C++ Truths

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for...

With my growing constexpr fascination, I thought of using it for something that would be really hard using template meta-programs. How about implementing a compile-time regular expression matcher? Fortunately, a simple regular expression matcher has already been written by Rob Pike . I just rewrote it using constexpr: single return statement in functions, no modifications to the parameters, abundant ternery operators, and recursion. Here we go... constexpr int match_c(const char *regexp, const char *text); constexpr int matchhere_c(const char *regexp, const char *text); constexpr int matchstar_c(int c, const char *regexp, const char *text); constexpr int matchend_c(const char * regexp, const char * text); constexpr int matchend_c(const char * regexp, const char * text) { return matchhere_c(regexp, text) ? 1 : (*text == '\0') ? 0 : matchend_c(regexp, text+1); } constexpr int match_c(const char *regexp, const char *text) { return (regexp[0] == '^') ? matchher...

It's official: C++11 has two meta-programming languages embedded in it! One is based on templates and other one using constexpr . Templates have been extensively used for meta-programming in C++03. C++11 now gives you one more option of writing compile-time meta-programs using constexpr . The capabilities differ, however. The meta-programming language that uses templates was discovered accidently and since then countless techniques have been developed. It is a pure functional language which allows you to manipulate compile-time integral literals and types but not floating point literals. Most people find the syntax of template meta-programming quite abominable because meta-functions must be implemented as structures and nested typedefs. Compile-time performance is also a pain point for this language feature. The generalized constant expressions (constexpr for short) feature allows C++11 compiler to peek into the implementation of a function (even classes) and perform optimization...

BoostCon'11 , held in Aspen, Colorado, was a fantastic conference this year. Not only because I got a chance to present my work on LEESA but also because of the breadth and depth of the topics covered . LEESA, as you may recall, is an embedded language in C++ to simplify XML programming. LEESA's programming model sits on top of the APIs generated by modern XML data binding tools . LEESA gives you XPath-like syntax (wildcards, child-axis, descendant-axis, tuples) to simplify data extraction from an XML object model. I had a privilege to talk at length about LEESA in BoostCon'11. In the 1hr 41 minutes long video, I'm talking everything from why you need LEESA, how its implemented using cool C++ techniques, such as templates, meta-programming, compile-time and run-time performance, and what direction it may take in future. Here are the slides of the presentation.