What does 'Error LNK2019: unresolved external symbol' mean? - c++

Question

I'm trying to run a program but it won't compile, I get errors. I have changed things, but doesn't seem to work. The code is this:
#include <iostream>
#include <string>
#include "StackLS.h"
using namespace std;
int main()
{
int answer;
char symbol;
char n, N;
StackLS stack;
bool balenced = true;
do {
cout << " ********** MENU ********** " << endl;
cout << " 1. Basic Brackets () " << endl;
cout << " 2. Standard Brackets ()[]{} " << endl;
cout << " 3. User-Defined brackets " << endl;
cout << " Please enter your choice: " << endl;
switch (answer){
case 1:
cout << "Current Setting: () " << endl;
cout << "Enter your expression followed by a ; : " << endl;
cin >> symbol;
do {
if (symbol = '(')
stack.push( '(' );
else
if (symbol = ')' )
{
if (stack.isEmpty())
balenced = false;
else {
symbol = stack.top();
stack.pop();
}
if (balenced)
cout << "Expression is well-formed" << endl;
else
cout << "Expression is not well-formed" << endl;
}
}
while (symbol != ';' && balenced);
stack.pop();
}
}
while (answer != 'n' || 'N');
} // end main
I haven't finished the program. I wanted to make sure that what I have so far will compile before I move on to the next case. Now I will post the errors I am getting. They are:
1>e:\c++ language 2\well-formed expression checker solution\well-formed expression checker project\main.cpp(11): warning C4101: 'n' : unreferenced local variable
1>e:\c++ language 2\well-formed expression checker solution\well-formed expression checker project\main.cpp(11): warning C4101: 'N' : unreferenced local variable
1>e:\c++ language 2\well-formed expression checker solution\well-formed expression checker project\main.cpp(22): warning C4700: uninitialized local variable 'answer' used
1>ManifestResourceCompile:
1> All outputs are up-to-date.
1>main.obj : error LNK2019: unresolved external symbol "public: int __thiscall StackLS::top(void)const " (?top#StackLS##QBEHXZ) referenced in function _main
1>main.obj : error LNK2019: unresolved external symbol "public: void __thiscall StackLS::push(int const &)" (?push#StackLS##QAEXABH#Z) referenced in function _main
1>E:\C++ language 2\Well-Form ed Expression Checker Solution\Debug\Well-Formed Expression Checker Project.exe : fatal error LNK1120: 2 unresolved externals
Thanks for the help.

Answer 1

The warnings are just that -- warnings. They don't stop your program from compiling, but you should look at them and try to fix them.
Your program actually compiles OK; the errors are keeping it from linking. That means that after your code has been compiled into machine code, and it's being built into a *.exe file,it turns out that some pieces are missing. It looks as if the StackLS.h file comes with either a C++ source file, or a *.lib or *.dll file; whatever you have, those need to be included when the executable is built, to supply those missing pieces.

Answer 2

It has compiled, you got some Warning about unused Variables.
Linking has failed and so what you are missing is the file where StackLS is implemented.
Just including the header does not makes them "implemented".
So you need something like
StackLS.cpp or the like
You don't have posted that.

Answer 3

The problem is (probably) not with your code, but with the way in which you are invoking your compiler/linker.
You need to compile the source file where int StackLS::top() const and void StackLS::push(int const &) are defined, and give the result to your linker that when linking your executable.

Answer 4

You program used a library called StackLS. This could either be precompiled library or some source code.
You add references to this library using the #include "StackLS.h", to allow the compiler to compile your code. This creates a compiled version of your code.
The next stage is linking your compiled code with the compiled StackLS library. This is the job of the Linker. These days the same program (compiler) usually makes all the calls needed to do this step too (though you can link your self), though technically it's a different step to compilation.
If StackLS is your code, then you must compile that too, or if it's a precompiled library, you need to tell the linker where to find it.
In your make file, you need to add a reference to the StackLS source code or library (it's usually a .dll or .lib type file in this case).

Answer 5

Remove the lines:
char n, N;
StackLS stack;
and see how you go.