Question

我在实施课程时遇到了麻烦。它应该能够从std :: string初始化。所以我写了一个副本（？）构造函数：

CVariable (std::string&, const int p_flags = 0);

我正在尝试制作一个CVariable的对象：

MCXJS::CVariable s_var = (string)"good job";

我收到以下错误：

F:\Projekty\MCXJS\src\main.cpp|8|error: conversion from 'std::string' to non-scalar type 'MCXJS::CVariable' requested|

如何解决这个问题？

我正在寻找能够完成完全的解决方案：

MCXJS::CVariable s_var = (string)"good job";

编辑：添加（几乎）完整的源代码：

cvariable.h

#ifndef CVARIABLE_H
#define CVARIABLE_H

#include <string>
#include <sstream>

namespace MCXJS
{
enum VARTYPE
{
    STRING = 0,
    INT = 1,
    FLOAT = 2
};

class CVariable
{
    public:
    VARTYPE Type () {return m_type;};
    std::string& Value () {return m_value;};
    bool SetType (VARTYPE);

    private:
    const int m_flags;
    VARTYPE m_type;
    std::string m_value;

    // ctors and operators
    public:
    CVariable (const int p_flags = 0);
    CVariable (CVariable&, const int);
    CVariable (std::string const&, const int);
    CVariable (const int&, const int);
    CVariable (const float&, const int);

    CVariable& operator= (const CVariable&);
    CVariable& operator= (const std::string&);
    CVariable& operator= (const int);
    CVariable& operator= (const float);
};
};

#endif // CVARIABLE_H

cvariable.cpp

#include "cvariable.h"

using namespace MCXJS;
using namespace std;

CVariable::CVariable (const int p_flags):
m_flags (p_flags)
{};

CVariable::CVariable (CVariable& p_var, const int p_flags = 0):
m_flags (p_flags),
m_type (p_var.Type()),
m_value (p_var.Value())
{};

CVariable::CVariable (std::string const& p_value, const int p_flags = 0):
m_flags (p_flags),
m_type (STRING),
m_value (p_value)
{};

CVariable::CVariable (const int p_value, const int p_flags = 0):
m_flags (p_flags),
m_type (INT)
{
std::ostringstream buffer;
buffer << p_value;
m_value = buffer.str();
};

CVariable::CVariable (const float p_value, const int p_flags = 0):
m_flags (p_flags),
m_type (FLOAT)
{
std::ostringstream buffer;
buffer << p_value;
m_value = buffer.str();
};

的main.cpp

#include "cvariable.h"
#include <iostream>

using namespace std;

int main()
{
MCXJS::CVariable s_var = (string)"good job"; // error
cout << s_var.Value() << '\n';
return 0;
}

编辑：添加枚举VARPARAM

编辑：好的，上面解决了，现在我有了这个：

cvariable.cpp|12|error: passing 'const MCXJS::CVariable' as 'this' argument of 'MCXJS::VARTYPE MCXJS::CVariable::Type()' discards qualifiers|
cvariable.cpp|13|error: passing 'const MCXJS::CVariable' as 'this' argument of 'std::string& MCXJS::CVariable::Value()' discards qualifiers|

Answer 1

你需要通过const引用

来实现

CVariable (std::string const&, const int p_flags = 0);

通过非const引用接受临时转换结果是没有意义的。之后对该参数的更改将会丢失。通过使它成为const引用使其工作很容易，因此标准C ++只是禁止它。

Answer 2

您是否省略了在代码示例中添加'='运算符重载的定义？您需要正确定义将字符串分配给对象时发生的情况。

Answer 3

MCXJS :: CVariable s_var =（string）“干得好”;

这是一个错字吗？应MCXJS::CVariable s_var = { 0, STRING, std::string("good job") };或更好，明确 - MCXJS::CVariable s_var(std::string("good job"), 0);。

从'std :: string'转换为请求的非标量类型

3 个答案: