Using has_include with variables in a loop to check libraries

Question

I'm trying to check if a library is available using __has_include from this post. Since I want to check each one I'm using a loop

/* Array of Strings */

const char* libraries[5] = { "iostream", "unistd.h", "stdlib.h", "Windows.h", "winaaasock2.h"};

/* Getting the length of the array */

int librariesSize = sizeof(libraries)/sizeof(libraries[0]);

/* Looping through them (5 Times) */

for (int i = 0; i < librariesSize; i++) {
  #if __has_include(<\"libraries[i]"\>)
      #include <\"libraries[i]"\>
      cout << "Ok";
  #else
    cout << "Error";
  #endif
}

It compiles but it's still telling me that all of them exist even for winaaasock2, which is made up from the original, winasock2.

It requires to be inside <> signs and quotes so I used back slashes

Using the same code without the loop works

#if __has_include("winaaasock2.h")
    #include "winaaasock2.h"
    cout << "Ok";
#else
    cout << "Error";
#endif

The output here is Error, with a library like unistd.h the output is ok since it exists

What I'm missing? Thanks in advance

Answer 1

It compiles

It shouldn't. The C++ language doesn't allow expression statements such as loops in the namespace scope. The example program is ill-formed.

Besides that, pre-processor has no knowledge of your loops. There are two possible ways that your program may be processed:

// if the header \"libraries[i]"\ exists
for (int i = 0; i < librariesSize; i++) {
    // content from header \"libraries[i]"\ 
    cout << "Ok";
}

// OR if the header \"libraries[i]"\ doesn't exist
for (int i = 0; i < librariesSize; i++) {
    cout << "Error";
}

---

If all that you want to do is to check whether all headers exist, and produce an error otherwise, then __has_include won't offer anything of use for you. You should simply include them. If a header is missing, there will be an error message that explains the issue.