C++ 中的简单代码中出现奇怪的语法错误

Question

这段代码中有一个奇怪的语法错误； 我不明白它或任何错误。

#pragma once
#include <iostream>
class Tree {
public:
    Node* root;
    void InputTree() {
        int n, father;
        int nd;
        int child;
        char dir;
        std::cin >> n;
        int** tree = new int* [n];
        for (int i = 0; i < n; i++) {
            std::cin >> nd;
            tree[i] = new int[3];
            tree[i][0] = nd;
            tree[i][1] = -1;
            tree[i][2] = -1;
        }

        for (int i = 0; i < n - 1; i++) {
            std::cin >> father >> child >> dir;
            if (dir == 'L')
                tree[father][1] = child;
            else
                tree[father][2] = child;
        }
        Initialize(tree, 0);
    }
private:
    void Initialize(int** tree, int headIndex) {
        if (headIndex != -1) {
            root->value = tree[headIndex][0];
            root->right_tree->Initialize(tree, tree[headIndex][2]);
            root->left_tree->Initialize(tree, tree[headIndex][1]);
        }
    }
};
class Node {
public:
    int value;
    Tree* left_tree;
    Tree* right_tree;
};

这是 Visual Studio 的构建 output：

Build started...
1>------ Build started: Project: CppMainProject, Configuration: Debug Win32 ------
1>CppMainProject.cpp
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(5,6): error C2143: syntax error: missing ';' before '*'
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(5,6): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(5,12): error C2238: unexpected token(s) preceding ';'
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(33,4): error C2065: 'root': undeclared identifier
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(34,4): error C2065: 'root': undeclared identifier
1>C:\Users\moata\OneDrive\Documents\Visual Studio Projects\CppMainProject\CppMainProject\tstream.h(35,4): error C2065: 'root': undeclared identifier
1>Done building project "CppMainProject.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

声明root时仍未定义Node 。 尝试将节点 Class 移动到树 class 中，我建议您将其更改为结构，如下所示：

#pragma once
#include <iostream>
class Tree {
public:

    void InputTree() {
        int n, father;
        int nd;
        int child;
        char dir;
        std::cin >> n;
        int** tree = new int* [n];
        for (int i = 0; i < n; i++) {
            std::cin >> nd;
            tree[i] = new int[3];
            tree[i][0] = nd;
            tree[i][1] = -1;
            tree[i][2] = -1;
        }

        for (int i = 0; i < n - 1; i++) {
            std::cin >> father >> child >> dir;
            if (dir == 'L')
                tree[father][1] = child;
            else
                tree[father][2] = child;
        }
        Initialize(tree, 0);
    }
private:
    struct Node {
        int value;
        Tree* left_tree;
        Tree* right_tree;
    };
    Node* root;
    void Initialize(int** tree, int headIndex) {
        if (headIndex != -1) {
            root->value = tree[headIndex][0];
            root->right_tree->Initialize(tree, tree[headIndex][2]);
            root->left_tree->Initialize(tree, tree[headIndex][1]);
        }
    }
};

Answer 2

如果您希望它们都在一个文件中，请转发声明Node并将Tree::Initialize(...)的定义移到Node的定义之后。 或者只是转发声明Node并使用典型的.h 文件/.cpp 文件拆分。

单个文件版本是这样的：

class Node;  // <= this is a forward declaration...

class Tree {
public:
    Node* root;
    void InputTree() {
        // ....
    }
private:
    void Initialize(int** tree, int headIndex);
};

class Node {
public:
    int value;
    Tree* left_tree;
    Tree* right_tree;
};

void Tree::Initialize(int** tree, int headIndex)
{
    // ...
}

Answer 3

在root->value = tree[headIndex][0];这样的行中 ，在您告诉编译器 class 是什么之前，您正在尝试使用Node class。 您可以在完整定义之前将变量声明为指向类的指针（但您需要class 的前向声明才能这样做），但在提供完整的 class 定义之前，您不能取消引用此类指针。

在您的代码中，您可以将Node class 的定义移动到Tree class之前； 但是，由于它使用Tree指针，您必须提供一个前向声明Tree是 class。

#pragma once
#include <iostream>

class Tree; // Forward declaration that this is a class...

class Node {
public:
    int value;
    Tree* left_tree; // ... so we can now declare pointers to "Tree"
    Tree* right_tree;
};

class Tree {
public:
    Node* root; // We can declare a pointer 
    void InputTree()
    {
    // ... and then the rest of your code as is ...

一些值得一读：我什么时候可以使用前向声明？