Can we declare constructor before member variables?

Can the constructer declared before the member variable alter its value?

I thought only the code below works,

struct test {
    int a; 
    test(int t): a(t) {}
};

but I found the code below also works.

struct test {
    test(int t): a(t) {}
    int a; 
};

Usually, in function, we cannot use the variable that is not declared. Why the code above is OK?

>Solution :

Actually in C++ there’s an exception that there’s no need for forward declaration of functions and variable of a class/struct.

You can see my such examples on the internet like this:

class foo
{
public:
    foo(int x) : my_var(x) {}
private:
    int my_var;
};

The above is 100% valid.

You can also call a function of a class before it is defined like:

class bar
{
public:
    bar() 
    {
        this->my_below_func();
    }

    int my_below_func()
    {
        return 1;
    }
};

Always remember that these tricks aren’t going to work outside C++ classes/structs, you will need forward declaration of your functions and variables.