Tag Archives: C++

Thunksgiving

Quoting Wikipedia:

The word thunk has at least three related meanings in computing science. A “thunk” may be:

  1. A piece of code to perform a delayed computation (similar to a closure)
  2. A feature of some virtual function table implementations (similar to a wrapper function)
  3. A mapping of machine data from one system-specific form to another, usually for compatibility reasons

In all three senses, the word refers to a piece of low-level code, usually machine-generated, that implements some detail of a particular software system.

In this post (whose name looks like an unrelated typo) we shall observe the need for a thunk of the second kind, in C++.

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Endianness and you

Programming is all about generalizations. We, as programmers, usually do not want to worry about all the small details; We will usually assume that there’s enough physical memory on the machine, we will knowingly use cross-platform libraries to make the operating system we’re running on irrelevant as well, and sometimes we will even resort to using programming languages that take these ideas to the extreme – like i.e. Java, which runs entirely on a virtual machine — making all above issues non-existent.

But there comes a time, especially in low-level programming languages (like C++ luckily is), when we simply cannot ignore certain low level details. One such example is the expected, architecture specific, Endianness.

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Overloading macros

The feature of function overloading can prove to be pretty useful: it allows us to define a few versions of the same function, which differ in argument types, or even in Arity (ignoring variadic functions for the moment). Unfortunately, the C\C++ pre-processor does not allow overloading macros in the same way; It treats such attempts as redefinitions.

While we do not really need to overload a macro in order to handle different argument types (since macros ignore type information), many times it would be desired to overload a macro such that each version is able to handle a different number of arguments. This goal can actually be achieved through invocation of the VA_NUM_ARGS macro mentioned in my previous post, as we will briefly demonstrate (the idea has also been mentioned under the comment section in the aforementioned post).

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Variadic macro to count number of arguments

The C-Preprocessor is a very powerful mechanism, which offers many different features. One of these features is called Variadic macros: macros that accept a varying number of arguments. It is interesting to note at this point, that such Variadic macros, despite being part of the C99 Standard, are not part of the C++ Standard at the moment. However, a big number of C++ compilers support it nevertheless.

While allowing the definition of Variadic macros, there is no built-in (preprocessor) way of obtaining the actual number of arguments that is passed to a specific Variadic macro. In this post we shall provide a possible macro implementation for such a query.

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