conversion constructor

In C++, if a class has a constructor which can be called with a single argument, then this constructor becomes conversion constructor because such a constructor allows automatic conversion to the class being constructed.
```
#include <iostream>
using namespace std;

class Y {
  int a, b;
public:
  Y(int i) { };
  Y(const char* n, int j = 0) { };
};

void add(Y) { };

int main() {
  // equivalent to obj1 = Y(2)
  Y obj1 = 2;

  // equivalent to obj2 = Y("somestring",0)
  Y obj2 = "somestring";

  // equivalent to obj1 = Y(10)
  obj1 = 10;

  // equivalent to add(Y(5))
  add(5);
}
```
We can avoid such implicit conversions as these may lead to unexpected results. We can make the constructor explicit with the help of an explicit keyword. 
```
class Y {
  int a, b;
public:
  explicit Y(int i) { };
  explicit Y(const char* n, int j = 0) { };
};
```
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