Question

In C++ I wanted to define a constant that I can use in another function, A short answer on how to do this will be fine..

Lets say at the beginning of my code I want to define this constant:

//After #includes
bool OS = 1; //1 = linux
if (OS) {
  const ??? = "clear";
} else {
  const ??? = "cls";
}

I don’t know what type to use to define the “clear” string… I’m so confused.

Later on I want to use it within a function:

int foo() {
 system(::cls); //:: for global

 return 0;
}

How would I define the string up top, and use the string down below? I heard char only had one character and things… I’m not sure how to use , since it says it’s converting string into const char or something.

Answer 1

char* isn’t quite a char. char* is basically a string (it’s what strings were before C++ came along).

For illustration:

int array[N];  // An array of N ints.
char str[N];   // An array of N chars, which is also (loosely) called a string.

char[] degrades to char*, so you’ll often see functions take a char*.

To convert std::string to const char*, you can simply call:

std::string s;
s.c_str()

In this case, it’s common to use the preprocessor to define your OS. This way you can use the compiler to do the platform specific stuff:

#ifdef OS_LINUX
const char cls[] = "clear";
#elif OS_WIN
const char cls[] = "cls";
#endif

One thing you may want to consider is making it a function. This avoids nasty dependencies of global construction order.

string GetClearCommand() {
  if (OS == "LINUX") {
    return "clear";
  } else if (OS == "WIN") {
    return "cls";
  }
  FAIL("No OS specified?");
  return "";
}

What it looks like you’re trying to do is this:

#include <iostream>
using namespace std;

#ifdef LINUX
const char cls[] = "LINUX_CLEAR";
#elif WIN
const char cls[] = "WIN_CLEAR";
#else
const char cls[] = "OTHER_CLEAR";
#endif

void fake_system(const char* arg) {
  std::cout << "fake_system: " << arg << std::endl;
}

int main(int argc, char** argv) {
  fake_system(cls);
  return 0;
}

// Then build the program passing your OS parameter.
$ g++ -DLINUX clear.cc -o clear
$ ./clear 
fake_system: LINUX_CLEAR