syntax - What is the explicit keyword for in c++?

explicit CImg(const char *const filename):_width(0),_height(0),_depth(0),_spectrum(0),_is_shared(false),_data(0) {
  assign(filename);

}

what's the difference with or without it?

Answer

It's use to decorate constructors; a constructor so decorated cannot be used by the compiler for implicit conversions.

C++ allows up to one user-provided conversion, where "user-provided" means, "by means of a class constructor", e.g., in :

 class circle {

   circle( const int r ) ;
 }

  circle c = 3 ; // implicit conversion using ctor

the compiler will call the circle ctor here, constructinmg circle c with a value of 3 for r.

explicit is used when you don't want this. Adding explicit means that you'd have to explicitly construct:

 class circle {
   explicit circle( const int r ) ;
 }

  // circle c = 3 ; implicit conversion not available now
  circle c(3); // explicit and allowed