Is it possible to make macros that are default arguments expand at call site?

Question

#include <stdio.h>

void print(int a = __LINE__){printf("hello %d\n", a);}

void main(){
  print();
  print();
  print();
  print();
}

The __LINE__ macro in this case expands to 3, so the print function is called 4 times with the same value. Is there a way to convince the compiler to expand this macro at the callsite, so that the print function would be called with 6,7,8,9 instead of 3,3,3,3 with features that exist in C++11?

My usecase:

In my application, I provide multiple functions that take a unique ID. The ID should be unique per callsite/location (so if the function is called two times throught the same statement, it should receive the same id). Currently, the user always has to manually type a LOCATION macro at the callsite, like this:

#define S1(x) #x //voodoo to concat __FILE__ and __LINE__
#define S2(x) S1(x)
#define LOCATION __FILE__ S2(__LINE__)

do_stuff1(arguments, LOCATION)
do_stuff2(arguments, LOCATION)

It would be more convenient if I could save them the typing, without creating a macro for each function like this:

#define do_stuff1(do_stuff1_imp(arguments, LOCATION))
#define do_stuff2(do_stuff2_imp(arguments, LOCATION))

Hence I thought a default argument would do the trick. Is there any way to achieve this?

Answer 1

The __LINE__ macro in this case expands to 3

Sure because your function declaration is done at line 3 and the prepreprocessor expands it from there.

Another macro instead of a function declaration would do the trick well (taken from @Ejay's comment):

#define print() printf("hello %d\n", __LINE__)

See a working example here.

Answer 2

You seem to be looking for std::experimental::source_location, which will be std::source_location in a future standard:

#include <experimental/source_location>

void print(std::experimental::source_location const& location
    = std::experimental::source_location())
{
    printf("hello %s:%d\n", location.file_name(), location.line());
}

Answer 3

You can write a macro which inserts the __FILE__ and __LINE__ into the argument list:

// Note: this doesn't work with a 0-argument function. Supporting 0-arg functions
// is much harder to do; I'd recommend just having a separate macro like as follows
#define LOCATED(fn, ...) fn(__VA_ARGS__, __FILE__, __LINE__)

#define LOCATED0(fn) fn(__FILE__, __LINE__)

void print(char const* file_name, int line)
{
    printf("hello %s:%d\n", file_name, line);
}

int do_something(int value, char const* file_name, int line);

int main() {
    LOCATED0(print);
    LOCATED(do_something, 42);
}

With a little more work, you can make it look nicer at the call-site:

#define LOCATED(...) LOCATED_##__VA_ARGS__

void print(char const* file_name, int line)
{
    printf("hello %s:%d\n", file_name, line);
}
#define LOCATED_print() ::print(__FILE__, __LINE__)

int do_something(int value, char const* file_name, int line);
#define LOCATED_do_something(value) ::do_something(value, __FILE__, __LINE__)

int main() {
    LOCATED(print());
    LOCATED(do_something(42));
}