Posts Tagged ‘visual c++


What is the size of a signed data type in Visual C++?

So, I was compiling some code the other day with warnings-as-errors enabled, like you do, and I hit a curious one:

 warning C4309: 'initializing' : truncation of constant value

This particular warning is generated when the code attempts to initialize (or assign, or convert) a variable (or parameter) to a value that is larger than the variable’s type can handle. The thing that makes this curious is that the line in question looks something like this:

short const tmp = 0xFFFF;

I’ll save you the bitmath and the Googling for the size of a short in Visual C++ and just tell you that, according to the documentation, a short is 2 bytes (16 bits) and so is 0xFFFF, which sets all 16 bits to 1. The above statement will generate the warning for any values of 0x8FFF and higher. Even more interesting, setting an unsigned short to 0xFFFF does not generate the warning. Does this mean that in VC++, signed shorts are actually 1 bit shorter than unsigned shorts??

Probably not. More likely, this just means that the compiler is being a bit aggressive with this warning. You see, Visual Studio (and g++, for that matter) has no objection to this code:

short const tmp = -0x0001

This is theoretically the exact same value as the first line, because VC++ uses two’s complement (which is not enforced by the C++ standard, by the way, but is used by most compilers) for the binary representation of signed numbers. My (mostly unsubstantiated) guess, though, is that the VC++ compiler throws this warning on the first statement but not the second because 65535, the unsigned value of 0xFFFF, would have to be be truncated to fit into a signed short (where the maximum allowable value is 32767), so the compiler would prefer it if you just used your negative sign to indicate your negativity, rather than the most significant bit, like any sane programmer would.

In this case, I think I’ll just suppress the warning.


My name is Maitland Lederer, and I’m a video game developer. I learn stuff you probably already knew and have opinions you've probably already heard. I figured it might be a good idea for me to start writing down the stuff I've learned so I don't have to relearn it. It's not, like, great wisdom or anything. It's just things I happened to learn, usually today.

Header photo by D Sharon Pruitt, used under a Creative Commons License.