Why is there an error when executing sizeof()?

Question

C++ / C#

When compiling, it complains about the int size() function:
error: invalid application of 'sizeof' to incomplete type 'test1'
return sizeof(test1);
^

class test1;
class test
{
private:
    int i;
    char str[20];
public:
    int size()
{
    return sizeof(test1);
}

};
class test1: public test
{
public:
    int testq()
    {
      return sizeof(test);
    }
};

but if I place this function outside the class itself (int test::size()), then everything is fine.

Answer 1

Everything is fair - at the time of accessing sizeof, the compiler does not have a complete declaration of the test1 class.
In C++, only templates should be placed in the body of a class.
**about inline-functions** - inline is only *advice to the compiler*, and not the command "put the body of the function into the call site"

Answer 2

Because sizeof(test1) is used before the test1 class is defined.
And try to place it above the definition of class test1.