C++ Programming - Explicit Constructors


class A {
public:
 A() { ... };
 A(int i) { ... };
};

main()
{
 A a;
 A b = 1; 
 ...
}


The compiler interprets the expression A b= 1; as if the programmer had written
A b = A(1);


You might encounter a similar problem when calling a function that takes A as argument. The following example can either be a cryptic coding style or simply a programmer's typographical error. However, due to the implicit conversion constructor of class A, it will pass unnoticed:


int f(A a);
int main()
{
  f(1); // without a an explicit constructor,
        //this call is expanded into:  f ( A(1) );
        //was that intentional or merely a programmer's typo?
}


In order to avoid such implicit conversions, a constructor that takes one argument needs to be declared explicit:


class A {
public:
 A() { ... };
 explicit A(int i) { ... };
};

An explicit constructor does not behave as an implicit conversion operator, which enables the compiler to catch the typographical error this time:

int main()
{
  A a; //OK
  A b = 1;// compile time error ; this time the compiler catches the typo
}

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