I have come across the following Macro and I cannot understand what it's doing exactly;
#include <iostream>
#define TEST(a1) \
int a1 = 5;
int main(){
TEST(a1);
std::cout << a1 << std::endl;
return 0;
}
It compiles and prints 5.
I know macros to a degree but I cannot wrap my head around what is happening here. To me TEST looks like a function which is redefining its argument a1?
A macro just puts the code defined in the macro wherever you call the macro through the preprocessor
What it basically comes down to is that this code:
#include <iostream>
#define TEST(a1) \
int a1 = 5;
int main(){
TEST(a1);
std::cout << a1 << std::endl;
return 0;
}
becomes this code:
#include <iostream>
int main(){
int a1 = 5;
std::cout << a1 << std::endl;
return 0;
}
in effect.
The advantage to a macro, is that you can use it multiple times. And, since our example is parameterized, we can use it with minor variations. So, for example, this code:
#include <iostream>
#define TEST(a1) \
int a1 = 5;
int main(){
TEST(a1);
std::cout << a1 << std::endl;
TEST(a2);
std::cout << a2 << std::endl;
TEST(a3);
std::cout << a3 << std::endl;
return 0;
}
becomes:
#include <iostream>
int main(){
int a1 = 5;
std::cout << a1 << std::endl;
int a2 = 5;
std::cout << a2 << std::endl;
int a3 = 5;
std::cout << a3 << std::endl;
return 0;
}
There are lots of things you can do with macros, and I recommend you research them, but this will get you started.
EDIT: Also worth mentioning is that, if you wanted to, you could change this macro to:
#define TEST(a1,val) \
int a1 = val;
That way, you could control the initialization value independently if you like. So:
TEST(a4,8)
becomes:
int a4 = 8;
int a1 = 5;as part of the macro expansion. If you're using GCC or Clang, you can try running just the preprocessor and looking at the output to see what's happening.-Eoption will make gcc/clang run just the preprocessor, and the will emit the final processed code (before compiliation). I recommend using gcc.godbolt.org as a simple way to explore macros and what they expand to (though heads up: if you#includea file you're in for a treat :)).