Function call inside another function [duplicate] - c++

Question

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 8 years ago.
I have two files : a .cpp and a .hpp file. In the .hpp file there is a class with name Knoten and a public function definition :
static void faerbe_algorithmus(bool jsp[5][5], std::list< std::list<int> > &liste_von_str_orth_spalten);
In the .cpp file I am trying to call the function in another function (compute_J) like this :
Knoten::faerbe_algorithmus(jsp, liste_von_str_orth_spalten);
but I get the following error from g++ :
In function `compute_J(double*, double (*) [5])':
11_3.cpp:(.text+0x3fc): undefined reference to `Knoten::faerbe_algorithmus(bool (*) [5], std::list<std::list<int, std::allocator<int> >, std::allocator<std::list<int, std::allocator<int> > > >&)'
collect2: error: ld returned 1 exit status
What am I doing wrong? I can post more of the code, when needed.

Answer 1

Undefined reference often means that you have forgotten to add an implementation for the function you are trying to call.
For example:
Bad Foo.cpp
void doSomething();
int main()
{
doSomething(); // undefined reference, there is no implementation
}
Good Foo.cpp
void doSomething();
int main()
{
doSomething(); // this is OK, as the function is implemented later on
}
void doSomething()
{
// insert code here
}
If you have implemented the function somewhere, check the the name is qualified correctly.
For example (applies to both namespaces and classes/structs):
MyClass.hpp
class MyClass
{
public:
static void doSomething();
};
Bad MyClass.cpp
#include "MyClass.hpp"
void doSomething() // not qualified properly
{
// insert code here
}
Good MyClass.cpp
#include "MyClass.hpp"
void MyClass::doSomething() // now qualified properly
{
// insert code here
}

Answer 2

undefined reference to Knoten::faerbe_algorithmus
Is the sign that you are missing the definition for the static public function. You either forgot to define it with:
void Knoten::faerbe_algorithmus(bool jsp[5][5], std::list< std::list<int> > &liste_von_str_orth_spalten) {
// ...
}
or you are not linking the definition correctly.
On a side note, I'd suggest you to drop C-style arrays and start using std::array instead. It will save you a lot of troubles, especially with array to pointer decay. Here's the corresponding version:
void Knoten::faerbe_algorithmus(const std::array<std::array<bool, 5>, 5>& jsp, std::list< std::list<int> > &liste_von_str_orth_spalten) {
// ...
}
I know it's probably harder to write, but you can create an alias to it:
template<class Type, std::size_t Size>
using bidim = std::array<std::array<Type, Size>, Size>
and use it like:
void Knoten::faerbe_algorithmus(const bidim<bool, 5>& jsp, std::list< std::list<int> > &liste_von_str_orth_spalten) {
// ...
}