How to fix C++ compiler-error "cannot convert 'Type' to 'const Type*'"?

Question

This is the complete error message:

error: cannot convert 'MyTime' to 'const MyTime*' for argument '1' to 'int DetermineElapsedTime(const MyTime*, const MyTime*)'|

And this is my code:

#include <iostream>
#include<cstdlib>
#include<cstring>

using namespace std;
struct MyTime { int hours, minutes, seconds; };
int DetermineElapsedTime(const MyTime *t1, const MyTime *t2);
const int hourSeconds = 3600;
const int minSeconds = 60;

int DetermineElapsedTime(const MyTime *t1, const MyTime *t2)
{
    long timeDiff = ((((t2->hours * hourSeconds) + (t2->minutes * minSeconds) + t2->seconds) -
                   ((t1->hours * hourSeconds) + (t1->minutes * minSeconds) + t1->seconds)));
    return(timeDiff);
}


int main(void)
{
    char delim1, delim2;
    MyTime tm, tm2;
    cout << "Input two formats for the time. Separate each with a space. Ex: hr:min:sec\n";
    cin >> tm.hours >> delim1 >> tm.minutes >> delim2 >> tm.seconds;
    cin >> tm2.hours >> delim1 >> tm2.minutes >> delim2 >> tm2.seconds;

    DetermineElapsedTime(tm, tm2);

    return 0;

}

Is there any way that I can fix? Please feel free to point out any other errors that you see. I do know about fixing DetermineTimeElapsed to properly output the hr:min:sec format. but right now I need to get past this.

Solution 1

The error should be at below line:

DetermineElapsedTime(tm, tm2);

You are passing MyTime objects to the above function when it expects const MyTime*.

Fix it by either passing the object addresses:

DetermineElapsedTime(&tm, &tm2);

Or better C++ way: by changing the function prototype to accept object references:

int DetermineElapsedTime(const MyTime &t1, const MyTime &t2);

the body also will change accordingly; e.g. -> will be replaced by . operator and so on.

Solution 2

The function is expecting a pointer to 2 variables but you are passing the variables themselves, that is the issue. You fix this by passing a pointer to the variables by just passing their memory addresses using the & operator as shown below

DetermineElapsedTime(&tm, &tm2);

Alternatively you can change the function to receive references to the variables as @iammilind suggests, which would mean you can leave the above line as it was. This would be a safer, cleaner more "C++" way.

Solution 3

Your funciton needs to take two const pointers:

int DetermineElapsedTime(const MyTime *t1, const MyTime *t2)
                         \_____one______/  \______two_____/

Now, here's how you are calling it:

MyTime tm, tm2;
DetermineElapsedTime(tm, tm2);

As you see you are passing the variables by value instead of passing them by a pointer as a function would expect. You can fix it in a couple of ways:

• change the function to expect a const reference:

  int DetermineElapsedTime(const MyTime &t1, const MyTime &t2)
  

• take the address of the variables that are being passed:

  MyTime tm, tm2;
  DetermineElapsedTime(&tm, &tm2);
  

• allocate stuff dynamically and pass pointers:

  MyTime *tm = new MyTime();
  MyTime *tm2 = new MyTime();
  DetermineElapsedTime(tm, tm2);
  

Solution 4

Your function DetermineElapsedTime expects pointers to MyTime.

Change your code to:

DetermineElapsedTime(&tm, &tm2);

The & operator in this context means "get the address of"

View more solutions

Comments

[iammilind]

Dynamic allocation might be an overkill for such problems.