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explicit

This keyword is a declaration specifier that can only be applied to in-class constructor declarations. An explicit constructor cannot take part in implicit conversions. It can only be used to explicitly construct an object.

The following program will fail to compile because of the explicit keyword. To resolve the error, remove the explicit keywords and adjust the code in g.

// spec1_explicit.cpp
// compile with: /EHsc
#include <stdio.h>
class C 
{
public:
   int i;
   explicit C(const C&)   // an explicit copy constructor
   {
      printf("\nin the copy constructor");
   }
   explicit C(int i )   // an explicit constructor
   {
      printf("\nin the constructor");
   }


   C()
   {
      i = 0;
   }
};

class C2
{
public:
   int i;
   explicit C2(int i )   // an explicit constructor
   {
   }
};

C f(C c)
{   // C2558
   c.i = 2;
   return c;   // first call to copy constructor
}

void f2(C2)
{
}

void g(int i)
{
    f2(i);   // C2558
   // try the following line instead
   // f2(C2(i));
}

int main()
{
   C c, d;
   d = f(c);   // c is copied
}
Note   explicit on a constructor with multiple arguments has no effect, since such constructors cannot take part in implicit conversions. However, for the purpose of implicit conversion, explicit will have an effect if a constructor has multiple arguments and all but one of the arguments has a default value.

See Also

C++ Keywords | Conversions

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