On 13 June 2012 03:45, T.J. Crowder <[email protected]> wrote:
> Is the reason for using ?: rather than ?? because we may want it for my > desired second ternary? E.g., from my other message: > Something else to consider; if we use (a ?: b), it would have to be equivalent to (a ? a : b) with the exception that a is only evaluated once, just as it is in gcc. Any other meaning would be confusing to end users, due to the strong resemblance to the ternary operator. The use-case here seems to be that it would have the value a whenever a is undefined, not when a is truey. This is a tricky enough difference that I think it mandates a different operator, like ??. That said, I don't think I have ever seen code like that in the field, where the caller sorts out the "default values" instead of the callee.* * later edit: I thought this was true, but see that I have been unwittingly using this idiom myself for years, see examples below. > Very much looking forward to ?: (however we spell it) and ?=. > Me too. They are all over my C code and Makefiles. Random usage (food for thought): memcpy(newHnd->buffer, thisHnd->buffer, min(length, newHnd->length ?: length)); const char *filename = gpsee_programRelativeFilename(cx, report->filename) ?: "<unknown filename>"; JS_SetGCParameter(rt, JSGC_MAX_BYTES, (size_t)strtol(rc_default_value(rc, "gpsee_gc_maxbytes", "0"), NULL, 0) ?: (size_t)-1); So, as you can see above, I tend to use ?: to supply default behaviour to functions when necessary, particularly when they are library functions out of my control. GPSEE_SRC_DIR ?= $(shell pwd) GREP ?= grep EGREP ?= egrep MAKEDEPEND ?= gcc -E -MM -MG NCURSES_LIB_NAME ?= ncurses BIN_DIR ?= $(GPSEE_PREFIX_DIR)/bin I use the ?= idiom quite heavily in my Makefiles as we don't use autoconf. We simply have a configure script which generates an include file of "stuff that's different for this platform", which gets included before the stuff that's assigned with the ?=. Also, the ?= allows the developer to override particular variables from environment variables passed to make, i.e. to install into a different target directory, use a different version of gcc, etc. This is idiom is especially powerful with Make's late evaluation of variables, but I don't think we should go there in ES. Wes -- Wesley W. Garland Director, Product Development PageMail, Inc. +1 613 542 2787 x 102
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