C++: Issues with Circular Buffer - c++

Question

I'm having some trouble writing a circular buffer in C++. Here is my code base at the moment:
circ_buf.h:
#ifndef __CIRC_BUF_H__
#define __CIRC_BUF_H__
#define MAX_DATA (25) // Arbitrary size limit
// The Circular Buffer itself
struct circ_buf {
int s; // Index of oldest reading
int e; // Index of most recent reading
int data[MAX_DATA]; // The data
};
/*** Function Declarations ***/
void empty(circ_buf*);
bool is_empty(circ_buf*);
bool is_full(circ_buf*);
void read(circ_buf*, int);
int overwrite(circ_buf*);
#endif // __CIRC_BUF_H__
circ_buf.cpp:
#include "circ_buf.h"
/*** Function Definitions ***/
// Empty the buffer
void empty(circ_buf* cb) {
cb->s = 0; cb->e = 0;
}
// Is the buffer empty?
bool is_empty(circ_buf* cb) {
// By common convention, if the start index is equal to the end
// index, our buffer is considered empty.
return cb->s == cb->e;
}
// Is the buffer full?
bool is_full(circ_buf* cb) {
// By common convention, if the start index is one greater than
// the end index, our buffer is considered full.
// REMEMBER: we still need to account for wrapping around!
return cb->s == ((cb->e + 1) % MAX_DATA);
}
// Read data into the buffer
void read(circ_buf* cb, int k) {
int i = cb->e;
cb->data[i] = k;
cb->e = (i + 1) % MAX_DATA;
}
// Overwrite data in the buffer
int overwrite(circ_buf* cb) {
int i = cb->s;
int k = cb->data[i];
cb->s = (i + 1) % MAX_DATA;
}
circ_buf_test.cpp:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
#include "circ_buf.h"
int main(int argc, char** argv) {
// Our data source
std::string file = "million_numbers.txt";
std::fstream in(file, std::ios_base::in);
// The buffer
circ_buf buffer = { .s = 0, .e = 0, .data = {} };
for (int i = 0; i < MAX_DATA; ++i) {
int k = 0; in >> k; // Get next int from in
read(&buffer, k);
}
for (int i = 0; i < MAX_DATA; ++i)
std::cout << overwrite(&buffer) << std::endl;
}
The main issue I'm having is getting the buffer to write integers to its array. When I compile and run the main program (circ_buf_test), it just prints the same number 25 times, instead of what I expect it to print (the numbers 1 through 25 - "million_numbers.txt" is literally just the numbers 1 through 1000000). The number is 2292656, in case this may be important.
Does anyone have an idea about what might be going wrong here?

Answer 1

Your function overwrite(circ_buf* cb) returns nothing (there are no return in it's body). So the code for printing of values can print anything (see "undefined behavior"):
for (int i = 0; i < MAX_DATA; ++i)
std::cout << overwrite(&buffer) << std::endl;
I expect you can find the reason of this "main issue" in the compilation log (see lines started with "Warning"). You can fix it this way:
int overwrite(circ_buf* cb) {
int i = cb->s;
int k = cb->data[i];
cb->s = (i + 1) % MAX_DATA;
return k;
}