The execution of program stops on windows seven abruptly..whats the error..!!

Hi!
I am new to C++ programming and was tinkering around with a program to learn about function pointers. I wrote the following code (using borland c++ compiler). The program compiles without error or warning but when I run the exe file it aborts..error message pops up on windows 7 saying execution of program has stopped working...
Below is the code..can someone tell me what have I done wrong..I am unable to figure out .. :(

Code:

---

#include <iostream>

float* Plus(float num1, float num2) {float* res; *res = num1 + num2; return res;}
typedef float*(*pFunc)(float, float);

pFunc GetPointer(char c)
{
    if(c == '+')
        return &Plus;
    else if(c == '-')
        return &Plus;
    else
        return &Plus;

}

float Calc(float num1, float num2, float* (*pCalcMethod)(float, float))

{
    float x=23;
   
    std::cout<< "result inside Calc method" << *pCalcMethod(num1, num2) << std::endl;
 
    return x;
}

int main()
{
    float result = Calc(50, 25, &Plus);
   
    std::cout << "Result: " << result << std::endl;
    system("PAUSE");       
    return 0;
}

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>>float* res; *res = num1 + num2;

For starters, you declare a pointer without assigning it then dereference it and try to change its value

The error is due to the line "std::cout<< "result inside Calc method" << *pCalcMethod(num1, num2) << std::endl;"

Quote:

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Originally Posted by Fordy

>>float* res; *res = num1 + num2;

For starters, you declare a pointer without assigning it then dereference it and try to change its value

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Hey thank you Fordy.. I added float* res=new float; .. After doing this the code is working fine :)
However I have one doubt..What is the best way to delete the memory assigned to the pointers in the code below

Code:

---

#include <iostream>

float* Plus(float num1, float num2) {float* res=new float; *res = num1 + num2; return res;}
typedef float*(*pFunc)(float, float);

pFunc GetPointer(char c)
{
    if(c == '+')
        return &Plus;
    else if(c == '-')
        return &Plus;
    else
        return &Plus;

}

float Calc(float num1, float num2, float* (*pCalcMethod)(float, float))

{
    float x=23;
   
    std::cout<< "result inside Calc method" << *pCalcMethod(num1, num2) << std::endl;
 
    return x;
}

int main()
{
    float result = Calc(50, 25, &Plus);
   
    std::cout << "Result: " << result << std::endl;
    system("PAUSE");       
    return 0;
}

---

Also in the below code if I use resultF pointer just like res in the earlier code(ie without initializing)..but it does not throw any error..why..??!!

Code:

---

#include <iostream>


float Plus(float num1, float num2) {return num1 + num2;}

typedef float(*pFunc)(float, float);

pFunc GetPointer(char c)
{
    if(c == '+')
        return &Plus;
    //else
    //    return &Multiply;
    else if(c == '-')
        return &Plus;
    else
        return &Plus;

}


float* Calc(float num1, float num2, char c)

{
    float (*pFuncRes)(float, float)=GetPointer(c);
   
    float *resultF;
    *resultF=pFuncRes(num1, num2);
   
    return resultF;
   
}

int main()
{
    float* result = Calc(50, 25, '+');
   
    std::cout << "Result: " << *result << std::endl;
    system("PAUSE");       
    return 0;
}

---

Why do you use pointers to return fundamental types, when they do not need to be shared?. Just pass them by value like you do with the parameters. To free memory you have to call the 'delete' operator on the pointer.

Quote:

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Originally Posted by Sourabh Dugar

Hey thank you Fordy.. I added float* res=new float; .. After doing this the code is working fine :)

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Except for a memory leak, yeah.

Quote:

---

Originally Posted by Sourabh Dugar

However I have one doubt..What is the best way to delete the memory assigned to the pointers in the code below

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Store the pointer returned by the function (Plus() or, indirectly, via a function pointer of type pFunc) and, when the value pointed to will not be needed any more, release it (using operator delete).

Better yet, change your Plus() function so it returns a float, rather than a pointer to float. That way you have no need to dynamically create a float, so no need to release it. Also change the typedef for pFunc accordingly, and code that uses them.

And try not to use Java techniques in C++.....

>>However I have one doubt..What is the best way to delete the memory assigned to the pointers in the code below

For floats, dont bother with pointers as you are using them. Just pass them and return them as variables.

>>float *resultF; *resultF=pFuncRes(num1, num2);

Same mistake as before...pointer pointing to nothing, then being dereferenced

Yeah Fordy I agree..I can return value.. but I am just a beginner..and was experimenting with pointers with this simple example :)
However the second time the program doesn't abort (the resultF case).. I wonder how its working the second time around o_O

Grumpy I am a total beginner so I have a doubt ..where is the best place to insert delete..?
Main func passes &Plus to Calc.. therefore the Plus method returns this pointer to Calc again..Now i need to pass the value of this pointer(from Calc to Main where it will be displayed) using return function..so how do i delete this pointer..??!! Sorry if I sound naive..and thanks for the help.. :)

Hey one more thing Grumpy..which technique do u think is java based..?!

As an example, inside your Calc function, you might do this

Code:

---

float* Calc(float num1, float num2, char c)

{
    pFunc pFuncRes =GetPointer(c);  // simpler way of getting the pointer
   
    float *resultF = pFuncRes(num1, num2);    // store the returned pointer
    float returnvalue = *resultF;    // store the value pointed to
    delete resultF;                        //  release the allocated memory
    return returnvalue;   
}

---

Anyway, the best thing you could do is change the return values of your functions from float * to float. Just because you CAN return a pointer, doesn't mean it's a good idea. As exhibited by the hoops jumped through in my example to make things work right.

It is a moderately common Java technique to dynamically allocate a value, and later forget about it. This works in Java for a number of reasons (eg garbage collection, semantics of "what is a variable") but such techniques are really bad karma in C++.

Quote:

---

Originally Posted by grumpy

As an example, inside your Calc function, you might do this

Code:

---

float* Calc(float num1, float num2, char c)

{
    pFunc pFuncRes =GetPointer(c);  // simpler way of getting the pointer
   
    float *resultF = pFuncRes(num1, num2);    // store the returned pointer
    float returnvalue = *resultF;    // store the value pointed to
    delete resultF;                        //  release the allocated memory
    return returnvalue;   
}

---

Anyway, the best thing you could do is change the return values of your functions from float * to float. Just because you CAN return a pointer, doesn't mean it's a good idea. As exhibited by the hoops jumped through in my example to make things work right.

It is a moderately common Java technique to dynamically allocate a value, and later forget about it. This works in Java for a number of reasons (eg garbage collection, semantics of "what is a variable") but such techniques are really bad karma in C++.

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Thank you Grumpy for the help..I wrote this code mainly to get a feel of function pointers..so just asking doubts to clarify things :)
If i use the code like u suggest..then return type of Calc should be made float not float*..right??
Also just one more thing..if u have seen my earlier posts..why is it that when I didn't initialize res in Plus the program aborted..but in the second or third post (inside the Calc method) when I use the resultF without initialising it works..!!
Thanks for the help again.. :)

Quote:

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Originally Posted by Sourabh Dugar

Also just one more thing..if u have seen my earlier posts..why is it that when I didn't initialize res in Plus the program aborted..but in the second or third post (inside the Calc method) when I use the resultF without initialising it works..!!
Thanks for the help again.. :)

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It doesn't. Failure to crash your computer does not mean "it works".

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