Constructor does not exist

Question

I am attempting to implement a circular buffer. So now I am testing some code.

raw_buffer.h

#include <algorithm> // for std::min

#ifndef RAWBUFFER_H
#define RAWBUFFER_H

class CircularBuffer
{
public:
    CircularBuffer(size_t capacity);
    ~CircularBuffer();

    size_t size() const { return size_; }
    size_t capacity() const { return capacity_; }
    // Return number of bytes written.
    size_t write(const char *data, size_t bytes);
    // Return number of bytes read.
    size_t read(char *data, size_t bytes);

private:
    size_t beg_index_, end_index_, size_, capacity_;
    char *data_;
};

extern CircularBuffer MyCircularBuffer;

#endif

main_raw.cpp

#include "raw_buffer.h"
#include <iostream>
#include <string>   

using namespace std;

    int main() {
        const char *data_test1 = "AAAA";
        const char *data_test2 = "BBBB";
        const char *data_test3 = "CCCC";

        CircularBuffer Buffer;//<---error

        return 0;
    }

The line CircularBuffer Buffer; is giving me the error: "no default constructor exists for class "CircularBuffer".

What is confusing about this? Your declaration and use are completely different. Simply don't do that. — Lightness Races in Orbit, Jul 20 '15 at 14:34
In this example how can I use this? I am trying to understand. Something like this? `CircularBuffer(12).write(data_teste1,4)` — jdscardoso, Jul 20 '15 at 14:55
You're the one who wrote the class. How come you don't know how to use it? That makes no sense. — Lightness Races in Orbit, Jul 20 '15 at 14:56
No, I am trying some samples to understand the implementation. "testing some code" — jdscardoso, Jul 20 '15 at 14:59
[link](http://www.asawicki.info/news_1468_circular_buffer_of_raw_binary_data_in_c.html) — jdscardoso, Jul 20 '15 at 15:02
That implementation has terrible bugs. You should learn C++ from [a good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list), not from random "tuts" on the internet. — Lightness Races in Orbit, Jul 20 '15 at 15:09

Cory Kramer · Accepted Answer · 2015-07-20T14:57:57.057

5

When you defined this parameterized constructor, you disabled the compiler generated default constructor

CircularBuffer(size_t capacity);

You can add the following to get the default constructor back (note that the default keyword is new as of C++11)

CircularBuffer() = default;

Though you should make sure that your class is instantiated in a valid state. If default initialized values do not fit the bill, then you should manually define the default constructor to initialize your member variables to a valid state.

If you intended to call your parameterized constructor, then you need to pass the argument that you declared. E.g.

CircularBuffer buffer{10};   // constructed with a capacity of 10

edited Jul 20 '15 at 14:57

answered Jul 20 '15 at 14:33

Cory Kramer

114,268
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4

It's unlikely a defaulted default ctor is the right solution here. You'd leave `capacity_` uninitialised... Are you sure this is good advice? – Lightness Races in Orbit Jul 20 '15 at 14:35
That is a good point, but I cannot answer that for the OP. They should either not use the default constructor if it doesn't make sense for this class, or use the compiler generated default, or define a default constructor that *does* leave the class in a valid state. Which they choose is up to them. – Cory Kramer Jul 20 '15 at 14:38
In general I agree but it is as least clear that leaving a member uninitialised after construction is a bad idea, which is unavoidable with your suggestion of `= default`. – Lightness Races in Orbit Jul 20 '15 at 14:41

score 3 · Answer 2 · answered Jul 20 '15 at 14:37

3

As @CoryKramer answered you didn t get a default constructor. If you want to change it you can also do this:

CircularBuffer(size_t capacity = 0);

by default if no capacity is send it will be are 0.

answered Jul 20 '15 at 14:37

Gabrielle de Grimouard

1,917
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Constructor does not exist

2 Answers2