C ++ fstream提示行为

Question

I am working on a C++ application in Visual Studio 2012 (32-bit). When I read a file using fstream and read four bytes twice, I get confusing values from tellg. I was expecting 0, 4 and 8.

std::fstream file;
file.open(filename, std::ios::in , std::ios::binary);
if ( !file.is_open())
{
    throw exception("Error opening file for reading");
}
int pos = file.tellg();  // pos is 0
boost::int32_t usedBlocks;
int size = sizeof (usedBlocks);
file.read(reinterpret_cast<char*>(&usedBlocks),sizeof(usedBlocks));
pos = file.tellg();      //pos is 3588
//Read reserved size
file.read(reinterpret_cast<char*>(&reservedSize),sizeof(reservedSize));
pos = file.tellg();     //pos is 3592

Why does this happen?

I have changed the code to use fopen, fread, and ftell, and then the pos values are 0, 4 and 8.

usedBlocks is a boost::int32 . boost::int32 is actually an int , not a struct. Even changing them to int gives the same result.

Answer 1

If you are looking at the values of pos in the debugger, they may be wrong due to optimization.

Try printing the values of pos into the standard output.