Implementing a four-bit adder with C++ template metaprogramming

I recently read a post by Phillip Larkson where the C preprocessor was used to implement a four-bit adder entirely at compile time. This got me wondering whether I could implement the same concept using C++ template metaprogramming. It seemed theoretically possible as all the components can be calculated at runtime, but I wanted to avoid making use of the preprocessor for anything but supplying the original values to be added. The goal was to compile something on the command line like:

clang -DNUMBER_1=5 -DNUMBER_2=7 foo.cpp

and have the value 12 written out to the command line.
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Interesting Note About the sizeof Operator

The expression used in a sizeof operator is an unevaluated expression in C and C++. This can make for some surprising situations if you are unaware of it. For instance:

#include <stdio.h>

int main( void ) {
  int a = 12;
  int b = sizeof( ++a );
  printf( "%d\n", a );
  return 0;

This code will print 12 instead of 13 because the expression ++a is unevaluated.

So what does it mean for an expression to be unevaluated? Basically, it means that the expression is used at compile time only as a way to determine a type, which is then used to evaluate the result of the sizeof operator. So in the above example, a’s type is determined, and the resulting type for ++ is determined, but no code is generated to execute the ++.

Here’s an abusive example that demonstrates this:

#include <stdio.h>

class c {
  double operator++();

int main(int argc, char *argv[]){
  c c1;
  int i = sizeof( ++c1 );
  int j = sizeof( c1 );
  ::printf( "%d, %d\n", i, j );
  return 0;

If you run this code, you will see 8, 1 printed and will not get a link error (because of the failure to define operator++)!

Posted in C/C++ | Tagged , | 3 Comments

MSVC Pointer Type Attributes

One of the lesser-known features of Visual Studio’s C/C++ compiler are the pointer type attributes __ptr32 and __ptr64. More information about them can be found on MSDN. These pointer type attributes are used to control the visible size and behavior of pointers in 32- and 64-bit applications. Their usage is a bit strange, but if you need to do interop between 32- and 64-bit mode, they can be handy features to have. Additionally, there are the __sptr and __uptr qualifiers which allow you to specify how the pointer types are extended. __sptr denotes sign extension, and __uptr denotes zero extension. Information about these qualifiers can also be found on MSDN.
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Posted in C/C++, Win32 | 1 Comment

Small Break in the Silence

Just because I’ve not written many posts lately doesn’t mean I’ve been silent. You should go check out the Tidbits page, it currently has over 20 little juicy pieces of information about C and C++. I’ve been using it as a training exercise for my coworkers, and it’s been very successful so far. I’ll likely be continuing with regular Friday updates of Tidbits for quite a while to come.

I do hope to write some more full-length blog posts now that life is starting to settle back down again. So I’m not dormant, honest!

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How Variable Argument Lists Work in C

Variable argument lists are very arcane in the world of C. You’ll see them expressed in function signatures as … at the end of the parameter list, but you may not understand how they work or what they do.
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Source Indexing SVN Repositories is Broken

Some of us spend a fair amount of time pouring through crash dumps generated on Windows. For us, the symbol server support provided by Microsoft’s debugging engines is a godsend. However, source indexing is an even bigger boon because it allows us to not only see the symbols within the crash, but be able to pull the exact source down for source-level debugging.

Unfortunately, it seems that SVN 1.7 broke this functionality! Since it is so critical to my daily workflow, I set about fixing it.
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Value Types in C++11

You may have heard these terms used for various programming languages before, but I wanted to discuss them in a bit more detail since they’re a fairly fundamental concept in compilers that spill over into the way you use the languages themselves.
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Now, With More Tidbits!

Sorry about the distinct lack of content lately, but I’ve been busy putting together a new training initiative for my day job. This initiative involves sending weekly snippets of information on C and C++ to many of our developers. The content is too small to justify a full blog post in many cases, but is interesting enough to warrant discussion. As a content management system, I have decided to collate these tidbits here on Ruminations. You can find all of them here: Be sure to check back every Friday, as that’s when I’ll be posting new tidbits. If you have suggestions for tidbits you’d like to see, or want clarification on existing tidbits, feel free to leave comments on the Tidbits post.

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Theory and Reality

One thing which I am pretty religious about is the placement of ++ and — in an expression. You have two options for where it can go. If it goes before the operand, it’s a pre-increment/decrement. If it goes after the operand, it’s a post-increment/decrement. They have different semantics, and so there are correctness issues with their placement. But what I want to talk about are the cases where there are not correctness issues, and what should happen.
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Memory Management in Frameworks

As a framework designer, you have a lot of things to worry about. Calling conventions, size compatibility, structure layout, etc. I’d like to briefly talk about another thing to worry about: memory management. I’m not just talking about “please don’t leak memory.” I’m talking about keeping a proper division of labor, and why it’s crucial for frameworks.
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Posted in Framework Design | Tagged | 1 Comment