You cannot. This is the constructor declaration, which has to be in the class definition:
// MyClass definition, possibly in .h file
class MyClass
{
public:
MyClass(); // default constructor declaration
...
};
and this is the constructor's definition
// MyClass default constructor definition, possibly in .cpp file
MyClass::MyClass() { }
What you are referring to is the constructor's definition. You can define it outside of the class definition to allow you to de-couple one from the other. Usually this means you put the class definition in a header file, to be included by client code, and the constructor definition (as well as other class member function definitions) in an implementation file that gets compiled. This way, users of your code have no compile time dependency on said definitions.