Unary operator overloading types in C++ (newbie)

Question

I'm trying to overload the shriek (!) operator to return the length of a vector like so: (! because it kind of looks like a lowercase "L", and I wanted a unary operator for the sake of brevity and readability.)

// Vector class in Vector.h
class Vector {
public:
  double x,y,z;
  /* ... */
  double operator ! (); /* Vector length */
  /* ... */
}

// Function definition in Vector.cpp
double Vector::operator ! (){
  return sqrt(x*x+y*y+z*z);
}

// Main function in main.cpp
#include "vector.h"
int main (int argc, char** argv){
  Vector a = Vector(1,2,3);
  cout << !a << endl;
}

And the error code that I'm getting from gcc is:

$ gcc -lstdc++ main.cpp
Undefined symbols for architecture ********:
  "Vector::operator!()", referenced from:
      _main in ********.o
ld: symbol(s) not found for architecture ********
collect2: ld returned 1 exit status

(Note: I couldn't find the answer to this question either due to inexperience with C++ or because I'm trying to do something that violates a very basic tenant of the language that I skipped over due to self-education.)

You probably forgot to add `Vector.cpp` in your compilation line, by the looks of it. — Nbr44, Aug 19 '13 at 15:31

score 3 · Answer 1 · edited May 23 '17 at 12:05

The compiler is complaining because it doesn't know where the vector method is defined. Try compiling with g++ main.cpp Vector.cpp.

Explination: In order for your code to work as expected, both your source files need to be compiled and then the compiled objects linked to create the final executable. This is that same as in C.

With C++ (or C with gcc), you can compile individual files using:

g++ -c my_file_1.cpp -o my_file_1.o
g++ -c my_file_2.cpp -o my_file_2.o

If you compile files individually you then need to link the files to create an executable:

g++ my_file_1.o my_file_2.o -o a.out

The command g++ main.cpp Vector.cpp does all this in one step and does away with the need to create multiple object files (the files with the .o extension).

Each method for compiling multiple files has its advantages. For example, compiling source files is usually one of the most time expensive exercises in programming. If you compile the files individually then you only have to recompile the object files when you change the source file associated with the object file.

The error in your question was caused in the linking stage (linking is preformed even if you only have 1 files, as long as you don't use the -c flag). The error basically said that the compiler had the name of the function but couldn't find where the actual compiled code was.

If you really want to use gcc, try using the command gcc -lstdc++ -lm main.cpp Vector.cpp.
Another note, change your declaration of a to Vector a(1, 2, 3);.
A program designed to make your life a bit easier is Make. Make can be set-up to check if files need to be rebuilt and run all the commands you need to build your executable. It does take some learning though, take a look at this post (https://stackoverflow.com/a/2481326/2372604).

Thanks! That really helped, but I'm not sure that I totally understand why that helped -- doesn't the compiler look through all of the files that I include and infer that `vector.cpp` is the right file to look in? I can see this being a problem/hassle if I have a lot of files, and this never seemed to be an issue in C. — eratosthenesia, Aug 19 '13 at 16:20
@eratosthenesia The compiler just compiles what you tell it to compile. Including `vector.h` does not imply that the compiler searches for a `vector.cpp` file. — Banex, Aug 19 '13 at 16:31

Unary operator overloading types in C++ (newbie)

1 Answers1