On Wed, Jan 21, 2015 at 4:52 PM, Matt Calabrese <rivorus@gmail.com> wrote:
I've found it useful, especially in tests, to easily be able to do expression validity checking via SFINAE in a static assert.
Bump, and also a couple of additional macros for checking constexpr-ness of an expression are linked farther down in this email, using similar tricks as the SFINAE checks int the previous email (pushing more complicated checks into the body of a lambda such that the macro expands to an expression). Motivation for this is that often when writing templated code it is difficult to be certain that a specific instantiation is actually constexpr even if the constexpr keyword is used with the function template. For instance, for a matrix type whose value type is a template parameter T, it can be difficult to know if matrix<T>'s multiplication operation actually results in a constexpr object, even if it has constexpr inputs. Particularly when designing such a library, it is useful to be able to write simple tests that check the constexpr-ness of a specified expression in the form of a static_assert. A proof-of-concept implementation of this is linked below: http://ideone.com/ko09KQ As an example of such a usage: //////////////////// int foo(int) { return 0; } STATIC_ASSERT_CONSTEXPR ( (constexpr int) var = 5, foo(var) ); //////////////////// Produces the error: prog.cpp:70:1: error: static assertion failed: ******************** Given: constexpr int var = 5; The following expression was expected to be constexpr but was not: foo(var) ******************** - Matt Calabrese