Le 04/02/2012 07:39, Jonathan Bober a écrit :
For another example: I recently tried to compile some of my own code using clang++ and discovered that I am not allowed to dovoid f(int j) { complex<double> x[j]; [...] } even though g++ accepts that. ( See http://clang.llvm.org/compatibility.html#vla ) Now, I have a few options. I could allocate everything on the heap explicitly; I could use vector<complex<double> >; or I could specify a maximum size for j. All of these options are bad for me, because I care very much about speed, and I know that j is small, and 99% of the time it will be less that 20, but I don't want to always allocate space for j = 100 because this is very rare. So I have to consciously decide what to do, and if I decide to stick with the nonstandard gcc extension it is not because I am an idiot but because it is probably the fastest/easiest option on what is likely to be the only compiler I ever care about.
If the "j" in question is always a constant in the code (I mean if you always have "f(2)", "f(156)" and the like), you may perhaps consider a template : http://stackoverflow.com/questions/499106/what-does-template-unsigned-int-n-mean
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