c++ - 错误 : declaration does not declare anything

Question

这是一个非常业余的问题，我确信这将是一个非常简单的答案，但我似乎无法弄清楚问题所在。我有一个带有相应 .cpp 文件的头文件，但出于某种原因，每当我尝试使用 g++ 进行编译时，我都会收到错误消息:

declaration does not declare anything

我很确定问题是我没有初始化文件中的(唯一)变量，但我不确定将其初始化为什么。如果有人可以提供帮助，我将不胜感激!这是我的文件:

符号表字典.h

#ifndef SymbolTable
#define SymbolTable
#include <new>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string>

using namespace std;

#pragma once

struct Symbol
{
    std::string Name;
    int Address;

    Symbol::Symbol()
    { }

    Symbol::Symbol(const string name, int address)
    {
        std::string sym(name);
        this->Name = sym;
        this->Address = address;
    }
};

extern map<std::string, Symbol> SymbolTable;

#endif

符号表词典.cpp

#include <new>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <map>

#include "SymbolTableDictionary.h"

using namespace std;

map<std::string, Symbol> SymbolTable;

编译错误:

In file included from SymbolTableDictionary.cpp:8:0:
SymbolTableDictionary.h:18:5: error: extra qualification ‘Symbol::’ on member ‘Symbol’ [-fpermissive]
SymbolTableDictionary.h:21:5: error: extra qualification ‘Symbol::’ on member ‘Symbol’ [-fpermissive]
SymbolTableDictionary.h:29:8: error: declaration does not declare anything [-fpermissive]
SymbolTableDictionary.cpp:12:1: error: declaration does not declare anything [-fpermissive]

Answer 1

问题是这段代码:

// header
#ifndef SymbolTable
#define SymbolTable

// implementation
map<std::string, Symbol> SymbolTable;

你#define SymbolTable 为空。因此建议

1. 始终对宏使用 ALL_UPPERCASE_NAMES(也用于包含守卫)

2. 仅为宏使用宏名称。