C++ concatenation operand ‘+’ different form ‘ ‘.

At school*, in C++, we always used the ‘+’ operator to concatenate strings.. for example:

string a = "hello"+" world.";

But today i was coding something like this:

#define filename "./temp"
[...]
function("something "+filename);

and the compiler was not okay with this:

invalid operands to binary expression ('const char[9]' and 'const char[7]')

I later found out that writing it like:

#define filename "./temp"
[...]
function("something" filename);

made it work.
Why is that??

P.S. i feel like i should also specify that my school often mixed C things into C++.. but ‘+’ only works with c++ strings, right?