simple C++ code linker error [duplicate] - c++

Question

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What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 5 years ago.
Im trying to run a very simple snippet of code. I've been getting a linker error. My code looks like this:
main.cpp -->
#include <iostream>
#include "Complex.h"
using namespace std;
int main()
{
Complex c1(1.0, 5.0); // this should create Complex object 1.0 + i5.0
return 0;
}
Complex.h -->
#include <iostream>
class Complex {
private:
double real;
double imaginary;
public:
Complex(double, double);
void setReal(double);
void setImaginary(double);
};
Complex.cpp -->
#include "Complex.h"
#include <cmath>
Complex::Complex(double x, double y) {
setReal(x);
setImaginary(y);
}
void Complex::setReal(double x) {
real = x;
}
void Complex::setImaginary(double x) {
imaginary = x;
}
The error I've been getting looks like this:
I have been trying to run my main for a while but I keep getting the linker error. I have no clue what's causing it. I would appreciate any suggestions.

Answer 1

Just do
g++ main-3.cpp complex.cpp
mind the filename main-3, this is inconsistent in your question.
You have to feed all cpp files you are using into the g++ commandline to tell g++ where the code for the functions defined in the header lies. Read up on .o files, static linking and understand what that means.
Here is a little guide I follow to understand file inclusion and other factors:
c++ compilation is moronically simple:
read .cpp file
replace every #include statement with the text of the specified file (just dump it in)
If the resulting text still has #include directives (now from the header files), goto step 2.
Compile the hughe messy blob into a ´.o´ object file, replace calls to functions with symbols and add to that file a table of known symbols and where they are defined.
if there are more .cpp files specified, start a new empty text blob. Goto step 1.
call the linker ´ld´ to link all object files together, replace symbols with the actual addresses.
Strictly speaking, above is a little bit of a lie nowadays and a lot is left out and no optimizations mentioned. But it is still a useful guide to the compiler's behaviour.
To use it to interpret your error:
Complex.h got dumped into your blob, via the #include in main.cpp , but Complex.cpp did not. g++ generated an internal temporary .o file for you that contained something along the lines of
PUT 1.0 on Stack
PUT 5.0 on Stack
JUMP Complex::Complex
... and ran the linker ´ld´ with that .o file.
ld Could not find the address of the symbol Complex::Complex,
it needs a memory address to jump to.
If you compile Complex.cpp as well, the resulting Complex.o will have a symbol table with, for example, this entry:
Complex::Complex = 0xaaff
The linker, given Complex.o can now replace the symbol in main.o with an address.
PUT 1.0 on Stack
PUT 5.0 on Stack
JUMP 0xaaff