What does the keyword 'explicit' do? Why is it used?

Data conversion from standard type to user-defined type is possible through conversion operator and he class’s constructor. But some conversion may take place which we don’t want. To prevent conversion operator from performing unwanted conversion, we avoid declaring it. But we may need constructor for building the object. Through the explicit keyword we can prevent unwanted conversions that may be done using the constructor.

#include<iostream.h>
class complex
{
  private:
    float real,imag;
  public: complex()
  {
    real=imag=0.0;
  }
  explicit complex(float r, float i = 0.0)
  {
    real = r;
    imag = i;
  }
  complex operator+(complex c)
  {
    complex t;
    t.real=real+c.real;
    t.imag=imag+c.imag;
    return t;
  }
  void display()
  {
    cout << endl << "real " 
          << real << ", imag " << imag;
  }
};
int main (int argc, char *argv[])
{
  complex c1(1.5,3.5);
  c2 = c1 + 1.25;
  c2.display();
}

In the statement: c2=c1+1.25
The compiler would find that here the overloaded operator +() function should get called. When it finds that there is a wrong type on the right hand side of + it would look for a conversion function which can convert float to complex. The two argument constructor can meet this requirement. Hence the compiler would call it. This is an implicit conversion and can be prevented by declaring the constructor as explicit.

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