How to call a function from header file in c++ - c++

Question

I'm new in C++
I was using java before, I'm trying to call a function in c++ from a hearder file.
Here is my code:
sum.h
int sum(int a, int b);
cur_time.h
#ifndef CUR_TIME_H
#define CUR_TIME_H
clock_t clock(void);
#endif /* CUR_TIME_H */
main.cpp
#include <iostream>
#include "sum.h"
#include "cur_time.h"
int main ()
{
int x;
int y;
x = sum(3,4);
std::cout << x;
y = clock(void);
y = std::cout << y;
return 0;
}
So in main.cpp I try to display the elapsed time of my system with this function: clock_t clock(void);
When I run the project, I have this error:
main.cpp:13:13: error: expected primary-expression before ‘void’
main.cpp:14:3: error: ‘__ostream_type’ was not declared in this scope
If I run the code without calling the second function it works for sum.

Answer 1

You don't have to (and mustn't) use void in parentheses on calling a function.
assigning std::cout << y to y is meaningless here and gcc gave me this error:
error: invalid user-defined conversion from 'std::basic_ostream' to 'int' [-fpermissive]
Therefore, main.cpp should be like this:
#include <iostream>
#include "sum.h"
#include "cur_time.h"
int main ()
{
int x;
int y;
x = sum(3,4);
std::cout << x;
y = clock(); // remove void
std::cout << y; // remove y =
return 0;
}

Answer 2

Have you defined your sum(int, int) function in Sum.h?
Simply doing int sum(int a, int b); is not good enough, you need to define that function.
int sum(int x, int y) {return x+y;}

Answer 3

Let's take that first error message as a clue. I encounter it a LOT, in various forms, when I write C++ code.
main.cpp:13:13: error: expected primary-expression before ‘void’
The phrase "expected primary-expression" almost always means that you have A) forgotten something, or B) typed something weird or unexpected.
For example, you will get the same kind of error if you were to try cout << "Hello, " < "world!" << endl; In that case, I put a < where I should have put a <<. (Actual example from my code two days ago.)
In your case, the error message is screaming about void, which means that void or whatever precedes it is out of place. In your line y = clock(void);, we know that the preceding token ( is correct...we're calling a function after all. That must mean that void is mistyped or out of place. Obviously it isn't mistyped. If you drop it, your code works.
What's up with that? Well, unlike some languages, C++ can accept an empty arg list, so long as the function wasn't expecting any arguments. y = clock(); will properly call the function.
[BONUS: If you're trying to call a constructor without any arguments, leave off the parenthesis altogether in the call. It's just one of those weird exceptions.]
So, what about that second error? In general, you should try to fix only the error at the top of the list (unless you know for certain what the other(s) are about). Many times, one error causes other weirder errors to happen as well. To see what I mean, try leaving off a semicolon and check out the errors you get as a result. Fix that error and recompile. If the second error shows up again, THEN you address it.
In regards to your comment to MikeCAT, this is a separate issue, but I'll address it the same anyway. Your clock() function returns clock_t, but you are assigning it to y, which is of type int. It appears that clock_t is based on int, so you should be able to get away with it. However, it may be a good idea to switch y to type clock_t as well, and see if that resolves it.
Beyond that, it may have to do with how you're using clock, though I'm not an expert on that class. If you continue to have problems, create a new question. On StackOverflow, you want to stick to ONE problem in ONE question. :)
(See my comment to your question for some additional tips on surviving and thriving around here.)