What is the explicit keyword for in c++?
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What does the explicit keyword in C++ mean?
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?
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", eg, 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
The explicit keyword prevents implicit conversions.
// Does not compile - an implicit conversion from const char* to CImg
CImg image = "C:/file.jpg"; // (1)
// Does compile
CImg image("C:/file.jpg"); // (2)
void PrintImage(const CImg& img) { };
PrintImage("C:/file.jpg"); // Does not compile (3)
PrintImage(CImg("C:/file.jpg")); // Does compile (4)
Without the explicit keyword, statements (1) and (3) would compile because the compiler can see that a const char* can be implicitly converted to a CImg (via the constructor accepting a const char* ). Sometimes this implicit conversion is undesirable because it doesn't always make sense.
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