switch/case statement in C++ with a QString type

Question

I want to use switch-case in my program but the compiler gives me this error:

switch expression of type \'QString\' is illegal

How can I us

Answer 1

If you can use a modern C++ compiler then you could compute a compile time hash value for your strings. In this answer there's an example of a rather simple constexpr hashing function.

So a solution can look like this:

// function from https://stackoverflow.com/a/2112111/1150303
// (or use some other constexpr hash functions from this thread)
unsigned constexpr const_hash(char const *input) {
    return *input ?
    static_cast<unsigned int>(*input) + 33 * const_hash(input + 1) :
    5381;
}

QString switchStr = "...";
switch(const_hash(switchStr.toStdString().c_str()))
{
case const_hash("Test"):
    qDebug() << "Test triggered";
    break;
case const_hash("asdf"):
    qDebug() << "asdf triggered";
    break;
default:
    qDebug() << "nothing found";
    break;
}

It is still not a perfect solution. There can be hash collisions (hence test your program whenever you add/change case) and you have to be careful in the conversion from QString to char* if you want to use exotic or utf characters, for instance.

For c++ 11 add CONFIG += c++11 to your project, for Qt5. Qt4: QMAKE_CXXFLAGS += -std=c++11

Answer 2

case "the":
    //^^^ case label must lead to a constant expression

I am not aware of qt, but you can give this a try. You can avoid switch and directly use == for comparison, if QString is no different than a normal std::string.

if( word == "the" )
{
   // ..
}
else if( word == "at" )
{
   // ..
}
// ....