Function execution time is weird - c++

Question

I am trying to evaluate the time difference between looping in a linked list with an operation on each element in these two scenarios:
1) Doing the operation inside a function
2) Doing the operation without a function call in the same place
I was expecting that the variation with the function call with be a lot costlier due to the OS overhead of creating and destroying a stackframe for every call, but the results I got was just the opposite. I could not understand why. Could someone please explain what happened?
This is my program:
// ConsoleApplication4.cpp : Defines the entry point for the console application.
#include "stdafx.h"
#include<iostream>
#include<chrono>
#include<stdlib.h>
#define _CRT_SECURE_NO_WARNINGS
class linked_list_node
{
int a;
public :
std::string var;
bool eval()
{
if (var == "abc")
return true;
return false;
}
linked_list_node() { a = rand() % 100; if (a % 2 == 0) var = "abc"; }
linked_list_node* nxt;
std::string getVar() { return var; }
linked_list_node* getNext()
{
return nxt;
}
};
int _tmain(int argc, _TCHAR* argv[])
{
linked_list_node *head = new linked_list_node();
linked_list_node *trav = head;
int len = 75000;
while (len != 0)
{
linked_list_node *n = new linked_list_node();
trav->nxt = n;
trav = n;
len--;
}
trav->nxt = NULL;
//traversal with function
int length = 0;
trav = head;
std::chrono::steady_clock::time_point begin = std::chrono::steady_clock::now();
while (trav != NULL)
{
length++;
if (trav->eval())
std::cout << "";
trav = trav->nxt;
}
std::chrono::steady_clock::time_point end = std::chrono::steady_clock::now();
std::cout << "Time difference with function == " << std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin).count() << std::endl;
//traversal without function
trav = head;
length = 0;
begin = std::chrono::steady_clock::now();
while (trav != NULL)
{
length++;
if (trav->var =="abc")
std::cout << "";
trav = trav->nxt;
}
end = std::chrono::steady_clock::now();
std::cout << "Time difference without function = " << std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin).count() << std::endl;
getchar();
return 0;
}
These are my results :
Time difference with function == 18100
Time difference without function = 33700000

Answer 1

First difference:
In the function, you are comparing the value of var to"abc".
In the non-function call code, you are comparing the value of var to "ram".
The second difference, the most important one:
In the first case, you are using std::chrono::microseconds.
In the second case, you are using std::chrono::nanoseconds
After I fix those errors, I get consistently lower value for the second number than the first one.

Answer 2

You're measuring time in different units: std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin) vs std::chrono::duration_cast<std::chrono::microseconds>(end - begin).