With the code you provided in it’s constructor your class accepts an integer with it’s default value set to 1, if no parameters has passed to the constructor, it will have the 1 as value, in your case if you try to do this one:

myClass *mycls = new MyClass();
*mycls = 100;

the a would get the value of 100 so also in this case you should use explicit keyword. But in the other hand, if you have a class that it’s constructor have two parameters (I extended your class below):

class MyClass
{
public:
MyClass(int a, int b = 1);
void printVals();
private:
int _a;
int _b;
};

and we initiate it’s instance like the code below:

MyClass *m = new MyClass(2,2);
*m = 111;

the b variable will take it’s default value and the a variable will have the value of 111, and as you might expect this is not correct because you cannot allow programmers to use your class in this way, and you should use explicit keywords again

class myClass {
public:
explicit myClass(int a = 1);
};

The constructor should be treated as a one parameter one, right? The default value should have no effect for what “explicit” is concerned…