它在if条件下给出了错误 [英] It is giving error on the if condition

问题描述

  bool  P（）
 {
  int  j =  0 ; 
  if （Token.CP ==  数据类型）
 {
  return   true ; 
} 
} 

它不是比较在struct和dataype中的字符串类型Token.cp，也是一个字符串请帮助

错误是

2没有运算符==匹配这些操作数

操作数类型是：std :: string == const char [9]

解决方案

试试这个

  bool  P （）
 {
  if （Token.CP.compare（ 数据类型）==  0 ）
 {
 返回  true ; 
} 
 返回  false ; 
} 

如果Token.CP真的是 std :: string 那么该行应该干净地编译。您是否在翻译单元中包含了std :: string的标题？如果不是，你的代码可能会从 iostream 这样的东西中获取 std :: string 的前向声明，而 operator == 在字符串中定义。

错误消息告诉您编译器尝试使用操作数类型 std :: string 查找比较运算符的匹配项（显然是指 Token.CP ）和 const char [9] （指字符串文字，数据类型）。



此运算符可以在头文件 string 中找到，其中还包括类的std :: string 。请参阅 http://www.cplusplus.com/reference/string/string/operators/ [< a href =http://www.cplusplus.com/reference/string/string/operators/target =_ blanktitle =New Window> ^ ]如果你确实包含了字符串，你就不应该'得到那个错误。如果你没有包含字符串，你应该得到更多的错误，表明编译器不知道 std :: string 的定义。



在后一种情况下，只需添加包含。

  #include   <   string  < span class =code-keyword>>   

在前一种情况下，我很难相信你没有得到任何其他错误 - 最有可能的错误只是您需要先修复的另一个错误的结果。但为了安全起见，您可以按照解决方案2中的建议尝试 std :: string :: compare （尽管我认为这仍然会让您在其他地方遇到某种问题）

bool P()
{
    int j=0;
    if(Token.CP=="Datatype")
    {
        return true;
    }
}

it is not comparing the Token.cp which is string type in struct and dataype which is also a string please help
error is
2 no operator "==" matches these operands
operand types are: std::string == const char [9]

解决方案

Try this

bool P()
{
    if(Token.CP.compare("Datatype")==0)
    {
        return true;
    }
    return false;
}

If Token.CP is really a std::string then that line should compile cleanly. Have you include the header for std::string in the translation unit? If not your code might be picking up a forward declaration of std::string from something like iostream while operator== is defined in string.

The error message tells you the compiler tries to find a match for the comparison operator with the operand types std::string (obviously referring to Token.CP) and const char[9] (referring to the string literal, "Datatype").

This operator can be found in the header file string, which also includes the class, std::string. See http://www.cplusplus.com/reference/string/string/operators/[^] If you did include string, you shouldn't get that error. If you didn't include string, you should get a lot more errors indicating that the compiler doesn't know the definition of std::string.

In that latter case, just add the include.

#include <string>

In the former case, I have a hard time believing you don't get any other error - most likely the error is just a consequence of another that you need to fix first. But to be safe, you could try std::string::compare as suggested in solution 2 (although I think that will still leave you with some kind of problem elsewhere)

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