從另一個類c ++中的構造函數調用方法

Question

我需要從另一個類的構造函數中的一個類調用一個方法。 我不知道如何在沒有得到“未在此范圍內聲明”錯誤的情況下執行此操作。 注意我只是學習C ++。

請參閱symboltable.cpp中的注釋，了解我在此處要完成的任務。 我不是在找任何人為我做這件事。 我可以使用一個例子或指向正確的方向，所以我可以想出來。

symboltable.h代碼：

class SymbolTable
{
public:
    SymbolTable() {}
    void insert(string variable, double value);
    void insert(string variable); // added for additional insert method
    double lookUp(string variable) const;
    void init(); // Added as mentioned in the conference area.
private:
    struct Symbol
    {
        Symbol(string variable, double value)
        {
            this->variable = variable;
            this->value = value;
        }
        string variable;
        double value;
    };
    vector<Symbol> elements;
};

symboltable.cpp代碼：

#include <string>
#include <vector>
#include <algorithm>

using namespace std;

#include "symboltable.h"

/* Implementing the "unreferenced variable" warning.
 * Modify the symbol table by adding another insert method
 * that supplies only the variable name.
 * This method should be called when the variable name
 * is encountered while building the arithmetic expression tree.
 * It would be called in the constructor of the Variable class.
 * The existing insert method, which is called when an assignment is encountered,
 * would first check to see whether it is already in the symbol table.
 * If it is not, then it is unreferenced.
 */

void SymbolTable::insert(string variable, double value)
{
    /* This existing insert method, which is called when an assignment is encountered,
     * first needs to check to see whether it is already in the symbol table.
     * If it is not, then it is unreferenced.
     * */

    //Need to check if variable is in the expression need to find out how the         expression is stored!

if (find(elements.begin(), elements.end(), variable)) {
    const Symbol& symbol = Symbol(variable, value);
        elements.push_back(symbol);
    } else
        throw string("Error: Test for output");
}

/* Adding another insert method that supplies only the variable name.
 * This method should be called when the variable name is encountered
 * while building the arithmetic expression tree.
 * It should be called in the constructor of the Variable class.
 */
void SymbolTable::insert(string variable)
{
    const Symbol& symbol = Symbol(variable, symbolTable.lookUp(variable));
    elements.push_back(symbol);
}


double SymbolTable::lookUp(string variable) const
{
    for (int i = 0; i < elements.size(); i++)
        if (elements[i].variable == variable)
             return elements[i].value;
        else
        throw "Error: Uninitialized Variable " + variable;
    return -1;
}

void SymbolTable::init() {
elements.clear(); // Clears the map, removes all elements.
}

variable.h代碼：

class Variable: public Operand
{
public:
    Variable(string name)  //constructor
    {
        // how do i call symbolTable.insert(name); here
        // without getting 'symboleTable' was not declared in this scope error

        this->name = name;
    }
    double evaluate();
private:
    string name;
};

variable.cpp代碼：

#include <string>
#include <strstream>
#include <vector>
using namespace std;

#include "expression.h"
#include "operand.h"
#include "variable.h"
#include "symboltable.h"

extern SymbolTable symbolTable;

double Variable::evaluate() {
    return symbolTable.lookUp(name);
}

Answer 1

extern SymbolTable symbolTable; 需要進入需要symbolTable的每個人都包含的頭文件。 然后，在variable.cpp中，您需要具有SymbolTable symbolTable;

Answer 2

有兩種解決方案：

1. 您使用全局變量 - 就像您的Variable::evaluate()示例一樣。 您當然可以將Variable::Variable()作為函數添加到“variable.cpp”而不是標題中。 或者您可以將extern SymbolTable symbolTable放到文件“variable.h”中。
2. 您symbolTable的引用傳遞給構造函數（並且可能將其存儲在Variable對象中 - 這樣，symbolTable根本不需要是全局變量。

順便說一下，在頭文件之前using namespace std添加通常被認為是不好的樣式。

Answer 3

您需要在構造函數中實例化第二個類，這將使其及其成員僅在第一個類的構造函數中或全局命名空間中可用。 例如：

MyFooClass CFoo;
MyBarClass CBar;

MyFooClass::MyFooClass()
{
  CBar = new MyBarClass();
  CBar.BarClassMemberFunction();
}