Macro for nested i, j loop

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How do you write a macro that can be used as follows:

FOR(n)
    FOR(j, n)
        //do stuff

and that expands to a valid nested C++ for loop, with:

  • FOR(n) will use a loop variable named i (default case) and with an invariant of less than (<) n,
  • FOR(j, n) will use a loop variable named as the first argument (j) and with an invariant of less than (<) n?

I saw this kind of macro used by a competitive programmer programming C++, but I have not been able to write such macros myself.

macros c++
1 Answers

I'm aware it's bad practice to do things like this [...]

Let's start off by emphasizing this once more: yes, this is horrible practice.

Now, what looks like macro overloading is actually macro dispatch, as covered in detail in e.g. the following Q&A:

Using the library code from the accepted answer:

#define CAT( A, B ) A ## B
#define SELECT( NAME, NUM ) CAT( NAME ## _, NUM )

#define GET_COUNT( _1, _2, _3, _4, _5, _6 /* ad nauseam */, COUNT, ... ) COUNT
#define VA_SIZE( ... ) GET_COUNT( __VA_ARGS__, 6, 5, 4, 3, 2, 1 )

#define VA_SELECT( NAME, ... ) SELECT( NAME, VA_SIZE(__VA_ARGS__) )(__VA_ARGS__)

we can define various FOR_N overloads e.g. as follows:

// Dispatch macro.
#define FOR( ... ) VA_SELECT( FOR, __VA_ARGS__ )

// Dispatch for 1 or 2 macro arguments for dispatch function.
#define FOR_1(N) for(int i = 0; i < N; ++i)
#define FOR_2(I, N) for(int I = 0; I < N; ++I)

used as follows:

int main() {
    int n = 3;
    FOR(n)
        FOR(j, n)
            std::cout << i << " " << j << "\n";
    /* 0 0
       0 1
       0 2
       1 0
       1 1
       1 2
       2 0
       2 1
       2 2 */
}

Note that what may look like (Pascalesque) scope of macroed for loops, is only limited to a single line, as they expand to

for(int i = 0; i < n; ++i)
    for(int j = 0; j < n; ++j)
        std::cout << i << " " << j << "\n";

and not

for(int i = 0; i < n; ++i) {
    for(int j = 0; j < n; ++j) {
        std::cout << i << " " << j << "\n";
    }
}

To achieve the latter, you need to similarly explicitly add block scopes:

FOR(n) {
    FOR(j, n) {
        std::cout << i << " " << j << "\n";
    }
}
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