将vector<unsigned char>转换为vector<unsigned short>

Question

我从一个二进制文件中获取数据，从文件中读取并写入一个无符号字符的向量。我不能编辑它，因为我正在使用外部库。

但是我从文件中读取的数据是一个16位的图像，我想把数据放在一个无符号短向量中

也许我可以给它做个演员？

Rgds。

Answer 1

通用方法(不是万无一失)：

#include <vector>
#include <iostream>
#include <iterator>
#include <algorithm>

typedef unsigned char u8;
typedef unsigned short u16;

u16 combine_two_bytes(u8 a, u8 b) {
    return a | (b << 8);
}

template<typename InIter, typename OutIter, typename InT, typename OutT>
void combine_pairs(InIter in, InIter in_end, OutIter out, OutT (*func)(InT, InT)) {
    while(1) {
        if(in == in_end) {
            break;
        }

        InT &left = *in++;

        if(in == in_end) {
            break;
        }

        InT &right = *in++;

        *out++ = func(left, right);
    }
}

int main() {
    using namespace std;    // lazy

    u8 input[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    const size_t in_size = sizeof(input) / sizeof(*input);
    u16 output[in_size / 2];

    cout << "Original: ";
    copy(input, input + in_size, ostream_iterator<int>(cout, " "));
    cout << endl;

    combine_pairs(input, input + in_size, output, combine_two_bytes);

    cout << "Transformed: ";
    copy(output, output + in_size / 2, ostream_iterator<int>(cout, " "));
    cout << endl;

    return 0;
}

Answer 2

如果您只想从一种类型转换为另一种类型，则使用标准构造函数。只要迭代器的值类型可以自动转换为目标向量值类型，编译器就会在这两种类型之间进行自动转换。只需使用标准构造函数

#include <vector>
#include <algorithm>
#include <iterator>

int main()
{
    std::vector<unsigned char>      a;
    a.push_back((unsigned char)12);
    a.push_back((unsigned char)13);
    a.push_back((unsigned char)14);

    std::vector<unsigned short>     b(a.begin(),a.end());

    // Print out the vector
    std::copy(b.begin(),b.end(),std::ostream_iterator<unsigned short>(std::cout,"\t"));
}

> g++ t.cpp
> ./a.out
12  13 14

如果你真的想把两个字节转换成一个字节，那么需要做一些工作。但这取决于输入数据实际上是否与您所在的机器具有相同的字节顺序。如果你知道它是相同的字节顺序，你只需要转换输入类型。

#include <vector>
#include <algorithm>
#include <iterator>

int main()
{
    std::vector<unsigned char>      a;

    // Make sure that the size is correct.
    // ie. An Odd number indicates that something is not quite correct.
    //
    std::vector<unsigned short>     b(static_cast<unsigned short*>(&a[0]),
                                      static_cast<unsigned short*>(&a[a.size()]));

    // Print out the vector
    std::copy(b.begin(),b.end(),std::ostream_iterator<unsigned short>(std::cout,"\t"));
}

或者，如果您实际上需要将两个值合并为一个值，其中字节顺序与目标体系结构不同，您可以编写一个特殊的迭代器。如下所示：

#include <Converter.h>

int main()
{
    std::vector<unsigned char>      a;

    // Make sure that the size is correct.
    // ie. An Odd number indicates that something is not quite correct.
    //
    std::vector<unsigned short>     b(make_Converter(a.begin()),make_Converter(a.end()));

    // Print out the vector
    std::copy(b.begin(),b.end(),std::ostream_iterator<unsigned short>(std::cout,"\t"));
}

Converter.h

#include <vector>
#include <iostream>
#include <iterator>

template<typename I>
struct Converter
{
    I   iterator;

    typedef typename std::input_iterator_tag                    iterator_category;
    typedef typename std::iterator_traits<I>::value_type        value_type;
    typedef typename std::iterator_traits<I>::difference_type   difference_type;
    typedef typename std::iterator_traits<I>::pointer           pointer;
    typedef typename std::iterator_traits<I>::reference         reference;

    Converter(I iter)
        :iterator(iter)
    {}

    Converter& operator++()
    {
        iterator++;
        return *this;
    }

    Converter operator++(int)
    {
        Converter   tmp(*this);
        this->operator++();

        return (tmp);
    }

    value_type operator*()
    {
        /*
         * The actual calculation done here will depend on the underlying hardware.
         */
        typename std::iterator_traits<I>::value_type val(*iterator);
        val << 8;
        iterator++;
        val |= (*iterator);

        return val;
    }

    bool operator!=(Converter const& rhs)
    {
        return iterator != rhs.iterator;
    }
};

template<typename I>
Converter<I> make_Converter(I iter)
{
    return Converter<I>(iter);
}

Answer 3

vector<unsigned char> a = ...;
vector<unsigned short> b(a.begin(), a.end());

但是对于简单的数据向量，您需要的是valarray。