STL const_iterator强制转换 - 编译器差异

Question

我正在将一大块代码从gcc移植到Visual Studio 2013.以下代码示例在gcc 4.4上工作正常(!),但在VS2013上编译begin()并end()失败:

错误C2440:'':无法从'unsigned char*'转换为'std :: _ Vector_const_iterator >>'

class foo {
    unsigned char* value;
    int length;

    std::vector<unsigned char>::const_iterator begin();
    std::vector<unsigned char>::const_iterator end();
};

std::vector<unsigned char>::const_iterator foo::begin() {
    return std::vector<unsigned char>::const_iterator(value);
}

std::vector<unsigned char>::const_iterator foo::end() {
    return std::vector<unsigned char>::const_iterator(value + length);
}

鉴于我不想重写整个事物,是否有可行的方法来创建这些const_iterators？

Answer 1

没有可移植的方法来执行您正在尝试的操作,因为不需要(const_)iterator可以从指向底层值类型的指针构造.libstdc ++恰好提供了这样的构造函数,但VS标准库实现却没有.相反,它的(const_)iterator构造函数接受一个指向底层值类型的指针和一个指向容器本身的指针,它用于在调试版本中执行额外的验证.

最简单的解决方案是替换std::vector<unsigned char>::const_iterator为unsigned char const *.原始指针属于RandomAccessIterator类别,与vector::(const_)iterators 相同.

unsigned char const *foo::begin() {
    return value;
}

unsigned char const *foo::end() {
    return value + length;
}

如果您需要将迭代器作为类类型,那么您将需要创建自定义迭代器.虽然这可以从头开始,但使用Boost.IteratorFacade要容易得多,它将提供构建自定义迭代器的一系列必要的样板.

#include <boost/iterator/iterator_facade.hpp>

struct const_foo_iterator : boost::iterator_facade<const_foo_iterator,
                                                    unsigned char const,
                                                    boost::random_access_traversal_tag>
{
  const_foo_iterator() = default;
  const_foo_iterator(unsigned char const *iter) : iter(iter) {}
private:
    friend class boost::iterator_core_access;

    void increment() { ++iter; }
    void decrement() { --iter; }
    void advance(std::ptrdiff_t n) { iter += n; }

    std::ptrdiff_t distance_to(const_foo_iterator const& other) const
    { return iter - other.iter; }

    bool equal(const_foo_iterator const& other) const
    { return this->iter == other.iter; }

    unsigned char const& dereference() const { return *iter; }
    unsigned char const* iter = nullptr;
};

const_foo_iterator foo::begin() {
    return value;
}

const_foo_iterator foo::end() {
    return value + length;
}

static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::value_type,
                           unsigned char>::value, "value_type");
static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::pointer,
                           unsigned char const *>::value, "pointer");
static_assert(std::is_same<std::iterator_traits<const_foo_iterator>::iterator_category,
                           std::random_access_iterator_tag>::value, "iterator_category");

现场演示