C preprocessor macro for returning a vector list from vec[0] to vec[n-1]

Question

I want to implement like this

#define XX ???

...

std::vector<int> vec;

Func(XX(vec));  --> Func(vec[0], vec[1], ... vec[n-1]);

I don't have any idea how to do this.

Very thanks if you can help me.

Answer 1

Your

Func(vec[0], vec[1], ... vec[n-1])

has a simple problem: if that n is not known at compile time , how many of those vec[i], should the compiler generate? In theory, we could think about runtime compilation or pre-generating all the possibilities and then dispatching over them at runtime, but that's both crazy and crazily useless.

So, if n isn't known beforehand, process your vec[i] s in a usual manner:

constexpr auto Func(auto&& range) { for (auto&& element: range) /* work */; }
int main() { Func(std::vector{1, 2, 3}); }

However, if n is a compile-time constant, you can easily get what you want and that Func is even written for you in the STL :

int main() { apply([](auto&&... elements) { /* work */ }, std::array{1, 2, 3}); }

And, yes, please try to stay away from #define s: they know nothing about namespace , constexpr and other basic C++ features. Instead, consider sticking to template - eg that's how std::apply is implemented.

Answer 2

If you provide the n as a parameter to the macro its possible (though very cludgy) and there are better techniques using templates.

Func(XX(n, vec));  --> Func(vec[0], vec[1], ... vec[n-1]);



#define EXTRACT_15(container)      container[14], EXTRACT_14(container)
#define EXTRACT_14(container)      container[13], EXTRACT_13(container)
#define EXTRACT_13(container)      container[12], EXTRACT_12(container)
#define EXTRACT_12(container)      container[11], EXTRACT_11(container)
#define EXTRACT_11(container)      container[10], EXTRACT_10(container)
#define EXTRACT_10(container)      container[9], EXTRACT_9(container)
#define EXTRACT_9(container)      container[8], EXTRACT_8(container)
#define EXTRACT_8(container)      container[7], EXTRACT_7(container)
#define EXTRACT_7(container)      container[6], EXTRACT_6(container)
#define EXTRACT_6(container)      container[5], EXTRACT_5(container)
#define EXTRACT_5(container)      container[4], EXTRACT_4(container)
#define EXTRACT_4(container)      container[3], EXTRACT_3(container)
#define EXTRACT_3(container)      container[2], EXTRACT_2(container)
#define EXTRACT_2(container)      container[1], EXTRACT_1(container)
#define EXTRACT_1(container)      container[0]
#define EXTRACT_0(container)

#define EXTRACT(size, container)   EXTRACT_ ## size(container)
#define XX(size, container)        EXTRACT(size, container)

I just noticed that I got that the wrong way around and the EXTRACT_<N> list the parameters the wrong way around. Which just shows how clunky this is (as it is too much work to fix).