Comparison of preprocessor variables

Question

What is the reason why this code does not compile? ``` #define THREADMODEL ASC #if THREADMODEL==NOASC // This block should be ignored and the code should compile #error #endif int main() { } ```

Accepted Answer

When the preprocessor interprets

```
#if THREADMODEL==NOASC
```

it will replace `THREADMODEL` with `ASC`:

```
#if ASC==NOASC
```

Unless you have `#define`d `ASC` and `NOASC` to have numeric values, the preprocessor will replace them with 0 values (it takes any undefined symbols and replaces them with 0):

```
#if 0==0
```

This then evaluates to `1` and so the preprocessor will evaluate the block.

To fix this, try giving different numeric values to `ASC` and `NOASC`, like this:

```
#define ASC    0
#define NOASC  (1 + (ASC))
```

Hope this helps!

Comparison of preprocessor variables

Solution

Problem