Exception thrown: write access violation. this was nullptr - c++

Question

So I am trying to make a buffer class. This buffer class contains a huge buffer of size 384*4. The plan was for every UDP datagram received, size(384), the buffer class is called and return a pointer to where the datagram should be written.
And there will be another listener pointer to which RtAudio playback will memcpy from. [The listening part is not entirely relevant yet as I still have a problem writing into the buffer]
When I try to call server_->getPointer() (shown below), the "Exception thrown: write access violation. this was nullptr." is thrown. Please help me!! and tell me if there is anything else that I should provide.
Buffer.h
#pragma once
#ifndef BUFFER_H
#define BUFFER_H
class Buffer {
private:
int bufferSize = 192 * 2; // one frame takes 2 Byte [int16]
int nBuffers = 4;
int *buffer_ = nullptr;
int* writerPointer = nullptr;
int* listenerPointer = nullptr;
int writerCounter = 0;
int listenerCounter = 0;
int* tempW = nullptr;
int* tempL = nullptr;
public:
Buffer();
~Buffer();
int* getWriterPointer();
int* getlistenerPointer();
int * getPointer();
};
#endif // !BUFFER_H
Buffer.cpp
#include"Buffer.h"
#include <iostream>
Buffer::Buffer() {
buffer_ = reinterpret_cast<int*>(malloc(bufferSize*nBuffers));
memset(buffer_, (int)5, bufferSize*nBuffers);
std::cout << "new Buffer" << bufferSize * nBuffers << std::endl;
listenerPointer = buffer_;
writerPointer = buffer_;
std::cout << "HERE " << *buffer_ << std::endl;
std::cout << "new Buffer" << bufferSize * nBuffers << " pointer a " << listenerPointer << " pointer b " << writerPointer << std::endl;
}
Buffer::~Buffer() {
delete buffer_;
}
...
//For teting purposes
int* Buffer::getPointer(){
bufferSize = 192 * 2;
std::cout << "get pointer asdfasdf::" << writerCounter << std::endl;
std::cout << "pointer's position offset: " << writerCounter - 1 << std::endl;
if (writerCounter == nBuffers - 1) {
writerCounter = 0;
return writerPointer + (bufferSize*(nBuffers - 1));
}
else {
writerCounter += 1;
return writerPointer + (bufferSize*(writerCounter - 1));
}
}
main.cpp
#include <iostream>
#include "Buffer.h"
int main()
{
std::cout << "Hello World!\n";
Buffer *buffer_ = new Buffer();
buffer_->getPointer();
}

Answer 1

Look up "zero copying" for the protocol part.
The problem you have is that your pointer is actually a nullptr at the time you are trying to use it. You need to check the return from malloc:
Buffer::Buffer() :
buffer_(reinterpret_cast<int*>(malloc(bufferSize*nBuffers)))
{
if(buffer_ == nullptr) throw std::bad_alloc();
}
But, you should use new instead which would do this check and throw bad_alloc automatically if it fails:
Buffer::Buffer() :
buffer_(new int[bufferSize*nBuffers])
{
// no need to check here
}
For every malloc you need one free, and for every new you need one delete - and you should never mix them.
Also, malloc allocates bytes, but an int is usually 4 or 8 bytes so you'd need to multiply the number of ints you want to allocate space for with sizeof(int)*CHAR_BIT/8 to get the correct size.
Just forget about malloc and free and use new and delete in their place.
This is how you delete your array allocated with new int[...]:
Buffer::~Buffer() {
delete[] buffer_; // delete array
}
An even better option is to use a std::unique_ptr which will do the delete[] for you when it goes out of scope:
class Buffer {
private:
std::unique_ptr<int[]> buffer_;
public:
Buffer() :
buffer_(std::make_unique<int[]>(bufferSize * nBuffers))
{}
// ~Buffer() no need to implement a destructor unless you manually handle resources
};