operator int()用法

operator int() 是類型轉換運算符，比如：

struct A
{
int a;
A(int i):a(i){}	
operator int() const { return a; }
};

void main()
{
A aa(1);
int i = int(aa);
int j = aa;     //作用一樣
}

該函數的返回值類型就是函數名，所以不用顯式地表示出。

什麼叫返回類型就是函數名？
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返回類型是int，函數名也是int，就是說不寫成 int operator int() const { return value; }，
返回類型被省去了。
operator int() is a conversion operator, which allows this class to be used in place of an int. If an object of this type is used in a place where an int (or other numerical type) is expected, then this code will be used to get a value of the correct type.

For example:

int i(1);
INT I(2); // Initialised with constructor; I.a == 2
i = I;    // I is converted to an int using `operator int()`, returning 2.