Question

I am new to C++ and I’m having trouble using a map correctly in class that I’m writing. Basically, when I make a new Block object in a test program and call its write method, the program totally shits itself and gives me a double free or corruption error. What is weird is that everything works fine if I uncomment that one line in the Block constructor. I’m guessing I’m missing some basic c++ knowledge but I’m not finding anything useful on google.

Block.h (includes not shown but they are there):

namespace SSDSim{
    class Block{
        public:
            Block(uint block_num);
            ~Block(void);
            void read(uint page_num);
            void write(uint page_num, void *data);
            void erase(void);
        private:
            uint block_num;
            std::map<uint, void *> page_data;
    };
}

Block.cpp:

#include "Block.h"

using namespace std;
using namespace SSDSim;

Block::Block(uint block){
    //page_data[4]= (void *) 0xfeedface;
    block_num= block;
}

...

void Block::write(uint page_num, void *data){
    if (page_data.find(page_num) == page_data.end()){
        page_data[page_num]= data;
    } else{
        cerr<<"Invalid write\n";
        exit(1);
    }
}

test.cpp:

#include <iostream>
#include "../Block.h"

using namespace std;
using namespace SSDSim;

int main(void){
    Block b= Block(0);
    b.write(0, (void *) 0xdeadbeef);
    b.read(0);
//  b.read(4);
    return 0;
}

Answer 1

Are you sure the error isn’t when you call read()? After all, your constructor assigns page[4] to 0xfeedface, an arbitrary place in memory that could point to anything. If you try to then read that location in memory, bad things may happen.

I would try to avoid creating pointers out of arbitrary addresses. Instead, test your program by making some object and passing its address to page[4].

The error could also be in your destructor. Since you are allocating a Block on the stack, its destructor is called when it goes out of scope.