??

Answer 1

Floating Point

While there is no C++ standard defining the sizes for formats of floating point values Microsoft has specified that they consistently use 4-byte and 8-byte IEEE floating point format for float and double types respectively.

Integrals

As for integral types, Microsoft does have compiler-specific defines for fixed length variables. Some non-Microsoft compilers define fixed-size integral types using the cstdint header. Neither of these are based on official standards.

Serialization

This will be terribly unportable and will most likely turn into a maintenance nightmare as your structs get more complicated. What you are effectively doing is defining an error-prone binary serialization format that must be complied with through convention. This problem has already been solved more effectively.

I would highly recommend using a serialization format like protocol buffers or maybe boost::serialization for communication between machines. If your data is hitting the wire, then the performance of serialization/deserialization is going to be an incredibly small fraction of transmission time.

Alignment

Another serious issue that you’ll have is how the struct is packed in memory. Your struct will most likely be laid-out in memory differently in a 32-bit process than it is in a 64-bit process.

In a 32-bit process, your struct members will be aligned on word boundaries, and on doubleword boundaries for 64-bit.

For example, this program outputs 20 on 32-bit and 24 on 64-bit platforms:

#include <iostream>                                                                                                                                             
#include <cstdint>

struct mystruct {
        uint32_t y;
        double z;
        uint8_t c;
        float v;
} mystruct_t;

int main() {
        std::cout << sizeof(mystruct_t);
}