TechOverflow

To get the filesize (in bytes) of any file (test.xml in our example) using the boost::filesystem library, use this snippet:

#include <experimental/filesystem>
#include <iostream>

using namespace std;
using namespace std::experimental::filesystem;

int main() {
    size_t filesize = file_size("test.xml");
    cout << filesize << endl;
}

You need to link the boost_filesystem and the boost_system libraries, i.e. compile like this:

g++ -o test test.cpp -lboost_filesystem -lboost_system

To get the filesize (in bytes) of any file (test.xml in our example) using only the C++17 filesystem library, use this snippet:

#include <experimental/filesystem>
#include <iostream>

using namespace std;
using namespace std::experimental::filesystem;

int main() {
    size_t filesize = file_size("test.xml");
    cout << filesize << endl;
}

With GCC/G++ you need to link the stdc++fs library, i.e. compile like this:

g++ -o test test.cpp -lstdc++fs

XML:

<?xml version="1.0" encoding="UTF-8"?>
<root-element>Test text</root-element>

C++:

#include <rapidxml/rapidxml_utils.hpp>
#include <iostream> 

using namespace rapidxml;
using namespace std;

int main() {
    file<> xmlFile("test.xml");
    // Create & parse document
    xml_document<> doc;
    doc.parse<0>(xmlFile.data());
    // Get root node
    xml_node<> *root = doc.first_node("root-element");
    // Print content
    if(root == nullptr) {
        cerr << "Element not found!" << endl;
    } else { // We found the root element
        cout << root->value() << endl;
    }
}

Problem:

You have a std::wstring that you want to print using cout

wstring w = L"Test: äöü";
cout << w << endl;

but you see a long error message that ends like this:

/usr/include/c++/7/ostream:682:5: note:   template argument deduction/substitution failed:
/usr/include/c++/7/ostream: In substitution of 'template<class _Ostream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_ostream<_Ostream>, std::__is_insertable<typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type, const _Tp&, void> >::value, typename std::__is_convertible_to_basic_ostream<_Tp>::__ostream_type>::type std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = std::__cxx11::basic_string<wchar_t>]':
test.cpp:9:13:   required from here
/usr/include/c++/7/ostream:682:5: error: no type named 'type' in 'struct std::enable_if<false, std::basic_ostream<char>&>'

Solution:

You need to use std::wcout instead of std::cout:

wstring w = L"Test: äöü";
wcout << w << endl

In order to declare a wstring literal, use the L prefix:

wstring w = L"Test: äöü";

In order to declare a wchar_t literal, you can also use the L prefix:

wchar_t wc = L'ö';

Problem:

You have a rapidxml::xml_node instance for which you want to access a specific attribute,  e.g. my-attribute

Solution:

Use first_attribute with the name argument set to a string:

rapidxml::xml_attribute<>* attr = node->first_attribute("my-attribute");

Remember that first_attribute() returns nullptr if no such attribute exists so be sure to check for that to avoid segmentation faults!

Full example:

XML:

<?xml version="1.0" encoding="UTF-8"?>
<root-element>
    <child my-attr="foo"></child>
</root-element>

C++:

#include <rapidxml/rapidxml_utils.hpp>
#include <string>
#include <iostream> 

using namespace rapidxml;
using namespace std;

int main() {
    rapidxml::file<> xmlFile("test.xml");
    // Create & parse document
    rapidxml::xml_document<> doc;
    doc.parse<0>(xmlFile.data());
    
    // Get root node
    rapidxml::xml_node<> *root = doc.first_node("root-element");
    rapidxml::xml_node<> *child = root->first_node("child");

    // Get & print attribute
    rapidxml::xml_attribute<>* attr = child->first_attribute("my-attr");
    if(attr == nullptr) {
        cout << "No such attribute!" << endl;
    } else {
        cout << attr->value() << endl;
    }
}

Or you can use this snippet function to get either the attribute value or a default value:

#include <rapidxml/rapidxml_utils.hpp>
#include <string>
#include <iostream> 

using namespace rapidxml;
using namespace std;

/**
 * Return either the ->value() of attr or default_value if attr == nullptr
 */
inline string attr_value_or_default(rapidxml::xml_attribute<>* attr, string default_value="") {
    if(attr == nullptr) {
        return default_value;
    } else {
        return attr->value();
    }
}

int main() {
    rapidxml::file<> xmlFile("test.xml");
    // Create & parse document
    rapidxml::xml_document<> doc;
    doc.parse<0>(xmlFile.data());
    
    // Get root node
    rapidxml::xml_node<> *root = doc.first_node("root-element");
    rapidxml::xml_node<> *child = root->first_node("child");

    // Get & print attribute+
    rapidxml::xml_attribute<>* attr = child->first_attribute("my-attr");
    cout << attr_value_or_default(attr, "No such attribute!") << endl;
}

If you are seeing an error message like fatal error: rapidxml_utils.hpp: No such file or directory in a line like

#include "rapidxml_utils.hpp"

or

#include <rapidxml_utils.hpp>

you likely need to replace that line by

#include <rapidxml/rapidxml_utils.hpp>

If this also doesn’t work, you might not have RapidXML installed. See How to install RapidXML on Ubuntu for details on how to do that on Ubuntu.