Strange "Undefined symbols" error? - c++

Question

I am learning C++ and I got this error:
Undefined symbols:
"_main", referenced from:
start in crt1.10.6.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
my code is this
#include <stdlib.h>
#include <iostream>
class Fraction {
private:
int num, den;
public:
void set(int n, int d) {num=n; den=d; normalize();}
int get_num(){return num;}
int get_den(){return den;}
private:
void normalize();
int gcf(int a, int b);
int lcm(int a, int b);
};
void Fraction::normalize() {
if (den == 0 || num == 0) {
num = 0;
den = 1;
}
if (den < 0) {
num *= -1;
den *+ -1;
}
int n = gcf(num, den);
num = num / n;
den = den / n;
}
int Fraction::gcf(int a, int b) {
if (a % b == 0)
return abs(b);
else return gcf(b, a % b);
}
int Fraction::lcm(int a, int b) {
return(a / gcf(a, b)) * b;
}
If it helps at all, I am using GCC with the command g++ -o.
Any help much appreciated!

Answer 1

Where is your main function? Every "ordinary" program in C++ starts from main function, which is why the linker is looking for one. You haven't provided it. Hence the error.

Answer 2

I think the problem is that you're compiling a source file that doesn't contain a main function. Not every source file has to have main defined, but every C++ program needs to have it somewhere, and since you didn't post any other source files I'll assume that this is your only file. If you try compiling and linking this code, you'll get an error because there is no entry point into the program.
To fix this, either link your code together with a file that contains a main function, or add a main function to your code, or compile the code without linking it (this depends on your compiler).
Also, you should probably split your code into a .h/.cpp pair. Typically speaking, classes are defined in header files so that they can be used by other parts of the program, while implementations are left int the .cpp file so that they aren't visible to clients.

Answer 3

Add a -c flag when using gcc; it stops the compiler looking for the main function

Answer 4

Almost every C++ function has a main function, which tells the computer which part of your code it should do first. Otherwise, if someone starts your program, where should it begin? It has a form like below:
int main(int argc, char **argv) {
//what should happen when someone runs the program?
//that goes here
return 0; //program all done
}

Answer 5

I think u must change the extension of the file u are using to ".h" instead of ".cpp"
for example :
if this file is named as abc.cpp and u r compiling it the way "gcc abc.cpp" then u must change it to "gcc abc.h" ......
this will instruct the compiler to compile a Header rather than an object file.... :)