Question

I have a ‘foreach’ macro I use frequently in C++ that works for most STL containers:

#define foreach(var, container) \   for(typeof((container).begin()) var = (container).begin(); \       var != (container).end(); \       ++var) 

(Note that ‘typeof’ is a gcc extension.) It is used like this:

std::vector< Blorgus > blorgi = ...; foreach(blorgus, blorgi) {   blorgus->draw(); } 

I would like to make something similar that iterates over a map’s values. Call it ‘foreach_value’, perhaps. So instead of writing

foreach(pair, mymap) {   pair->second->foo(); } 

I would write

foreach_value(v, mymap) {   v.foo(); } 

I can’t come up with a macro that will do this, because it requires declaring two variables: the iterator and the value variable (‘v’, above). I don’t know how to do that in the initializer of a for loop, even using gcc extensions. I could declare it just before the foreach_value call, but then it will conflict with other instances of the foreach_value macro in the same scope. If I could suffix the current line number to the iterator variable name, it would work, but I don’t know how to do that.

Answer 1

You can do this using two loops. The first declares the iterator, with a name which is a function of the container variable (and you can make this uglier if you’re worried about conflicts with your own code). The second declares the value variable.

#define ci(container) container ## iter #define foreach_value(var, container) \     for (typeof((container).begin()) ci(container) = container.begin(); \          ci(container) != container.end(); ) \         for (typeof(ci(container)->second)* var = &ci(container)->second; \              ci(container) != container.end(); \              (++ci(container) != container.end()) ? \                  (var = &ci(container)->second) : var) 

By using the same loop termination condition, the outer loop only happens once (and if you’re lucky, gets optimized away). Also, you avoid calling ->second on the iterator if the map is empty. That’s the same reason for the ternary operator in the increment of the inner loop; at the end, we just leave var at the last value, since it won’t be referenced again.

You could inline ci(container), but I think it makes the macro more readable.