QT Basis (ii) explicit usage

recently in Review Qt, ready to do the project, QT Creator default generated code explicit Dialog (Qwidget *parent = 0), there is such a keyword Explicit , which is used to modify the constructor. In the past when writing programs under Windows, basically did not encounter this keyword, then what is the keyword to do?

the keyword Explicit can prevent the "single-argument constructor" from being used for automatic type conversions . It doesn't seem easy to see this, but here's a simple example.

//main.cpp
#include <iostream>

             using namespace std;

             class Test
             {
            public:
                 Test (int a)
                 {
                       m_data = A;
                }

                  void Show ()
                  {
                        cout << "m_data =" << m_data << Endl;
                 }

             private:
                  int m_data;
            };

void Main (void)
{
Test T = 2; Assigns a constant to an object
T.show ();
}

compile to pass, execution result: M_data = 2.

Why is that? The original C + + passed the implicit conversion, constructs a temporary object Test (2), assigns it to T (here calls the default constructor, not the overloaded "=", because this is when the object is created). Then, if the constructor is appended with the keyword explicit, the constructor becomes the explicit Test (int a) and compiles again, the compiler will error. At this point, you can only explicitly use the constructor, Test t = Test (2) .

QT Basis (ii) explicit usage