This boils down to whether or not a lambda’s type differs across translation units. If it does, it may affect template argument deduction and potentially cause different functions to be called – in what are meant to be consistent definitions. That would violate the ODR (see below). However, that isn’t intended. In fact, this problem … Read more
The one-definition rule (3.2, [basic.def.odr]) applies differently to classes and functions: 1 – No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template. […] 4 – Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program […] … Read more
You are violating C’s “one definition rule”, and the result is undefined behavior. The “one definition rule” is not formally stated in the standard as such. We are looking at objects in different source files (aka, translation units), so we concerned with “external definitions”. The “one external definition” semantic is spelled out (C11 6.9 p5): … Read more
Does it mean that a static constexpr variable is not odr-used (and therefore can be in-class initialized) if it is returned from a function? Yes. Essentially, as long as you treat it as a value, rather than an object, then it is not odr-used. Consider that if you pasted in the value, the code would … Read more
In C, the statement int a; when made at file scope, is a declaration and a tentative definition. You can have as many tentative definitions as you want, as long as they all match each other. If a definition (with an initializer) appears before the end of the translation unit, the variable will be initialized … Read more
TryMe::arr is odr-used but you don’t provide a definition (see it live): constexpr int TryMe::arr[1]; Why is the result inconsistent between gcc and clang? This is because odr violations do not require a disagnostic, from both the C++11 and C++14 draft standard (emphasis mine): Every program shall contain exactly one definition of every non-inline function … Read more
It looks like defect report 1400: Function pointer equality deals with this issue and seems to me to say that it is okay to do this optimization but as comments indicate, there is disagreement. It says (emphasis mine): According to 5.10 [expr.eq] paragraph 2, two function pointers only compare equal if they point to the … Read more
It’s just an arbitrary definition, used by the standard to specify when you must provide a definition for an entity (as opposed to just a declaration). The standard doesn’t say just “used”, because this can be interpreted diversely depending on context. And some ODR-use doesn’t really correspond to what one would normally associate with “use”; … Read more
Oh man, one of my pet peeves. inline is more like static or extern than a directive telling the compiler to inline your functions. extern, static, inline are linkage directives, used almost exclusively by the linker, not the compiler. It is said that inline hints to the compiler that you think the function should be … Read more