Empty string is interpreted as bool in Constructor

Question

Im am working on a QT project and have found a strange behaviour:

I have a class with several constructors that look like

DB_Variable(QString name, QString newValue):
name(name),value_string(newValue), var_type(DB_STRING){}

DB_Variable(QString name, bool newValue):
    name(name), value_bool(newValue), var_type(DB_BOOL){}

I now want to use the first constructor to create an object like this:

DB_Variable foo("some_name"," ");

I'd expect the empty string to be interpreted as a QString, but the second (bool) constructor is called. Can someone tell my why? Is the " " a pointer to an empty string and the then somehow rather a bool than a string?

Foo

Answer 1

This problem results from implicit conversions going on in the constructor. String literals, such as the one in your code, are stored as const char types. Because you didn't have a constructor taking this type the compiler tries to find the conversion to a type that it can find in one of your constructors.

In this case const char* converts to bool easier that QString so when you do:

DB_Variable foo("some_name"," ");

The constructor

DB_Variable(QString name, bool newValue):

Is called.

Note that the behavior you are seeing isn't due to " " getting treated differently than any other string literal, it's just that you most likely didn't have a constructor with the types bool, bool (did all your constructors take a QString as the first argument?). Chances are if you had a constructor such as the following:

DB_Variable(bool test1, bool newValue):

Then this would have been called instead when you did something such as DB_Variable foo("some_name"," ");

To get the results you wanted you could pass in QStrings like so:

DB_Variable foo(QString("some_name"), QString());

Or perhaps define a constructor that takes const char* for the second parameter.