正确使用C变量模板

Question

我目前正在从事一个项目，该项目需要一个生成简单XML的对象。我对XML很陌生，还在学习c
我试图实现的是一个可以在代码中这样调用的函数：

std::string xml = createXML("name", "attribute1", 10, "attribute2",
                            "stringForAttrib2");

在这一行之后，xml字符串应该包含如下内容：

该函数必须接受尽可能多的属性，并且名称不是可选的。我尝试过这样的东西：

template<typename ...Attributes>
void XMLGenerator::createXML(std::string param_name, Attributes... attributes)
{
    std::ostringstream xmlString;
    xmlString << "<parameter name=\"" << param_name << "\" ";
    createXML(xmlString, attributes...);
    xmlString << "/>";
    m_xmlString << xmlString;
}

template<typename ...Attributes, typename DataType>
void XMLGenerator::createXML(std::ostringstream &xmlString,
                             std::string attribut_name, DataType data,
                             Attributes... attributes)
{
    xmlString << attribut_name << "=\"" << data << "\" ";
    createXML(xmlString, attributes...);
}

void XMLGenerator::addParameter(std::ostringstream &xmlString){ }

G不喜欢：

对“void XMLGenerator:：createXML”的未定义引用

为什么不调用第一个createXML方法？

另外，如果我的问题有一个更简单的解决方案，我会感兴趣的。




Answer 1

这里是一个解决方案，转发声明很重要，请注意std:：forward与通用引用结合使用

#include <iostream>
#include <string>

// =================================
// print_attribs parameter usage : ( os, name0, val0, ..., nameN, valN )
// name is char const* or std::string
// val can be anything compatible with os operator<<
// print_attribs is specialize for char const* and std::string values to enclose them with double quotes.

template < typename T, typename... Ts> void print_attribs( std::ostream & os, char const * name, T&& val, Ts&&... ts );
template < typename... Ts>             void print_attribs( std::ostream & os, char const * name, char const * val, Ts&&... ts );
template < typename... Ts>             void print_attribs( std::ostream & os, char const * name, std::string const & val, Ts&&... ts );

template < typename T, typename... Ts> void print_attribs( std::ostream & os, std::string const & name, T&& val, Ts&&... ts );
template < typename... Ts>             void print_attribs( std::ostream & os, std::string const & name, char const * val, Ts&&... ts );
template < typename... Ts>             void print_attribs( std::ostream & os, std::string const & name, std::string const & val, Ts&&... ts );

void print_attribs( std::ostream & os ) { }

template < typename... Ts>
void print_attribs( std::ostream & os, char const * name, char const * val, Ts&&... ts ) { 
    os << " " << name << "=\"" << val << "\"";
    print_attribs( os, std::forward<Ts>(ts)... );
}

template < typename... Ts>
void print_attribs( std::ostream & os, std::string const & name, char const * val, Ts&&... ts ) { 
    print_attribs( os, name.c_str(), val, std::forward<Ts>(ts)... );
}

template < typename... Ts>
void print_attribs( std::ostream & os, char const * name, std::string const & val, Ts&&... ts ) { 
    os << " " << name << "=\"" << val << "\"";
    print_attribs( os, std::forward<Ts>(ts)... );
}

template < typename... Ts>
void print_attribs( std::ostream & os, std::string const & name, std::string const & val, Ts&&... ts ) { 
    print_attribs( os, name.c_str(), val, std::forward<Ts>(ts)... );
}

template < typename T, typename... Ts>
void print_attribs( std::ostream & os, char const * name, T&& val, Ts&&... ts ) { 
    os << " " << name << "=" << std::forward<T>(val);
    print_attribs( os, std::forward<Ts>(ts)... );
}

template < typename T, typename... Ts>
void print_attribs( std::ostream & os, std::string const & name, T&& val, Ts&&... ts ) { 
    print_attribs( os, name.c_str(), std::forward<T>(val), std::forward<Ts>(ts)... );
}

template < typename... Ts>
void print( std::ostream & os, char const * name, Ts&&... ts ) {
    static_assert( sizeof...(ts) % 2 == 0, "arguments must be paired of string and value" );
    os << "<parameter name=\"" << name << "\"";
    print_attribs( os, std::forward<Ts>(ts)... );
    os << " />";
}

template < typename... Ts>
void print( std::ostream & os, std::string const & name, Ts&&... ts ) {
    static_assert( sizeof...(Ts) % 2 == 0, "Ts must be even, as they are pairs of name and value" );
    print( os, name.c_str(), std::forward<Ts>(ts)... );
}

int main() {
    auto endl = [] { std::cout << std::endl; };
    print( std::cout, "example", "foo", 10 ); endl();
    print( std::cout, "example", "foo", 10, "bar", "bare" ); endl();
    print( std::cout, "example", "foo", 10, "bar", 3.14f ); endl();
    print( std::cout, "example", "foo", 10, "bar", 3.14f, "bar", 3.14f ); endl();
    print( std::cout, "example", "foo", 10, "bar", "bare", "baz", 3.14f ); endl();
}

编辑：添加了参数计数和char const*的静态断言，以替代不必要的std:：string构造。