Question

I have a strong use case for pre-allocating all the memory I need upfront and releasing it upon completion.

I have came out with this real simple buffer pool C++ implementation which I have to test but I am not sure that the pointer arithmetic I am trying to use will allow me to do that. Basically the bit where I do next and release. I would prefer some trick around this idea and not relying on any sort of memory handler which just makes the client code more convoluted.

#include <stdio.h>
#include <queue>

#include "utils_mem.h"

using namespace std;

template <class T>
class tbufferpool {
private:
    const int m_initial;
    const int m_size;
    const int m_total;
    T*        m_buffer;
    vector<T*> m_queue;

public:
    // constructor
    tbufferpool(int initial, int size) : m_initial(initial), m_size(size), m_total(initial*size*sizeof(T)) {
        m_buffer = (T*) malloc(m_total);
        T* next_buffer = m_buffer;
        for (int i=0; i < initial; ++i, next_buffer += i*size) {
            m_queue.push_back(next_buffer);
        }
    }

    // get next buffer element from the pool
    T* next() {
        // check for pool overflow
        if (m_queue.empty()) {
            printf("Illegal bufferpool state, our bufferpool has %d buffers only.", m_initial);
            exit(EXIT_FAILURE);
        }
        T* next_buffer = m_queue.back();
        m_queue.pop_back(); 
        return next_buffer;
    }

    // release element, make it available back in the pool
    void release(T* buffer) {
        assert(m_buffer <= buffer && buffer < (buffer + m_total/sizeof(T)));
        m_queue.push_back(buffer);
    }

    void ensure_size(int size) {
        if (size >= m_size) {
            printf("Illegal bufferpool state, maximum buffer size is %d.", m_size);
            exit(EXIT_FAILURE);
        }
    }

    // destructor
    virtual ~tbufferpool() {
        free(m_buffer);
    }
};

Answer 1

First, when you increase a pointer to T, it will point the next element of T in the memory.

m_queue.push(m_buffer + (i*size*sizeof(T)));

This should be like

m_buffer = (T*) malloc(m_total);
T* next = m_buffer;
for (int i=0; i < initial; ++i) {
    m_queue.push(next++);
}

Second,

assert(m_buffer <= buffer && buffer < m_total);

It should be like

assert(m_buffer <= buffer && buffer <= m_buffer + m_total/sizeof(T));

Hope it helps!