Removing class name causes segmentation fault - c++

Question

When I define a function of a class, I call another function of the same class within it. But when I do not type the class name it gives segmentation fault. Check below.
Header file
class DomainSolver
{
int fnc1 (UserDefinedType & var);
int fnc2 (UserDefinedType & var);
};
C file
int DomainSolver::fnc2 (UserDefinedType & var)
{
return 0;
}
int DomainSolver::fnc1 (UserDefinedType & var)
{
// fnc2 (var); // does not work
DomainSolver::fnc2(var); // works
return 0;
}

Answer 1

Wild guess… since the code you presented does not have any issues…
The function being called is declared virtual in a base class, so even if the virtual keyword is not present in the declaration here it is virtual.
The function being called does not access any member of the object.
You are calling the function on an invalid pointer or reference (for example through a null pointer or on an object that has already been deleted.
If all those guesses are right, the use of the qualification inhibits the dynamic dispatch mechanism, avoiding the dereference of an invalid pointer to the vtable. The code is still wrong (due to the third point above), but it seems to work.
The solution is not to call a member function through an invalid pointer or reference.

Answer 2

Although as pointed out by Zac's reply, the functions as you present them are not properly formed, there shouldn't be a difference between calling the scoped version; if you are getting a segfault in one case and not the other it's possibly because of code elsewhere.
Here is an example that works just fine:
dsolver.hh
#ifndef DSOLVER_HH
#define DSOLVER_HH
class DomainSolver
{
public:
int fnc1 (int& var);
int fnc2 (int& var);
};
#endif
dsolver.cc
#include <iostream>
#include "dsolver.hh"
int DomainSolver::fnc1 (int& var)
{
std::cout << "fnc1\n";
fnc2( var );
var = -1;
return var;
}
int DomainSolver::fnc2 (int& var)
{
std::cout << "fnc2\n";
var = 100;
return var;
}
main.cc
#include <iostream>
#include "dsolver.hh"
int main()
{
DomainSolver my_dsolver;
int my_int = 5;
my_dsolver.fnc1(my_int);
return 0;
}

Answer 3

Assuming this is close to your actual code, you have undefined behavior in fnc1:
int DomainSolver::fnc1 (UserDefinedType & var)
{
// fnc2 (var); // does not work
DomainSolver::fnc2(var); // works
// missing return!!!
}
You declare it to return an int, but then never return anything (in either case). Both cases are UB, so anything they do is technically "valid", since your code is not.
This code should be:
int DomainSolver::fnc1 (UserDefinedType & var)
{
return fnc2 (var);
}
As a side note: This is a good example of why you should never ignore the warnings given by the compiler (as you should have received a warning with both versions).
EDIT
With your latest edit adding a return value to fnc1, you'll need to show more of your actual code as there is not enough there to properly diagnose the problem (with the return being there, there is nothing wrong with your shown code).