Cpp

shared_ptr memory layout

Struct

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class __shared_ptr {
    element_type*	   _M_ptr;         // Contained pointer.
    __shared_count<_Lp>  _M_refcount;    // Reference counter.
}
class __shared_count {
	_Sp_counted_base<_Lp>*  _M_pi;
}


class _Sp_counted_ptr final : public _Sp_counted_base<_Lp>
class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>

class _Sp_counted_base {

    _Atomic_word  _M_use_count;     // #shared
    _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)

};

template<typename _Ptr, _Lock_policy _Lp>
class _Sp_counted_ptr final : public _Sp_counted_base<_Lp> {
    _Ptr             _M_ptr;
}

1. std::make_shared Function

Source

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// memory.cpp
#include <memory>
#include <vector>
#include <iostream>

using VecUL = std::vector<unsigned long>;

void func(std::shared_ptr<VecUL> datas) {
    auto count = datas.use_count();
    std::cout << count << "\n";
    for (auto &data : *datas) {
        data++;
    }
}

void func(std::shared_ptr<unsigned int> data) {
    auto count = data.use_count();
    std::cout << count << "\n";
}

int main() {
    std::shared_ptr<unsigned int> ptr = std::make_shared<unsigned int>();
    auto count = ptr.use_count();
    std::cout << count << "\n";
    *ptr = 65535;
    func(ptr);

    std::shared_ptr<VecUL> ptr1 = std::make_shared<VecUL>();
    auto count1 = ptr1.use_count();
    std::cout << count1 << "\n";
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    func(ptr1);

    return 0;
}

Debug

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> g++ -g -O1 memory.cpp -o memory
> gdb memory

(gdb) br func
Breakpoint 1 at 0x12e9: func. (2 locations)
(gdb) run

(gdb) x /3gx  $rdi
                # element_type*       # _Sp_counted_base<_Lp>*
0x7fffffffdaa0: 0x000055555556aec0      0x000055555556aeb0
0x7fffffffdab0: 0x00007ffff7dc2fc8

(gdb) x /2gx 0x000055555556aec0
                # Contained
0x55555556aec0: 0x000000000000ffff      0x0000000000000411

(gdb) x /4gx 0x000055555556aeb0 # _Sp_counted_base<_Lp>*
                # vtable pointer   #_M_weak_count #_M_use_count     
0x55555556aeb0: 0x0000555555557c78      0x0000000100000002
			   # Contained
0x55555556aec0: 0x000000000000ffff      0x0000000000000411
(gdb) c
Continuing.

(gdb) x /2gx  $rdi
                # element_type*       # _Sp_counted_base<_Lp>*
0x7fffffffdaa0: 0x000055555556b2f0      0x000055555556b2e0
(gdb) x /4gx  0x000055555556b2f0
			  # vector Contained
0x55555556b2f0: 0x000055555556b380      0x000055555556b3a8
0x55555556b300: 0x000055555556b3c0      0x0000000000000021
(gdb) x /6gx 0x000055555556b380   # vector data
0x55555556b380: 0x000000000000ffff      0x000000000000ffff
0x55555556b390: 0x000000000000ffff      0x000000000000ffff
0x55555556b3a0: 0x000000000000ffff      0x0000000000000000

(gdb) x /6gx 0x000055555556b2e0  # _Sp_counted_base<_Lp>*
                # _Sp_counted_ptr_inplace
                # vtable pointer      #_M_weak_count #_M_use_count 
0x55555556b2e0: 0x0000555555557cb0      0x0000000100000002
                # Contained
0x55555556b2f0: 0x000055555556b380      0x000055555556b3a8
0x55555556b300: 0x000055555556b3c0      0x0000000000000021

2. shared_ptr( Y* ptr ) Constructor

Source

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#include <memory>
#include <vector>
#include <iostream>

using VecUL = std::vector<unsigned long>;

void func(std::shared_ptr<VecUL> datas) {
    auto count = datas.use_count();
    std::cout << count << "\n";
    for (auto &data : *datas) {
        data++;
    }
}

void func(std::shared_ptr<unsigned int> data) {
    auto count = data.use_count();
    std::cout << count << "\n";
}

int main() {
    auto * p0 = new unsigned int{34};
    std::shared_ptr<unsigned int> ptr{p0};
    auto count = ptr.use_count();
    std::cout << count << "\n";
    *ptr = 65535;
    func(ptr);

    auto *p1 = new VecUL{0xff, 0xffff, 0xffffff, 0xffffffff};
    std::shared_ptr<VecUL> ptr1{p1};
    auto count1 = ptr1.use_count();
    std::cout << count1 << "\n";
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    ptr1->push_back(65535);
    func(ptr1);

    return 0;
}

Debug

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> g++ -g -O1 memory.cpp -o memory
> gdb memory

(gdb) br func
Breakpoint 1 at 0x1329: func. (2 locations)
(gdb) run
Breakpoint 1, func (data=std::shared_ptr<unsigned int> (use count 2, weak count 0) = {...}) at memory1.cpp:15

(gdb) x /2gx $rdi
0x7fffffffda90: 0x000055555556aeb0      0x000055555556aed0
(gdb) x /2gx 0x000055555556aeb0
0x55555556aeb0: 0x000000000000ffff      0x0000000000000000
(gdb) x /4gx 0x000055555556aed0 # _Sp_counted_base<_Lp>*
                # _Sp_counted_ptr
                # vtable pointer      #_M_weak_count #_M_use_count 
0x55555556aed0: 0x0000555555557c68      0x0000000100000002
                # Contained pointer
0x55555556aee0: 0x000055555556aeb0      0x0000000000000411

(gdb) c
Continuing.

(gdb) x /2gx $rdi
0x7fffffffda90: 0x000055555556b300      0x000055555556b350

(gdb) x /4gx 0x000055555556b300
0x55555556b300: 0x000055555556b3c0      0x000055555556b408
0x55555556b310: 0x000055555556b440      0x0000000000000031
(gdb) x /6gx 0x000055555556b3c0
0x55555556b3c0: 0x00000000000000ff      0x000000000000ffff
0x55555556b3d0: 0x0000000000ffffff      0x00000000ffffffff
0x55555556b3e0: 0x000000000000ffff      0x000000000000ffff
(gdb) x /4gx 0x000055555556b350 # _Sp_counted_base<_Lp>*
                # _Sp_counted_ptr
                # vtable pointer      #_M_weak_count #_M_use_count
0x55555556b350: 0x0000555555557ca0      0x0000000100000002
0x55555556b360: 0x000055555556b300      0x0000000000000051

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