Reallocating memory correctly in c++

Question

#include <iostream>
#include <stdlib.h>

class circuitTypes{

protected:
    static int size;
    circuitTypes **Matrix;
    int input1,input2;
    int output1, output2;

public:

    circuitTypes() {};
    static int getSize() { return size; };
    static void upSize() { size++; };
    void ItemRegistry();
    virtual void setTruthTable()=0;
    void setInputAndCalculateOutput(int a, int b);
    int *getOutput();
};

int circuitTypes::size=0;

int *circuitTypes::getOutput(){
    int Output[2];
    Output[0]=output1;
    Output[1]=output2;
    return Output;
}
void circuitTypes::ItemRegistry(){

    circuitTypes::upSize();
    int circuitSize=circuitTypes::getSize();

    if(circuitSize==1)
        Matrix=(circuitTypes **)malloc(circuitSize*sizeof(circuitTypes *));
    else
        Matrix=(circuitTypes **)realloc(Matrix,circuitSize*sizeof(circuitTypes *));
    if(Matrix==0){
        std::cout <<"No available memory \n";
        exit(1);
    }
    Matrix[circuitSize-1]=this;
}


void circuitTypes::setInputAndCalculateOutput(int a, int b){
    input1=a;
    input2=b;
    setTruthTable();
}


class TypeA : private circuitTypes{

public:
    TypeA() { ItemRegistry(); };
    void setTruthTable();
};

void TypeA::setTruthTable(){
    if (input1==0){
        if (input2==0){
            output1=0;
            output2=0;
        }
        else{
            output1=0;
            output2=1;
        }
    }
    else{
        if (input2==0){
            output1=0;
            output2=1;
        }
        else{
            output1=1;
            output2=1;
        }
    }
}


class TypeB : private circuitTypes{
public:
    TypeB() { ItemRegistry(); };
    void setTruthTable();
};

void TypeB::setTruthTable(){
    if (input1==0){
        if (input2==0){
            output1=0;
            output2=0;
        }
        else{
            output1=0;
            output2=1;
        }
    }
    else{
        if (input2==0){
            output1=1;
            output2=1;
        }
        else{
            output1=0;
            output2=1;
        }
    }
}

void circuit (circuitTypes **Example, int a, int b){
    std::cout << "NIKKK";
    Example[0]->setInputAndCalculateOutput(a,b);
    int Size=Example[0]->getSize();
    for (int i=1;i<Size;i++){

        Example[i]->setInputAndCalculateOutput(Example[i-1]->getOutput()[0],Example[i-1]->getOutput()[1]);

    }
    std::cout << "For input a= " << a << " and b= " << b << " the result is c= " << Example[Size-1]->getOutput()[0] << " and d=" << Example[Size-1]->getOutput()[1] << "\n";
}


int main (){

    circuitTypes **Example;
    TypeA A1,A2,A3;
    TypeB B1,B2,B3;
    for (int i=0;i<2;i++){

        for (int j=0;j<2;j++){
            circuit (Example,i,j);
        }

    }

}

I 'm sorry for the size of the code, but as I have no idea where the problem is I decided to post the whole programm. The .cpp file is compiled without any problem in Ubuntu 11.10 (with g++) but when I try to execute the a.out file I get this error:

* glibc detected ./a.out: realloc(): invalid pointer: 0x003c2ff4 **

======= Backtrace: =========

/lib/i386-linux-gnu/libc.so.6(+0x721a2)[0x2b71a2]

/lib/i386-linux-gnu/libc.so.6(realloc+0x2a5)[0x2bb245]

./a.out[0x8048835]

./a.out[0x8048ca9]

./a.out[0x8048b2a]

/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0x25e113]

./a.out[0x80486c1]

======= Memory map: ========

00110000-0012c000 r-xp 00000000 08:06 1311674 /lib/i386-linux-gnu/libgcc_s.so.1

0012c000-0012d000 r--p 0001b000 08:06 1311674 /lib/i386-linux-gnu/libgcc_s.so.1

etc.

Can anyone help me?

My advice: forget that malloc or realloc exists at all. Use an std::vector instead. As for your truth table, for TypeA it seems to be just output1 = input1 & input2, output2 = input1 | input2. Your getoutput also returns the address of a local variable, so using it leads to undefined behavior (again, easily cured by returning a vector instead of an array). — Jerry Coffin
Right off the top, look at your main() carefully, then ask, and answer, this question of yourself: What is the value of Example that is being passed in to the function circuit()? — WhozCraig
realloc(0,size) is the same as malloc(size). So you could just null Matrix initially, and then do Matrix = realloc(Matrix, n); if (!Matrix) oom(); — Rob N

user207421 user207421 · Accepted Answer · 2013-02-26T02:09:31

   if(circuitSize==1)
       Matrix=(circuitTypes **)malloc(circuitSize*sizeof(circuitTypes *));
   else
       Matrix=(circuitTypes **)realloc(Matrix,circuitSize*sizeof(circuitTypes *));

I don't understand the if condition here. Surely it should be if (Matrix == 0)? Otherwise you run the risk of trying to realloc something that you have never malloc-ed in the first place.

But I agree with @JerryCoffin that you should throw it out and use a std::vector.

Reallocating memory correctly in c++

2 Answers