boost::adaptors::transformed for 类 没有 const begin/end
boost::adaptors::transformed for classes without const begin/end
我正在尝试将对象传递给 boost::adaptors::transformed
。但是,这似乎只有在该对象的 class 定义了 begin
和 end
的 const
版本时才有效。然而,对我来说情况并非如此,因为遍历 this class 的对象会修改对象本身的内部状态。这是一个使用虚拟 class Vector
的最小示例,它仅公开非 const 版本 begin
和 end
或其 _v
成员,我可以更改为让这个工作?
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
template<typename T>
class Vector
{
public:
using value_type = typename std::vector<T>::value_type;
using reference = typename std::vector<T>::reference;
using iterator = typename std::vector<T>::iterator;
Vector(std::initializer_list<T> init)
: _v(init)
{}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
private:
std::vector<T> _v;
};
int main()
{
Vector<int> v{1, 2, 3, 4};
auto t = [](int i){ return 2 * i; };
auto range(v | boost::adaptors::transformed(t)); // does not compile
}
一般来说,迭代修改集合是一种代码异味。
当然,某些东西可以逻辑上 const,这就是我们使用 mutable
关键字的目的。我大致可以看到两种方法
Keep in mind that threading-aware libraries might assume that const
operations are threadsafety guarantees (so either bit-wise immutable OR e.g. only operation on synchronization primitives like a member mutex).
使容器存储可变
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
#include <fmt/ranges.h>
using boost::adaptors::transformed;
template<typename T>
class Vector {
using Cont = std::vector<T>;
public:
using value_type = typename Cont::value_type;
using reference = typename Cont::reference;
using iterator = typename Cont::iterator;
using const_iterator = typename Cont::const_iterator;
Vector(std::initializer_list<T> init) : _v(init) {}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
const_iterator begin() const { return _v.begin(); }
const_iterator end() const { return _v.end(); }
//const_iterator cbegin() const { return _v.begin(); }
//const_iterator cend() const { return _v.end(); }
private:
Cont mutable _v;
};
static auto twice(int i) { return 2 * i; }
int main() {
fmt::print("{} -> {}\n",
Vector {1, 2, 3, 4},
Vector {1, 2, 3, 4} | transformed(twice));
}
版画
{1, 2, 3, 4} -> {2, 4, 6, 8}
更纯粹的方法:可变元素数据
为了好玩,让我们制作一个元素来跟踪观察其值的次数:
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
#include <fmt/ranges.h>
using boost::adaptors::transformed;
struct Element {
Element(int value) : value(value) {}
operator int() const { ++usage_counter; return value; }
long usages() const { return usage_counter; }
private:
mutable long usage_counter = 0;
int value;
};
template<typename T>
class Vector {
using Cont = std::vector<T>;
public:
using value_type = typename Cont::value_type;
using reference = typename Cont::reference;
using iterator = typename Cont::iterator;
using const_iterator = typename Cont::const_iterator;
Vector(std::initializer_list<T> init) : _v(init) {}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
const_iterator begin() const { return _v.begin(); }
const_iterator end() const { return _v.end(); }
//const_iterator cbegin() const { return _v.begin(); }
//const_iterator cend() const { return _v.end(); }
private:
Cont _v;
};
static auto twice(int i) { return 2 * i; }
int main() {
Vector<Element> const v {1, 2, 3, 4}; // note const
fmt::print("{} -> {} (usages {})\n",
v,
v | transformed(twice),
v | transformed(std::mem_fn(&Element::usages))
);
}
版画
{1, 2, 3, 4} -> {2, 4, 6, 8} (usages {3, 3, 3, 3})
我正在尝试将对象传递给 boost::adaptors::transformed
。但是,这似乎只有在该对象的 class 定义了 begin
和 end
的 const
版本时才有效。然而,对我来说情况并非如此,因为遍历 this class 的对象会修改对象本身的内部状态。这是一个使用虚拟 class Vector
的最小示例,它仅公开非 const 版本 begin
和 end
或其 _v
成员,我可以更改为让这个工作?
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
template<typename T>
class Vector
{
public:
using value_type = typename std::vector<T>::value_type;
using reference = typename std::vector<T>::reference;
using iterator = typename std::vector<T>::iterator;
Vector(std::initializer_list<T> init)
: _v(init)
{}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
private:
std::vector<T> _v;
};
int main()
{
Vector<int> v{1, 2, 3, 4};
auto t = [](int i){ return 2 * i; };
auto range(v | boost::adaptors::transformed(t)); // does not compile
}
一般来说,迭代修改集合是一种代码异味。
当然,某些东西可以逻辑上 const,这就是我们使用 mutable
关键字的目的。我大致可以看到两种方法
Keep in mind that threading-aware libraries might assume that
const
operations are threadsafety guarantees (so either bit-wise immutable OR e.g. only operation on synchronization primitives like a member mutex).
使容器存储可变
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
#include <fmt/ranges.h>
using boost::adaptors::transformed;
template<typename T>
class Vector {
using Cont = std::vector<T>;
public:
using value_type = typename Cont::value_type;
using reference = typename Cont::reference;
using iterator = typename Cont::iterator;
using const_iterator = typename Cont::const_iterator;
Vector(std::initializer_list<T> init) : _v(init) {}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
const_iterator begin() const { return _v.begin(); }
const_iterator end() const { return _v.end(); }
//const_iterator cbegin() const { return _v.begin(); }
//const_iterator cend() const { return _v.end(); }
private:
Cont mutable _v;
};
static auto twice(int i) { return 2 * i; }
int main() {
fmt::print("{} -> {}\n",
Vector {1, 2, 3, 4},
Vector {1, 2, 3, 4} | transformed(twice));
}
版画
{1, 2, 3, 4} -> {2, 4, 6, 8}
更纯粹的方法:可变元素数据
为了好玩,让我们制作一个元素来跟踪观察其值的次数:
#include <initializer_list>
#include <vector>
#include <boost/range/adaptors.hpp>
#include <fmt/ranges.h>
using boost::adaptors::transformed;
struct Element {
Element(int value) : value(value) {}
operator int() const { ++usage_counter; return value; }
long usages() const { return usage_counter; }
private:
mutable long usage_counter = 0;
int value;
};
template<typename T>
class Vector {
using Cont = std::vector<T>;
public:
using value_type = typename Cont::value_type;
using reference = typename Cont::reference;
using iterator = typename Cont::iterator;
using const_iterator = typename Cont::const_iterator;
Vector(std::initializer_list<T> init) : _v(init) {}
iterator begin() { return _v.begin(); }
iterator end() { return _v.end(); }
const_iterator begin() const { return _v.begin(); }
const_iterator end() const { return _v.end(); }
//const_iterator cbegin() const { return _v.begin(); }
//const_iterator cend() const { return _v.end(); }
private:
Cont _v;
};
static auto twice(int i) { return 2 * i; }
int main() {
Vector<Element> const v {1, 2, 3, 4}; // note const
fmt::print("{} -> {} (usages {})\n",
v,
v | transformed(twice),
v | transformed(std::mem_fn(&Element::usages))
);
}
版画
{1, 2, 3, 4} -> {2, 4, 6, 8} (usages {3, 3, 3, 3})