我如何将定义（#define SOME_DEFINE）作为函数的参数传递？

Question

Well, I was experimented with language features, and didn't find the way, how can I pass define to the function as param. I know, that it's not the best practice, just pure interest.

I've already tried to do it in several ways, at least generics didn't help.

So, what I'd like to do is:

#define SOME_DEFINE

template <class T>
void foo(T param)
{
    if(param == SOME_DEFINE) do stuff
}

int main
{
    foo(SOME_DEFINE);
}

Appreciate you.

Answer 1

All that's missing is a value to your definition.

Just add it and you can start testing the feature.

#include <iostream>

#define SOME_DEFINE 42

void do_all_the_things() {
  std::cout << "woohoo!" << std::endl;
}

void do_none_of_the_things() {
  std::cout << "darn" << std::endl;
}

template <class T>
void foo(T param) {
  if (param == SOME_DEFINE) {
    do_all_the_things();
  } else {
    do_none_of_the_things();
  }
}

int main() {
  foo(42);
  foo(3);
  return 0;
}

For strings, it gets a tad bit more complicated. String literals, const char* can't be compared very easily. For example:

const char* a = "value a";
const char* b = "value b";
return a == b;

May seem like it works, but it's actually just comparing the pointers and not the content. You could fix this by either defining your macro as std::string("an example value") or using a specialized compare function.

#include <iostream>
#include <cstring>

#define SOME_DEFINE "test value"

void do_all_the_things() {
  std::cout << "woohoo!" << std::endl;
}

void do_none_of_the_things() {
  std::cout << "dang it" << std::endl;
}

template <class T>
struct AreEqual final {
  bool operator () (T a, T b) const {
    return a == b;
  }
};

template <>
struct AreEqual<const char*> final {
  bool operator () (const char* a, const char* b) const {
    return std::strcmp(a, b) == 0;
  }
};

template <class T, class AreEqualTo = AreEqual<T>>
void foo(T param, AreEqualTo are_equal_to = AreEqualTo()) {
  if (are_equal_to(param, SOME_DEFINE)) {
    do_all_the_things();
  } else {
    do_none_of_the_things();
  }
}

int main() {
  foo("test");
  foo("test value");
  return 0;
}

This is another worthy route to explore if you're learning the language. There are a lot of functions in the C++ library that use template functions.