c++ - 如何读取值为可变长度 char* 数组(即 c_strings？)的 HDF5 标量属性

Question

我已经成功地创建了一个标量值属性，它的值是一个可变长度的 const char* 数组。但是我不明白如何读取这个属性!

这是我用来创建属性的代码:

    void create_attribute_with_vector_of_strings_as_value()
{
    using namespace H5;

    // Create some test strings.
    std::vector<std::string> strings;
    for (int iii = 0; iii < 10; iii++)
    {
        strings.push_back("this is " + boost::lexical_cast<std::string>(iii));
    }

    // Part 1: grab pointers to the chars
    std::vector<const char*> chars;
    for (auto si = strings.begin(); si != strings.end(); ++si)
    {
        std::string &s = (*si);
        chars.push_back(s.c_str());
    }
    BOOST_TEST_MESSAGE("Size of char* array is:  " << chars.size());

    // Part 2: create the variable length type
    hvl_t hdf_buffer;
    hdf_buffer.p = chars.data();
    hdf_buffer.len = chars.size();

    // Part 3: create the type
    auto s_type = H5::StrType(H5::PredType::C_S1, H5T_VARIABLE);
    auto svec_type = H5::VarLenType(&s_type);

    try
    {
        // Open an existing file and dataset.
        H5File file(m_file_name.c_str(), H5F_ACC_RDWR);

        // Part 4: write the output to a scalar attribute
        DataSet dataset = file.openDataSet(m_dataset_name.c_str());

        std::string filter_names = "multi_filters";

        Attribute attribute = dataset.createAttribute( filter_names.c_str(), svec_type, H5S_SCALAR);
        attribute.write(svec_type, &hdf_buffer);
        file.close();
    }

这是从 h5dump 中看到的具有属性的数据集:

    HDF5 "d:\tmp\hdf5_tutorial\h5tutr_dset.h5" {
GROUP "/" {
DATASET "dset" {
DATATYPE  H5T_STD_I32BE
DATASPACE  SIMPLE { ( 4, 6 ) / ( 4, 6 ) }
DATA {
(0,0): 1, 7, 13, 19, 25, 31,
(1,0): 2, 8, 14, 20, 26, 32,
(2,0): 3, 9, 15, 21, 27, 33,
(3,0): 4, 10, 16, 22, 28, 34
}
ATTRIBUTE "multi_filters" {
DATATYPE  H5T_VLEN { H5T_STRING {
STRSIZE H5T_VARIABLE;
STRPAD H5T_STR_NULLTERM;
CSET H5T_CSET_ASCII;
CTYPE H5T_C_S1;
}}
DATASPACE  SCALAR
DATA {
(0): ("this is 0", "this is 1", "this is 2", "this is 3", "this is 4", "this is 5", "this is 6", "this is 7", "this is 8", "this is 9")
}
}
}
}
}

我不明白如何读取这些数据。到目前为止我试验过的代码如下。它可以编译，但是我已经将数组大小硬连接到已知长度并且可变长度 cstrings 是空的？有人对我哪里出错有任何建议吗？具体来说，如何查询 const char* 数组的长度以及如何读取数组中包含的实际 const char* cstring？

    void read_attribute_with_vector_of_strings_as_value()
{
    using namespace H5;

    std::vector<std::string> strings;

    try
    {
        // Open an existing file and dataset readonly
        H5File file(m_file_name.c_str(), H5F_ACC_RDONLY);

        // Part 4: Open the dataset
        DataSet dataset = file.openDataSet(m_dataset_name.c_str());

        // Atribute_name
        std::string filter_names = "multi_filters";

        Attribute attribute = dataset.openAttribute(filter_names.c_str());
        size_t sz = attribute.getInMemDataSize();
        size_t sz_1 = attribute.getStorageSize();
        auto t1 = attribute.getDataType();
        VarLenType t2 = attribute.getVarLenType();
        H5T_class_t type_class = attribute.getTypeClass();
        if (type_class == H5T_STRING)
            BOOST_TEST_MESSAGE("H5T_STRING");

        int length = 10;
        std::vector<char*> tmp_vec(length);
        auto s_type = H5::StrType(H5::PredType::C_S1, H5T_VARIABLE);
        auto svec_type = H5::VarLenType(&s_type);

        hvl_t hdf_buffer;
        hdf_buffer.p = tmp_vec.data();
        hdf_buffer.len = length;
        attribute.read(svec_type, &hdf_buffer);
        //attribute.read(s_type, &hdf_buffer);
        //attribute.read(tmp_vec.data(), s_type);

        for(size_t x = 0; x < tmp_vec.size(); ++x)
        {
            fprintf(stdout, "GOT STRING [%s]\n", tmp_vec[x] );
            strings[x] = tmp_vec[x];
        }

        file.close();
    }

Answer 1

如果您不需要使用特定技术来实现您的想法，您可以考虑 HDFql ( http://www.hdfql.com )，这是一种可以轻松管理 HDF 文件的高级语言(想想 SQL)。这样，您就可以从操作您描述的 HDF 文件的所有低级细节中解脱出来。在 C++ 中使用 HDFql，读取可变长度 char 的数组是这样完成的:

// include HDFql C++ header file (make sure it can be found by the C++ compiler)
#include <iostream>
#include "HDFql.hpp"

int main(int argc, char *argv[])
{

    // create an HDF file named "example.h5" and use (i.e. open) it
    HDFql::execute("CREATE AND USE FILE example.h5");

    // create an attribute named "multi_filters" of type varchar of one dimension (size 5)
    HDFql::execute("CREATE ATTRIBUTE multi_filters AS VARCHAR(5)");

    // insert (i.e. write) values "Red", "Green", "Blue", "Orange" and "Yellow" into attribute "multi_filters"
    HDFql::execute("INSERT INTO multi_filters VALUES(Red, Green, Blue, Orange, Yellow)");

    // select (i.e. read) attribute "multi_filters" into HDFql default cursor
    HDFql::execute("SELECT FROM multi_filters");

    // display content of HDFql default cursor
    while(HDFql::cursorNext() == HDFql::Success)
    {
        std::cout << "Color " << HDFql::cursorGetChar() << " has a size of " << HDFql::cursorGetSize() << std::endl;
    }

    return 0;

}