实参时丢弃了类型 discards qualifiers discards qualifiers问题

百思不得其解,于是百度,google吧。。

发现*上也有人有相同的问题

下面是他的问题:

For my compsci class, I am implementing a Stack template class, but have run into an odd error:

Stack.h: In member function ‘const T Stack<T>::top() const [with T = int]’:

Stack.cpp:10: error: passing ‘const Stack<int>’ as ‘this’ argument of ‘void Stack<T>::checkElements() [with T = int]’ discards qualifiers

Stack<T>::top() looks like this:

const T top() const {
    checkElements();
    return (const T)(first_->data);
}

Stack<T>::checkElements() looks like this:

void checkElements() {
    if (first_==NULL || size_==0)
        throw range_error("There are no elements in the stack.");
}

The stack uses linked nodes for storage, so first_ is a pointer to the first node.

Why am I getting this error?

正确答案是

Your checkElements() function is not marked as const so you can't call it on const qualified objects.

top(), however is const qualified so in top()this is a pointer to a const Stack (even if theStack instance on which top() was called happens to be non-const), so you can't callcheckElements() which always requires a non-const instance.

意思是说  在一个加了const限定符的成员函数中,不能够调用 非const成员函数。

因为如果调用了非const成员函数,就违反了const成员函数不改变对象的规定。

而error:...discards qualifiers 的意思就是缺少限定符

因此,他的解决办法很简单了 ,只要将checkElements()函数申明改为  checkElements()const就行了

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