从二进制文件中将 unsigned int 读入 uint64_t

Question

I am trying to read a binary file using fstream. I know the types of everything in this file but there are two different versions, an old and a new version. The old version contains variables of type 'unsigned int' deemed to have insufficient range, and are replaced by 'uint64_t' in the new version. I'd like one file reader capable of reading both formats(for backwards compatibility). The read result should always be a uint64_t.

I am sure this is a common problem. I've tried simply reading 4 bytes from an ifstream into the memory of the uint64_t, comparable to the snippet below(without checking for any problems):

std::uint64_t buff=0;
std::ifstream is(filePath, std::ios_base::binary)
std::is.read(&buff,sizeof(unsigned int));

This gives me the correct value but I realize this might be subject to endianness. Is there a common method to solve this problem?

Answer 1

You need to read it into a variable of the correct type and then you can assign it to your uint64_t variable.

static_assert(std::endian::native == std::endian::big ||
              std::endian::native == std::endian::little);
                
constexpr auto file_endianess = std::endian::big; // or little

std::uint64_t buff = 0;
std::ifstream is(filePath, std::ios_base::binary);
if (is) {
    std::uint32_t temp;
    if (is.read(reinterpret_cast<char*>(&temp), sizeof temp)) {
        
        // ... deal with endianess here ...
        // (example from C++23 below)
        #ifdef __cpp_lib_byteswap
        if constexpr(std::endian::native != file_endianess) {
            temp = std::byteswap(temp);
        }
        #else
        // an alternative endianess solution goes here
        #endif

        buff = temp;
    }
}