字符串常量前的unqualified-id [英] Unqualified-id before string constant

问题描述

在编译以下代码时，出现错误字符串常量之前的预期非限定ID

On compiling following code I get error "expected unqualified-id before string constant"

In file "Notification_Constants.h"

namespace NOTIFICATION_CONSTANTS
{
    #define SERVICE_EMAIL "[e-mail]"
}

文件SendEmail.cpp

In file SendEmail.cpp

#include "Notification_Constants.h"

void UserPreferences::get_senders_email(String &_email)
{
    _email = NOTIFICATION_CONSTANTS::SERVICE_EMAIL;
}

如果我按照以下说明进行分配，那么编译的原因是什么错误。

If i assign like following it works properly, what is the reason for the compilation error.

_email = SERVICE_EMAIL;

有一个类似的，但未提及原因。

There is a similar question but the reason is not mentioned.

带有相关方法的字符串类声明

String class declaration with relevant methods

class String
{
public:

String();
String(const String& src);
String(const char *new_str);
String& operator=(const String& src);
String& operator=(const char *new_str);
};

推荐答案

首先，您应该在电子邮件地址周围加上引号：

First, you should put quotation marks around the email address:

#define SERVICE_EMAIL "[e-mail]"

第二，您完全不应使用 #define 。请使用 const 变量代替：

Second, you should not use #define at all. Use a const variable instead:

const String SERVICE_EMAIL = "[e-mail]";

#define 类型不安全，具有没有范围，通常都是邪恶的。

#defines are type unsafe, have no scope and are generally evil.

最后，您可能要考虑使用 std :: string 而不是您的 String 类。

Last, you may want to consider using std::string instead of your String class.

更新：

问题在于预处理器 #define s只不过是文本替换。完成预处理程序后，编译器将看到

The problem is that the preprocessor #defines are nothing more than text substitutions. When the preprocessor is done, your compiler will see

_email = NOTIFICATION_CONSTANTS::"[e-mail]";

该命名空间中没有字符串常量。 SERVICE_EMAIL 不是任何类型的标识符-只是预处理器的一个指示，用任何出现的 SERVICE_EMAIL 替换为 [e-mail] 。

There is no string constant in that namespace. SERVICE_EMAIL is not an identifier of any kind - it is just an indication to the preprocessor to substitute any occurrence of SERVICE_EMAIL with "[e-mail]".

解决方案是删除名称空间限定符：

The solution is to remove the namespace qualifier:

_email = SERVICE_EMAIL;

更好的解决方案：

如果您无权访问 #define ，则应将其包装在头文件中，如果可能的话：

If you do not have access to the #define, you should wrap it in a header file, if possible:

#include "Notification_Constants.h"

namespace NOTIFICATION_CONSTANTS {
    const String REAL_SERVICE_EMAIL = SERVICE_EMAIL;
}

，然后使用 NOTIFICATION_CONSTANTS :: REAL_SERVICE_EMAIL 具有范围，是类型，是名称空间的适当成员，等等。

and then use NOTIFICATION_CONSTANTS::REAL_SERVICE_EMAIL instead, which has scope, is a type, is a proper member of the namespace, and so on.

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