explicit_constructor.md

Explicit Constructor

The compiler is allowed to make one implicit conversion to resolve the parameters to a function. Suppose, you have a class String:

class String 
{
public:
// allocate n bytes to the String object
    String(int n)
    {
        std::cout<<"allocate "<<n<<" bytes to the String object" <<std::endl;
    }

// initializes object with char *p
    String(const char *p)
    {
        std::cout<<"initializes object with char *p" <<std::endl;
    }
};

You can write:

String mystring = 'a';

This is actually possible and character 'a' will be implicitly converted to int (ascii code for 'a' is 97) and then the String(int) constructor will be called:

allocate 97 bytes to the String object

But, perhaps this is not what you might have intended. explicit keyword will specify that a constructor or conversion function is explicit, which means, it cannot be used for implicit conversions and copy-initialization and the conversion will be disabled.

class String 
{
public:
 //allocate n bytes
    explicit String (int n);
// initialize sobject with string p    
    String(const char *p); 
};

explicit keyword will disable the above conversion (allocate 97 bytes instead of 'a'), so we should do a casting first:

Explicit::String s2=(const char*)'a';

code