From #define to function

Question

I have this code inside a function but I am not able to understand what it does.

....
#define ascend(i) do {\
            int h = nodes[i].heavyindex;\
            int p = nodes[i].heavypos;\
            m##i = max(m##i + paths[h].ftree.sum(p), paths[h].stree.max_(0, p));\
            i = paths[h].parent;\
        } while (0)
    while (nodes[a].heavyindex != nodes[b].heavyindex) {
        if (nodes[a].heavyindex > nodes[b].heavyindex) {
            ascend(a);
        } else {
            ascend(b);
        }
    }
#undef ascend
...

The code of #define, I think, is:

#define ascend(i) do {\
            int h = nodes[i].heavyindex;\
            int p = nodes[i].heavypos;\
            m##i = max(m##i + paths[h].ftree.sum(p), paths[h].stree.max_(0, p));\
            i = paths[h].parent;\
        } while (0)

so the real code inside the function is only this:

while (nodes[a].heavyindex != nodes[b].heavyindex) {
        if (nodes[a].heavyindex > nodes[b].heavyindex) {
            ascend(a);
        } else {
            ascend(b);
        }
    }

1) It is right?
2) I want to move the code of the #define inside a function to better understand what it does, but how I translate the following line?

m##i = max(m##i + paths[h].ftree.sum(p), paths[h].stree.max_(0, p));\

`m##i` is using the token-pasting operator, it will become either `ma` or `mb`. You'll need to pass the function a pointer (or in C++, a reference) to it, in order to read and write it from inside the function. — Ben Voigt, Mar 20 '15 at 00:10
Most compilers can output the preprocessed source code. For Gcc, it's `gcc -E`. — mafso, Mar 20 '15 at 00:16
Regardomg the `do{} while(0)` construct, read here http://stackoverflow.com/questions/154136/do-while-and-if-else-statements-in-c-c-macros — leonbloy, Mar 20 '15 at 00:24
[zenith](http://stackoverflow.com/users/3425536/zenith) `ma` and `mb` are int — user72708, Mar 20 '15 at 11:45

score 1 · Accepted Answer · edited May 23 '17 at 11:43

1

Yes.
As mentioned by Ben Voigt in the comments, ## is the token-pasting operator. So with #define f(i) m##i defined, f(a) will expand to ma, f(b) will expand to mb, etc.

Since that's only possible with the preprocessor, you have to think of something else to implement it as a function. Passing ma and mb by reference would be a good idea. It could look something like this:
```
ascend(T& mi) {
    ...
    mi = max(mi + paths[h].ftree.sum(p), paths[h].stree.max_(0, p)); 
    ...
}
```
Where T is the type of ma and mb. If they're of different types, you need to make it a function template.

edited May 23 '17 at 11:43

Community

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answered Mar 20 '15 at 00:22

Emil Laine

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It's important to note that this is a C++ answer. If OP is really looking for a C answer, this will need some (minor) semantic cleanup, and a function template is not possible. – Degustaf Mar 20 '15 at 00:40

From #define to function

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