C++: Using boolalpha

Question

I am using a function (TinyXML's TiXmlElement::QueryValueAttribute(const std::string &name, T * outValue) that attempts to read a string into the data type that is passed. In my case I am passing a bool. So I want to use the boolalpha flag so that the input can be true or false instead of 0 or 1.

How do I do this?

Thanks.

Answer 1

TiXmlElement::QueryValueAttribute uses a std::istringstream to parse the value. So, you can create a wrapper class around bool that overloads operator >> to always set boolalpha before extraction:

class TinyXmlBoolWrapper
{
public:
    TinyXmlBoolWrapper(bool& value) : m_value(value) {}

    bool& m_value;
};

std::istream& operator >> (std::istream& stream, TinyXmlBoolWrapper& boolValue)
{
    // Save the state of the boolalpha flag & set it
    std::ios_base::fmtflags fmtflags = stream.setf(std::ios_base::boolalpha);
    std::istream& result = stream >> boolValue.m_value;
    stream.flags(fmtflags);  // restore previous flags
    return result;
}

...

bool boolValue;
TinyXmlBoolWrapper boolWrapper(boolValue);
myTinyXmlElement->QueryAttribute("attributeName", &boolWrapper);
// boolValue now contains the parsed boolean value with boolalpha used for
// parsing