依赖范围和嵌套模板 [英] Dependent scope and nested templates

问题描述

编译时：

#ifndef BTREE_H
#define BTREE_H
#include <QList>

template <class T, int degree>
class btree
{
public:
    class node
    {
    public :
        node();
    private:
        node* parent;
        QList<T> values;
        QList<node*> children;
    };
public:
    btree();
    void insert(const T& value);
    node* findLeaf(const T& value);
    void performInsertion(const T& value, node& place);
    //
    node* root;
};
#endif // BTREE_H

findLeaf的实现是：

Implementation of findLeaf is this:

template <class T, int degree>
btree<T,degree>::node* btree<T,degree>::findLeaf(const T &value)
{
    if(root == NULL)
        return root;
}

发生此错误：

 error: need ‘typename’ before ‘btree<T, degree>::Node’
 because ‘btree<T, degree>’ is a dependent scope

推荐答案

C ++语法是可怕的，当给定一个模板类时，要知道你所引用的 :: node 是一个变量/常量还是一个类型。

The C++ grammar is horrendous, and as such it is not possible, when given a template class, to know whether the ::node you refer to is a variable/constant or a type.

因此，标准要求您在类型之前使用 typename 删除此歧义，并将所有其他用法视为变量。

The Standard therefore mandates that you use typename before types to remove this ambiguity, and treats all other usages as if it was a variable.

因此

template <typename T, int degree>
typename btree<T,degree>::node* btree<T,degree>::findLead(T const& value)
^~~~~~~~

是定义的正确签名。

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