When do I need a default constructor?

Question

class Test
{
private :
    int i;
public:
    Test(int m)
    {
      i = m;
    }
    void restart(int k)
    {
        Test(k);
    }
};

However, the compiler (VS17) send me an error saying that "no default constructor exists for class Test", but I don't think I need a default constructor since all functions in this class need a int type argument.

Answer 1

In

class Test {
// ...
    void restart(int k)
    {
        Test(k);
    }
};

the statement Test(k); declares a variable of type Test named k. This variable k is initialized by calling the default constructor which doesn't exist.

I don't think I need a default constructor since all functions in this class need a int type argument.

That is neither a reason for nor against a class having/needing a default constructor or not.

If what you want is to set the value of Test::i inside Test::reset() then just do so:

class Test
{
private:    
    int i;

public:    
    Test(int m) : i{ m }  // you should use initializer lists instead of 
    {}                    // assignments in the constructors body.

    void restart(int k) { i = k; }
};