Uninitialised value was created by a stack allocation,segmentation fault - c++

Question

I am writing to perform and test a binary search. The aim is to get the maximum x when y<=0 for the function y=x-1, and the search range is [-2,2),obviously the answer should be 1.
for test part, I should test if the invocations of function f is less than the calculated max_invoke
a segmentation fault appeared, and when I valgrind it, the error is:
==126667== Use of uninitialised value of size 8
==126667== at 0x400C8A: binarySearchForZero(Function<int, int>*, int,
int) (in /home/jw562/ece551/092_tests_binsrch/test)
==126667== by 0x400DD5: check(Function<int, int>*, int, int, int,
char const*) (in /home/jw562/ece551/092_tests_binsrch/test)
==126667== by 0x400E81: main (in
/home/jw562/ece551/092_tests_binsrch/test)
==126667== Uninitialised value was created by a stack allocation
==126667== at 0x400E41: main (in
/home/jw562/ece551/092_tests_binsrch/test)
below is my code:
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
using namespace std;
template<typename R, typename A>
class Function {
public:
virtual R invoke(A arg) = 0;
virtual ~Function() {}
};
class CountedIntFn : public Function<int,int>{
protected:
unsigned remaining;
Function<int,int> * f;
const char * mesg;
public:
CountedIntFn(unsigned n, Function<int,int> * fn, const char * m):
remaining(n),f(fn),mesg(m) {}
virtual int invoke(int arg) {
if (remaining == 0) {
fprintf(stderr,"Too many function invocations in %s\n", mesg);
exit(EXIT_FAILURE);
}
remaining--;
return f->invoke(arg);
}
class linearFunction : public Function<int, int> {
public:
virtual int invoke(int arg) {
int ans = arg-1;
return ans;
}
virtual ~linearFunction(){}
};
int binarySearchForZero(Function<int, int> * f, int low, int high){
if (high <= low){
cout << "high less or equal than low" << endl;
return EXIT_FAILURE;
}
int low_ans = f->invoke(low);
int high_ans = f->invoke(high);
int mid = (high-low)/2+low;
int mid_ans = f->invoke(mid);
if (low_ans > 0){
return low;//all positive
}
if (high_ans < 0){
return high-1;//all negtive
}
if (low_ans = high_ans){
return low;//all zero
}
if (mid_ans <= 0){
low = mid;
}
if (mid_ans >= 0){
high = mid;
}
return binarySearchForZero(f,low,high);
}
void check(Function<int,int> * f,int low,int high,int
expected_ans,const char * mesg){
int max_invoke = 0;//the maximum number of invocations allowed
if (high > low){
max_invoke = (int)(log2(high-low))+1;
}
else {
max_invoke = 1;
}
CountedIntFn count(max_invoke,f,mesg);
int ans = binarySearchForZero(f,low,high);
if (ans != expected_ans){
cout << "wrong answer" <<endl;
}
}
int main(void){
linearFunction *fl;
const char * message = "linearFunction";
int low = -2;
int high = 2;
int expected_ans = 1;
check(fl,low,high,expected_ans,message);
return EXIT_SUCCESS;
}

Answer 1

This is because variable fl of type linearFunction* is uninitialized.
You can change it to a non-pointer, and invoke with address-of operator:
linearFunction fl;
...
check(&fl, low, high, expected_ans, message);