c++ - 创建boost::shared_ptr的深层拷贝

Question

我有shared_ptr的两个 vector ；

typedef boost::shared_ptr <A> someptr;
std::vector<someptr>src;
std::vector<someptr>dest;

如果我使用dest = src复制，这两个 vector 元素共享相同的内存位置，这会增加指针的ref计数。它们都指向相同的位置，并且一个 vector 元素的任何变化都会影响另一个。我知道这是浅拷贝，这是预期的行为。
但是，如果我想为具有不同存储位置的dest vector 元素创建深拷贝，该怎么办？如何实现呢？
Boost是否具有实现此目的的功能？

Answer 1

当然。最简单的:

#include <vector>
#include <memory>

struct T {};

int main() {
    std::vector<T> a{100};
    auto b = a; // deep copy all T
}

当然，您拥有shared_ptr是有原因的。例如。当对象不可移动和/或运行时多态时。

输入指针容器
Boost Pointer Container满足了这一需求。它允许您自定义克隆元素的方式(请参见How does boost::ptr_vector deep copy the underlying objects?或the docs)。
简单示例: Live On Coliru

#include <vector>
#include <memory>
#include <boost/ptr_container/ptr_vector.hpp>
#include <iostream>

struct Base {
  virtual ~Base() = default; // runtime polymorphic
  virtual char const* foo() const = 0;
};

struct T : Base {
  virtual char const* foo() const override { return "T"; }
};
struct U : Base {
  virtual char const* foo() const override { return "U"; }
};

static inline Base* new_clone(Base const& obj) {
    if (auto* p = dynamic_cast<T const*>(&obj))
        return new T{*p};
    if (auto* p = dynamic_cast<U const*>(&obj))
        return new U{*p};
    return nullptr;
}

int main() {
    boost::ptr_vector<Base> a;
    std::generate_n(std::back_inserter(a), 5, [] { return new T{}; });
    // polymorphic
    a.insert(a.begin()+2, new U{});

    auto b = a; // deep copy all elements, derived from Base

    // not sharing the instances:
    assert(&a.front() != &b.front());

    std::cout << "\na:";
    for (auto& el : a) std::cout << " " << el.foo(); 

    std::cout << "\nb:";
    for (auto& el : b) std::cout << " " << el.foo(); 
}

版画

a: T T U T T T
b: T T U T T T

std::vector<unique_ptr>这在概念上类似，但是需要您做更多的工作:
Live On Coliru

#include <algorithm>
#include <vector>
#include <memory>
#include <iostream>
#include <cassert>

struct Base {
  virtual ~Base() = default; // runtime polymorphic
  virtual char const* foo() const = 0;
};

struct T : Base {
  virtual char const* foo() const override { return "T"; }
};
struct U : Base {
  virtual char const* foo() const override { return "U"; }
};

struct Cloner {
    using Ptr = std::unique_ptr<Base>;
    Ptr operator()(Ptr const& pb) const {
        if (auto* p = dynamic_cast<T const*>(pb.get()))
            return std::make_unique<T>(*p);
        if (auto* p = dynamic_cast<U const*>(pb.get()))
            return std::make_unique<U>(*p);
        return nullptr;
    }
};

int main() {
    std::vector<std::unique_ptr<Base> > a;
    a.push_back(std::make_unique<T>());
    a.push_back(std::make_unique<U>());
    a.push_back(std::make_unique<T>());

    std::vector<std::unique_ptr<Base> > b;

    // deep copy all elements, derived from Base
    Cloner clone;
    std::transform(begin(a), end(a), back_inserter(b), clone);

    // not sharing the instances:
    assert(&*a.front() != &*b.front());

    std::cout << "\na:";
    for (auto& p : a) std::cout << " " << p->foo(); 

    std::cout << "\nb:";
    for (auto& p : b) std::cout << " " << p->foo(); 
}

打印品:

a: T U T
b: T U T

vector<unique_ptr>但是“更轻松”？
如果您愿意，可以使用一些“魔术”使之更简单:

使用加速范围
Live On Coliru

  // deep copy all elements, derived from Base
  auto b = boost::copy_range<upvec>(a | transformed(Cloner{}));

使用标准范围(c++ 20)
改用Ranges v3，因为to_vector尚未标准化(尚未)
Live On Compiler Explorer

  // deep copy all elements, derived from Base
  auto b = ranges::to_vector(a | ranges::views::transform(Cloner{}));

全自动:Boost PolyContainer
这不会保留顺序，但是它的优点是您不必提出克隆逻辑，并且可以按类型等进行智能迭代:
Live On Coliru

#include <boost/poly_collection/base_collection.hpp>
#include <memory>
#include <iostream>
#include <cassert>

struct Base {
  virtual ~Base() = default; // runtime polymorphic
  virtual char const* foo() const = 0;
};

struct T : Base {
  virtual char const* foo() const override { return "T"; }
};
struct U : Base {
  virtual char const* foo() const override { return "U"; }
};

int main() {
    using C = boost::poly_collection::base_collection<Base>;
    
    C a;
    a.insert(T{});
    a.insert(U{});
    a.insert(T{});

    // deep copy all elements, derived from Base
    auto b = a;

    // not sharing the instances:
    assert(&*a.begin() != &*b.begin());

    std::cout << "\na:";
    for (auto& p : a) std::cout << " " << p.foo(); 

    std::cout << "\nb:";
    for (auto& p : b) std::cout << " " << p.foo(); 
}

版画

a: U T T
b: U T T