Preventing Implicit Conversions with Single-Argument Constructors

The reason we’ve been declaring every single-argument contructor preceded by the keyword explicit is to suppress implicit conversions via conversion constructors when such conversions should not be allowed. A constructor that’s declared explicit cannot be used in an implicit conversion. In the example of Figure 10.13, we use the original version of Array.h from Fig. 10.10, which included the keyword explicit in the declaration of the single-argument constructor in line 14

explicit Array( int = 10 ); // default constructor

Figure 10.13 presents a slightly modified version of the program in Fig. 10.12. When this program in Fig. 10.13 is compiled, the compiler produces an error message indicating that the integer value passed to outputArray in line 13 cannot be converted to a const Array &. The compiler error message (from Visual C++) is shown in the output window. Line 14 demonstrates how the explicit constructor can be used to create a temporary Array of 3 elements and pass it to function outputArray.

Click here to view code image

 1   // Fig. 10.13: fig10_13.cpp
 2   // Demonstrating an explicit constructor.
 3   #include <iostream>
 4   #include "Array.h"
 5   using namespace std;
 6
 7   void outputArray( const Array & ); // prototype
 8
 9   int main()
10   {
11      Array integers1( 7 ); // 7-element Array
12      outputArray( integers1 ); // output Array integers1
13      outputArray( 3 ); // convert 3 to an Array and output Array's contents
14      outputArray( Array( 3 ) ); // explicit single-argument constructor call
15   }  // end main
16
17   // print Array contents
18   void outputArray( const Array &arrayToOutput )
19   {
20      cout << "The Array received has " << arrayToOutput.getSize()
21         << " elements. The contents are: " << arrayToOutput << endl;
22   } // end outputArray

c:ooks2012cpphtp9examplesch10fig10_13fig10_13.cpp(13): error C2664:
'outputArray' : cannot convert parameter 1 from 'int' to 'const Array &'
          Reason: cannot convert from 'int' to 'const Array'
          Constructor for class 'Array' is declared 'explicit'

Fig. 10.13. Demonstrating an explicit constructor.

Image Error-Prevention Tip 10.4

Always use the explicit keyword on single-argument constructors unless they’re intended to be used as conversion constructors.

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