for loop as a macros

[aryansmit3754]

could somebody tell wats wrong with this
#define FOR(x,n)for(typeof(x);x<n;x++)

[MosaicFuneral]

Just don't use macros for loops.

[grumpier]

Firstly, there is no space between the closing bracket at the end of FOR(x,n) and preceding the rest. That's a stylistic concern, but badly hurts readability.

Second, and more significantly, typeof() is not a standard function or operator in C++. It is an extension specific to g++ (GNU c++) in which typeof(x) expands to be type of x and will trigger a compiler error with every compiler that does not support that extension. Within a for loop (assuming you're using g++), if x is an int, then FOR(x,n) would expand to for (int; x < n; x++) which is invalid code.

[ArkM]

Apart from anything else this useless macros must be

#define FOR(x,n)for(typeof(x)x=0;x<(n);x++)

[bobpaul]

"#define FOR(x,n)for(typeof(x)x=0;x<(n);x++)" won't even work. In order for typeof() to function, x has to already exist, and if x already exists the compiler will complain that you're redeclaring x. EG:

int x;

FOR(x,10)
   cout<<"Looping"<<endl;

will expand to "for(int x=0; x<(10);x++", but you already declared x... The closest you might want is "FOR(n) for(int x=0; x<(n); x++)" but I fail to see much point. If you're going to macro a for loop, do so because you're controlling the entire loop for a specific set. Such as:

#define ITERATE_DEVICE_LIST    for(int i=0; i < NUM_DEVICES; i++)

At least this ensures that indexing errors need only be fixed in the macro and prevents stray typos from causing problems in the future.

[mrnutty]

Why would anyone do this? Ouch my eyes. Use either std::for_each, BOOST_FOR_EACH,
or even manually.