#define a function?

Question

I was looking at the #define keyword and I wondered if it could be used on a function so I made this small bit of code:
#include <iostream>
using namespace std;

#define int sumfunc(int x){return x;};

int main() {
cout<<sumfunc(5);
return 0;
}

my question is why does this work and am I actually defining sumfunc()?

Answer 1

Hmm, this was a confusing one to figure out but I think I got it.
First of all you should realise that stuff like:
main(){ return 0; }
f(){ return 0; }
is allowed in C/C++. If a return type is missing the compiler assumes it returns an int. Although it's not a good practice to do.

When you do #define int sumfunc(int x){return x;} ( I removed the last ; as it doesn't matter in this case ) you literally replace all ints with that function. ( except that 1 int in the function parameter )

So int main(){} becomes
sumfunc(int x){return x;} main(){}
basically 2 functions on 1 line.
This is the place where your function gets defined.

This is also the reason why int main(){} seems to work but
int f(){}
int main(){}
doesn't. Because the function would be defined twice.

If you changed
#define int sumfunc(int x){ return x; }
to
#define double sumfunc(int x){ return x; }
then you suddenly get an undefined error. Because this time the int in main isn't replaced by the function and therefore there is no function definition. ( there is no double anywhere else in the code to replace )

So in short, don't do this, at least not with keywords.
#define a int sumfunc(int x){ return x; } is the one you want, where a is the name, not that this has much use though.

Lastly:
#define int sumfunc(int x){return x;}
int main()
{
int i = 0; // Can't use int anymore :(
}

Answer 2

Yea, you're right swim, I spontaneously forgot that's a gcc thing during me figuring out what was going on and confused it since it is allowed in C.

Answer 3

It seems this conversation is way beyond my level, i think ill go back to working with pointers and stuff until im good with them

Answer 4

Dennis that was an interesting read, it's probably gonna take me a little while to fully understand whats going on here but anyways thanks for the in-depth explanation

Answer 5

Dennis One more thing why doesn't it work when i try to make it a template function?

Answer 6

Do you have the code that you used? Because it does work for me:
#define int template<typename T> somefunc(const T& x){ return x; }
or
#define int template<typename T> \
somefunc(const T& x){ return x; }
for multiline.

( still returns an int tho )

Answer 7

Ah that explains it i didn't define the template