Dev Notes

Various Cheat Sheets and Resources by David Egan/Carawebs.

Explicit Conversion Constructor in C++


cpp
David Egan

Prepending the keyword explicit to a C++ constructor declaration (i.e. adding the explicit function specifier to the constructor within the class declaration) prevents unwanted type conversions.

Without the explicit function specifier, a constructor is a converting constructor - the type can be initialised by assigning a variable of the appropriate type. Prior to C++11, the constructor needed to be called with a single non-default parameter. For example:

class A
{
	A(int) {}		// Converting constructor
	A(int, double) {}	// Converting constructor
};

class B
{
	explicit B(int) {}
	explicit B(int, double) {}
}

void main()
{
	// Converting constructors
	// -----------------------
	A x = 42;		// Copy initialisation
	A x1 = {42, 3.14};	// Copy list initialisation
	A x2(42);		// Direct initialisation
	A x3{42, 3.14};		// Direct list initialisation

	// Explicit conversion constructors
	// --------------------------------
	B y = 42;		// NOT ALLOWED: Error
	B y1 = {42, 3.14};	// NOT ALLOWED: Error
	B y2(42);		// Direct initalisation OK
	B y3{42, 3.14};		// Direct list initialisation OK
	B y4 = (B)1		// OK - exlicit cast does a static cast, direct initialisation
}

Note that prior to C++11, converting constructors needed to have a single non-default parameter.

References


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