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sandbox implementation of shared_ptr #1

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6ab859c
sandbox implementation of shared_ptr
jroelofs Jan 27, 2020
eee93b8
Fix leak
jroelofs Mar 5, 2020
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2 changes: 1 addition & 1 deletion Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -7,4 +7,4 @@ clean:

undo: undo.cpp
@mkdir -p bin
clang++ undo.cpp -o bin/undo --std=c++14
clang++ undo.cpp -o bin/undo --std=c++14 -O0 -g -fsanitize=address,undefined
118 changes: 118 additions & 0 deletions shared.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,118 @@
#ifndef SHARED_H
#define SHARED_H

#include <cassert>
#include <cstddef>
#include <utility>
#include <cstdio>

namespace sandbox {

#if DEBUG_RETAIN
static int cb_count = 0;
#endif

template <typename T>
struct shared_ptr {
struct control_block {
template<typename U>
control_block(U *u_, size_t c_) : t(u_), strong_count(u_)
{
#if DEBUG_RETAIN
printf("CB #%d ctor\n", id = ++cb_count);
#endif
}

control_block(T *t_, size_t c_) : t(t_), strong_count(c_) {
#if DEBUG_RETAIN
printf("CB #%d ctor\n", id = ++cb_count);
#endif
}

~control_block() {
#if DEBUG_RETAIN
printf("CB #%d dtor\n", id = cb_count--);
#endif
}

#if DEBUG_RETAIN
int id;
#endif
T *t;
size_t strong_count;
};

template<class U> explicit shared_ptr(U *u)
: cb(new control_block(static_cast<T*>(u), 0)) {
retain(cb);
}

shared_ptr(const shared_ptr &rhs) : cb(rhs.cb) {
retain(cb);
}

template<class U>
shared_ptr(shared_ptr<U> &&rhs) {
// FIXME: this seems like an ugly hack / potential UB.
cb = reinterpret_cast<control_block*>(rhs.cb);
retain(cb);
}

shared_ptr &operator=(const shared_ptr &rhs) {
if (this != &rhs) {
release(cb);
cb = rhs.cb;
retain(cb);
}
return *this;
}

~shared_ptr() {
release(cb);
}

void release(control_block *&cb) {
#if DEBUG_RETAIN
printf("release %d --> %zu\n", cb->id, cb->strong_count);
#endif

if (--cb->strong_count)
return;

assert(cb->strong_count == 0 && "releasing, but there are still referers");

delete cb->t;
cb->t = nullptr;

delete cb;
cb = nullptr;
}

void retain(control_block *cb) {
cb->strong_count++;
#if DEBUG_RETAIN
printf("retain %d --> %zu\n", cb->id, cb->strong_count);
#endif
}

T *operator->() {
assert(cb && cb->strong_count && "dereferencing without a pointee");
return cb->t;
}

const T *operator->() const {
assert(cb && cb->strong_count && "dereferencing without a pointee");
return cb->t;
}

control_block *cb;
};

template <typename T, typename ...Args>
shared_ptr<T> make_shared(Args... args) {
return shared_ptr<T>(new T(std::forward<Args>(args)...));
}

}

#endif
6 changes: 4 additions & 2 deletions undo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,8 @@
* https://www.youtube.com/watch?v=bIhUE5uUFOA
*/

#include "shared.h"

#include <iostream>
#include <memory>
#include <string>
Expand All @@ -17,7 +19,7 @@ void draw(const T &t, std::ostream &out, size_t position);
class object_t {
public:
template <typename T>
object_t(T x) : self_(std::make_shared<model<T>>(std::move(x))) {}
object_t(T x) : self_(sandbox::make_shared<model<T>>(std::move(x))) {}

friend void draw(const object_t &x, std::ostream &out, size_t position) {
x.self_->draw_(out, position);
Expand All @@ -39,7 +41,7 @@ class object_t {
T data_;
};

std::shared_ptr<const concept_t> self_;
sandbox::shared_ptr<const concept_t> self_;
};

using document_t = std::vector<object_t>;
Expand Down